firebird-mirror/src/jrd/cch.cpp

/*
 *	PROGRAM:	JRD Access Method
 *	MODULE:		cch.c
 *	DESCRIPTION:	Disk cache manager
 *
 * The contents of this file are subject to the Interbase Public
 * License Version 1.0 (the "License"); you may not use this file
 * except in compliance with the License. You may obtain a copy
 * of the License at http://www.Inprise.com/IPL.html
 *
 * Software distributed under the License is distributed on an
 * "AS IS" basis, WITHOUT WARRANTY OF ANY KIND, either express
 * or implied. See the License for the specific language governing
 * rights and limitations under the License.
 *
 * The Original Code was created by Inprise Corporation
 * and its predecessors. Portions created by Inprise Corporation are
 * Copyright (C) Inprise Corporation.
 *
 * All Rights Reserved.
 * Contributor(s): ______________________________________.
 * 2001.07.06 Sean Leyne - Code Cleanup, removed "#ifdef READONLY_DATABASE"
 *                         conditionals, as the engine now fully supports
 *                         readonly databases.
 *
 * 2002.10.29 Sean Leyne - Removed obsolete "Netware" port
 *
 */
#include "firebird.h"
#include "../jrd/ib_stdio.h"
#include <string.h>
#include <stdlib.h>
#include "../jrd/jrd.h"
#include "../jrd/que.h"
#include "../jrd/lck.h"
#include "../jrd/ods.h"
#include "../jrd/os/pio.h"
#include "../jrd/cch.h"
#include "gen/codes.h"
#include "../jrd/jrn.h"
#include "../jrd/lls.h"
#include "../jrd/req.h"
#include "../jrd/sdw.h"
#include "../jrd/tra.h"
#include "../jrd/sbm.h"
#include "../jrd/iberr.h"
#include "../jrd/rse.h"
#include "../jrd/btr.h"
#include "../jrd/gdsassert.h"
#include "../jrd/all_proto.h"
#include "../jrd/cch_proto.h"
#include "../jrd/err_proto.h"
#include "../jrd/gds_proto.h"
#include "../jrd/iberr_proto.h"
#include "../jrd/isc_proto.h"
#include "../jrd/isc_s_proto.h"
#include "../jrd/jrd_proto.h"

#include "../jrd/lck_proto.h"
#include "../jrd/misc_proto.h"
#include "../jrd/mov_proto.h"
#include "../jrd/pag_proto.h"
#include "../jrd/os/pio_proto.h"
#include "../jrd/sbm_proto.h"
#include "../jrd/sch_proto.h"
#include "../jrd/sdw_proto.h"
#include "../jrd/shut_proto.h"
#include "../jrd/thd_proto.h"
#include "../jrd/tra_proto.h"
#include "../jrd/ail.h"
#include "../wal/wal_proto.h"
#include "../common/config/config.h"
#include "../jrd/jrd_time.h"

/* In the superserver mode, no page locks are acquired through the lock manager.
   Instead, a latching mechanism is used.  So the calls to lock subsystem for
   database pages in the original code should not be made, lest they should cause
   any undesirable side-effects.  The following defines help us achieve that.  */

#ifdef SUPERSERVER
#define	CACHE_WRITER
#define PAGE_LATCHING	1
#endif

#ifdef SUPERSERVER_V2
#define CACHE_READER
#endif

#ifdef PAGE_LATCHING
#define PAGE_LOCK(lock, lock_type, wait)
#define PAGE_LOCK_RELEASE(lock)  
#define PAGE_LOCK_ASSERT(lock)
#define PAGE_LOCK_OPT(lock, lock_type, wait)
#define PAGE_LOCK_RE_POST(lock) 
#define PAGE_OVERHEAD	(sizeof (bcb_repeat) + sizeof (struct bdb) + \
			 (int) dbb->dbb_page_size)
#else
#define PAGE_LOCK(lock, lock_type, wait)	LCK_lock (tdbb, lock, lock_type, wait)
#define PAGE_LOCK_RELEASE(lock)			LCK_release (tdbb, lock)
#define PAGE_LOCK_ASSERT(lock)			LCK_assert (tdbb, lock)
#define PAGE_LOCK_OPT(lock, lock_type, wait)	LCK_lock_opt (tdbb, lock, lock_type, wait)
#define PAGE_LOCK_RE_POST(lock)			LCK_re_post (lock)
#define PAGE_OVERHEAD	(sizeof (bcb_repeat) + sizeof (struct bdb) + \
			 sizeof (struct lck) + (int) dbb->dbb_page_size)
#endif

extern "C" {


static BDB alloc_bdb(TDBB, BCB, UCHAR **);
#ifndef PAGE_LATCHING
static int blocking_ast_bdb(void *);
#endif
static void btc_flush(TDBB, SLONG, BOOLEAN, ISC_STATUS *);
static void btc_insert(DBB, BDB);
static void btc_remove(BDB);
static void cache_bugcheck(int);
#ifdef CACHE_READER
static void THREAD_ROUTINE cache_reader(DBB);
#endif
#ifdef CACHE_WRITER
static void THREAD_ROUTINE cache_writer(DBB);
#endif
static void check_precedence(TDBB, WIN *, SLONG);
static void clear_precedence(DBB, BDB);
static BDB dealloc_bdb(BDB);
#ifndef PAGE_LATCHING
static BOOLEAN down_grade(TDBB, BDB);
#endif
static void expand_buffers(TDBB, ULONG);
static BDB get_buffer(TDBB, SLONG, LATCH, SSHORT);
static void journal_buffer(ISC_STATUS *, BDB);
static SSHORT latch_bdb(TDBB, LATCH, BDB, SLONG, SSHORT);
static SSHORT lock_buffer(TDBB, BDB, SSHORT, SSHORT);
static ULONG memory_init(TDBB, BCB, ULONG);
static void page_validation_error(TDBB, struct win *, SSHORT);
#ifdef CACHE_READER
static void prefetch_epilogue(PRF, ISC_STATUS *);
static void prefetch_init(PRF, TDBB);
static void prefetch_io(PRF, ISC_STATUS *);
static void prefetch_prologue(PRF, SLONG *);
#endif
static SSHORT related(BDB, BDB, SSHORT);
static void release_bdb(TDBB, BDB, BOOLEAN, BOOLEAN, BOOLEAN);
static BOOLEAN writeable(BDB);
static int write_buffer(TDBB, BDB, SLONG, USHORT, ISC_STATUS *, BOOLEAN);
static BOOLEAN write_page(TDBB, BDB, USHORT, ISC_STATUS *, BOOLEAN);
static void unmark(TDBB, WIN *);

#define MIN_BUFFER_SEGMENT	65536L

#ifndef DEBUG_PRINTF
#define DEBUG_PRINTF(msg)	ib_fprintf (ib_stderr, (msg))
#endif

/* Given pointer a field in the block, find the block */

#define BLOCK(fld_ptr, type, fld) (type)((SCHAR*) fld_ptr - OFFSET (type, fld))

#ifdef MULTI_THREAD
#ifndef VMS
#define INTERLOCK_CACHE
#endif
#endif

#define BCB_MUTEX_ACQUIRE
#define BCB_MUTEX_RELEASE

#define PRE_MUTEX_ACQUIRE
#define PRE_MUTEX_RELEASE

#define BTC_MUTEX_ACQUIRE
#define BTC_MUTEX_RELEASE

#define LATCH_MUTEX_ACQUIRE
#define LATCH_MUTEX_RELEASE

#define JOURNAL_PAGE		-1
#define SHADOW_PAGE		-2
#define FREE_PAGE		-3
#define CHECKPOINT_PAGE		-4
#define MIN_PAGE_NUMBER		-5

#define PRE_SEARCH_LIMIT	100
#define PRE_EXISTS		-1
#define PRE_UNKNOWN		-2

#define DUMMY_CHECKSUM        12345

bool set_write_direction(DBB dbb, BDB bdb, SSHORT direction) {
#ifdef SUPERSERVER
	if (bdb->bdb_write_direction == BDB_write_normal ||
		bdb->bdb_write_direction == BDB_write_both) 
	{
		if (direction != BDB_write_normal && direction != BDB_write_both)		
			dbb->backup_manager->release_sw_database_lock();
	} else {
		if (direction == BDB_write_normal || direction == BDB_write_both)		
			dbb->backup_manager->get_sw_database_lock(true, true);
	}
	bdb->bdb_write_direction = direction;
#else
	LCK_ast_inhibit();
	switch(bdb->bdb_write_direction) {
	case BDB_write_normal:
	case BDB_write_both:
		switch (direction) {
		case BDB_write_diff:
			dbb->backup_manager->increment_diff_use_count();
			dbb->backup_manager->release_sw_database_lock();
			break;
		case BDB_write_undefined:
			dbb->backup_manager->release_sw_database_lock();
			break;
		}
		break;
	case BDB_write_diff:
		switch(direction) {
		case BDB_write_normal:
		case BDB_write_both:
			dbb->backup_manager->decrement_diff_use_count();
			bdb->bdb_write_direction = direction;
			// We ask this function to enable signals
			if (!dbb->backup_manager->get_sw_database_lock(true, true)) {
				bdb->bdb_write_direction = BDB_write_undefined;
				return false;
			}
			return true;
		case BDB_write_undefined:
			dbb->backup_manager->decrement_diff_use_count();
		}
		break;
	case BDB_write_undefined:
		switch (direction) {
		case BDB_write_diff:
			dbb->backup_manager->increment_diff_use_count();
			break;			
		case BDB_write_normal:
		case BDB_write_both:
			bdb->bdb_write_direction = direction;
			// We ask this function to enable signals
			if (!dbb->backup_manager->get_sw_database_lock(true, true)) {
				bdb->bdb_write_direction = BDB_write_undefined;
				return false;
			}
			return true;
		}
		break;
	}
	bdb->bdb_write_direction = direction;
	LCK_ast_enable();
#endif
	return true;
}

void CCH_flush_database(TDBB tdbb) {
/**************************************
 *
 *	C C H _ f l u s h _ d a t a b a s e
 *
 **************************************
 *
 * Functional description
 *	Flush all buffers coming from database file.
 *	May be called from AST
 *
 **************************************/
	SET_TDBB(tdbb);
	DBB dbb = tdbb->tdbb_database;

	BCB bcb = dbb->dbb_bcb;

#ifdef SUPERSERVER
	// This is called on architectures with shared buffer cache (like SuperServer)

	// Determine a list of pages to flush
	Firebird::BePlusTree<ULONG, ULONG, Firebird::MemoryPool> pages(tdbb->tdbb_default);
	BCB_MUTEX_ACQUIRE;
	try {
		for (ULONG i = 0; (bcb = dbb->dbb_bcb) && i < bcb->bcb_count; i++) {
			BDB bdb = bcb->bcb_rpt[i].bcb_bdb;
			if (bdb->bdb_length)
				continue;
			if (!(bdb->bdb_flags & (BDB_dirty | BDB_db_dirty)) || 
				bdb->bdb_write_direction == BDB_write_diff) continue;
			// As the result we can add a number of redundant pages to flush
			// that were marked as dirty, but not released yet. This should
			// not create problems except that we'll loose some time later
			// during the flush itself
			pages.add(bdb->bdb_page);
		}
	} catch(std::exception&) {
		// Handle out-of-memory error
		BCB_MUTEX_RELEASE;
		throw;
	}
	BCB_MUTEX_RELEASE;
		
	// Now, apply force to flush this pages. Flush pages in order.
	if (pages.getFirst()) do {
		WIN window;
		window.win_page = pages.current();
		window.win_flags = 0;
		PAG page = CCH_FETCH(tdbb, &window, LCK_write, pag_undefined);
		CCH_MARK_MUST_WRITE(tdbb, &window);
		CCH_RELEASE(tdbb, &window);
	} while(pages.getNext());
#else		
	/* Do some fancy footwork to make sure that pages are
	   not removed from the btc tree at AST level.  Then
	   restore the flag to whatever it was before. */

	bool keep_pages = bcb->bcb_flags & BCB_keep_pages;
	dbb->dbb_bcb->bcb_flags |= BCB_keep_pages;
	for (ULONG i = 0; (bcb = dbb->dbb_bcb) && i < bcb->bcb_count; i++) {
		BDB bdb = bcb->bcb_rpt[i].bcb_bdb;
		if (bdb->bdb_length)
			continue;
		if (!(bdb->bdb_flags & (BDB_dirty | BDB_db_dirty)) || 
			bdb->bdb_write_direction == BDB_write_diff) continue;
		down_grade(tdbb, bdb);
	}
	if (!keep_pages)
		bcb->bcb_flags &= ~BCB_keep_pages;
#endif
}

USHORT CCH_checksum(BDB bdb)
{
/**************************************
 *
 *	C C H _ c h e c k s u m
 *
 **************************************
 *
 * Functional description
 *	Compute the checksum of a page.
 *
 **************************************/
#ifdef NO_CHECKSUM
	return DUMMY_CHECKSUM;
#else

	ULONG checksum, *p, *end;
	USHORT old_checksum;
	PAG page;
	DBB dbb;

	dbb = bdb->bdb_dbb;
#ifdef WIN_NT
/* ODS_VERSION8 for NT was shipped before page checksums
   were disabled on other platforms. Continue to compute
   checksums for ODS_VERSION8 databases but eliminate them
   for ODS_VERSION9 databases. The following code can be
   deleted when development on ODS_VERSION10 begins and
   NO_CHECKSUM is defined for all platforms. */

	if (dbb->dbb_ods_version >= ODS_VERSION9)
		return DUMMY_CHECKSUM;
#endif
	page = bdb->bdb_buffer;

	end = (ULONG *) ((SCHAR *) page + dbb->dbb_page_size);
	old_checksum = page->pag_checksum;
	page->pag_checksum = 0;
	p = (ULONG *) page;
	checksum = 0;

	do {
		checksum += *p++;
		checksum += *p++;
		checksum += *p++;
		checksum += *p++;
		checksum += *p++;
		checksum += *p++;
		checksum += *p++;
		checksum += *p++;
	}
	while (p < end);

	page->pag_checksum = old_checksum;

	if (checksum)
		return (USHORT) checksum;

/* If the page is all zeros, return an artificial checksum */

	for (p = (ULONG *) page; p < end;)
		if (*p++)
			return (USHORT) checksum;

/* Page is all zeros -- invent a checksum */

	return 12345;
#endif
}


void DLL_EXPORT CCH_do_log_shutdown(TDBB tdbb, SSHORT force_archive)
{
/**************************************
 *
 *    C C H _ d o _ l o g _ s h u t d o w n
 *
 **************************************
 *
 * Functional description
 *    Clear recover flag in the log page and flush buffers
 *
 **************************************/
	DBB dbb;
	LIP logp;
	WIN window;
	SLONG seqno, offset, p_offset;
	SCHAR walname[MAXPATHLEN];
	SSHORT w_len;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

	if (!dbb->dbb_wal)
		return;

	window.win_page = LOG_PAGE;
	window.win_flags = 0;
	logp = (LIP) CCH_FETCH(tdbb, &window, LCK_write, pag_log);
	logp->log_flags &= ~log_recover;

	AIL_shutdown(walname, &seqno, &offset, &p_offset, force_archive);

	w_len = strlen(walname);

/* This will update control points and current file */

	AIL_upd_cntrl_pt(walname, w_len, seqno, offset, p_offset);
	AIL_upd_cntrl_pt(walname, w_len, seqno, offset, p_offset);

	CCH_MARK_MUST_WRITE(tdbb, &window);
	CCH_RELEASE(tdbb, &window);
}


int CCH_down_grade_dbb(void *ast_object)
{
/**************************************
 *
 *	C C H _ d o w n _ g r a d e _ d b b
 *
 **************************************
 *
 * Functional description
 *	Down grade the lock on the database in response to a blocking
 *	AST.
 *
 **************************************/
	DBB dbb = reinterpret_cast<DBB>(ast_object);
	BCB bcb;
	LCK lock;
	struct tdbb thd_context, *tdbb;
	bcb_repeat *tail, *end;
	ISC_STATUS_ARRAY ast_status;

/* Ignore the request if the database or lock block does not appear
   to be valid . */

	if ((MemoryPool::blk_type(dbb) != type_dbb) ||
		!(lock = dbb->dbb_lock) ||
		(MemoryPool::blk_type(lock) != type_lck) || !(lock->lck_id))
		return 0;

/* Since this routine will be called asynchronously, we must establish
   a thread context. */

	SET_THREAD_DATA;

	tdbb->tdbb_database = dbb;
	tdbb->tdbb_attachment = lock->lck_attachment;
	tdbb->tdbb_quantum = QUANTUM;
	tdbb->tdbb_request = NULL;
	tdbb->tdbb_transaction = NULL;
	tdbb->tdbb_status_vector = ast_status;

	dbb->dbb_ast_flags |= DBB_blocking;

/* Database shutdown will release the database lock; just return. */

	if (SHUT_blocking_ast(dbb)) {
		dbb->dbb_ast_flags &= ~DBB_blocking;
		RESTORE_THREAD_DATA;
		return 0;
	}

/*
if (dbb->dbb_use_count)
    {
    RESTORE_THREAD_DATA;
    return;
    }
*/

/* If we are already shared, there is nothing more we can do.
   If any case, the other guy probably wants exclusive access,
   and we can't give it anyway */

	if ((lock->lck_logical == LCK_SW) || (lock->lck_logical == LCK_SR)) {
		RESTORE_THREAD_DATA;
		return 0;
	}

	if (dbb->dbb_flags & DBB_bugcheck) {
		LCK_convert(tdbb, lock, LCK_SW, LCK_WAIT);
		dbb->dbb_ast_flags &= ~DBB_blocking;
		RESTORE_THREAD_DATA;
		return 0;
	}

/* If we are supposed to be exclusive, stay exclusive */

	if (dbb->dbb_flags & DBB_exclusive) {
		RESTORE_THREAD_DATA;
		return 0;
	}

/* Assert any page locks that have been requested, but not asserted */

	ISC_ast_enter();

	dbb->dbb_ast_flags |= DBB_assert_locks;
	if ( (bcb = dbb->dbb_bcb) ) {
		if (bcb->bcb_count)
			for (tail = bcb->bcb_rpt, end = bcb->bcb_rpt + bcb->bcb_count; tail < end;
				 tail++)
				PAGE_LOCK_ASSERT(tail->bcb_bdb->bdb_lock);
	}

/* Down grade the lock on the database itself */

	if (lock->lck_physical == LCK_EX)
		LCK_convert(tdbb, lock, LCK_PW, LCK_WAIT);	/* This lets waiting cache manager in first */
	else
		LCK_convert(tdbb, lock,
					(dbb->dbb_flags & DBB_cache_manager) ? LCK_SR : LCK_SW,
					LCK_WAIT);

	dbb->dbb_ast_flags &= ~DBB_blocking;
	ISC_ast_exit();

/* Restore the prior thread context */

	RESTORE_THREAD_DATA;
	return 0;
}


BOOLEAN CCH_exclusive(TDBB tdbb, USHORT level, SSHORT wait_flag)
{
/**************************************
 *
 *	C C H _ e x c l u s i v e
 *
 **************************************
 *
 * Functional description
 *	Get exclusive access to a database.  If we get it, return TRUE.
 *	If the wait flag is FALSE, and we can't get it, give up and
 *	return FALSE. There are two levels of database exclusivity: LCK_PW
 *	guarantees there are  no normal users in the database while LCK_EX
 *	additionally guarantes background database processes like the
 *	shared cache manager have detached.
 *
 **************************************/
	LCK lock;
	DBB dbb;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

#ifdef SUPERSERVER
	if (!CCH_exclusive_attachment(tdbb, level, wait_flag))
		return FALSE;
#endif

	if (!(lock = dbb->dbb_lock))
		return FALSE;

	dbb->dbb_flags |= DBB_exclusive;

	switch (level) {
	case LCK_PW:
		if ((lock->lck_physical >= LCK_PW)
			|| LCK_convert(tdbb, lock, LCK_PW, wait_flag))
			return TRUE;
		break;

	case LCK_EX:
		if (lock->lck_physical == LCK_EX ||
			LCK_convert(tdbb, lock, LCK_EX, wait_flag)) return TRUE;
		break;

	default:
		break;
	}

/* If we are supposed to wait (presumably patiently),
   but can't get the lock, generate an error */

	if (wait_flag == LCK_WAIT)
		ERR_post(gds_deadlock, 0);

	dbb->dbb_flags &= ~DBB_exclusive;

	return FALSE;
}


BOOLEAN CCH_exclusive_attachment(TDBB tdbb, USHORT level, SSHORT wait_flag)
{
/**************************************
 *
 *	C C H _ e x c l u s i v e _ a t t a c h m e n t
 *
 **************************************
 *
 * Functional description
 *	Get exclusive access to a database. If we get it, return TRUE.
 *	If the wait flag is FALSE, and we can't get it, give up and
 *	return FALSE.
 *
 **************************************/
	DBB dbb;
	ATT attachment, *ptr;
	SLONG timeout, remaining;
	BOOLEAN found;

#define CCH_EXCLUSIVE_RETRY_INTERVAL	1	/* retry interval in seconds */

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;
	attachment = tdbb->tdbb_attachment;
	if (attachment->att_flags & ATT_exclusive)
		return TRUE;

	attachment->att_flags |=
		(level == LCK_none) ? ATT_attach_pending : ATT_exclusive_pending;

	timeout =
		(SLONG) (wait_flag <
				 0) ? -wait_flag : ((wait_flag ==
									 LCK_WAIT) ? 1L << 30 :
									CCH_EXCLUSIVE_RETRY_INTERVAL);

/* If requesting exclusive database access, then re-position attachment as the
   youngest so that pending attachments may pass. */

	if (level != LCK_none) {
		for (ptr = &dbb->dbb_attachments; *ptr; ptr = &(*ptr)->att_next)
			if (*ptr == attachment) {
				*ptr = attachment->att_next;
				break;
			}
		attachment->att_next = dbb->dbb_attachments;
		dbb->dbb_attachments = attachment;
	}

	for (remaining = timeout; remaining > 0;
		 remaining -= CCH_EXCLUSIVE_RETRY_INTERVAL) {
		if (tdbb->tdbb_attachment->att_flags & ATT_shutdown)
			break;

		found = FALSE;
		for (attachment = tdbb->tdbb_attachment->att_next; attachment;
			 attachment = attachment->att_next) {
			if (attachment->att_flags & ATT_shutdown)
				continue;

			if (level == LCK_none) {	/* Wait for other attachments requesting exclusive access */
				if (attachment->att_flags &
					(ATT_exclusive | ATT_exclusive_pending)) {
					found = TRUE;
					break;
				}
			}
			else {				/* Requesting exclusive database access */

				found = TRUE;
				if (attachment->att_flags & ATT_exclusive_pending) {
					tdbb->tdbb_attachment->att_flags &=
						~ATT_exclusive_pending;
					if (wait_flag == LCK_WAIT)
						ERR_post(gds_deadlock, 0);
					else
						return FALSE;
				}
				break;
			}
		}

		if (!found) {
			tdbb->tdbb_attachment->att_flags &=
				~(ATT_exclusive_pending | ATT_attach_pending);
			if (level != LCK_none)
				tdbb->tdbb_attachment->att_flags |= ATT_exclusive;
			return TRUE;
		}

		/* Our thread needs to sleep for CCH_EXCLUSIVE_RETRY_INTERVAL seconds. */

		if (remaining > CCH_EXCLUSIVE_RETRY_INTERVAL)
		{
			THREAD_EXIT;
			THREAD_SLEEP(CCH_EXCLUSIVE_RETRY_INTERVAL * 1000);
			THREAD_ENTER;
		}

#ifdef CANCEL_OPERATION
		if (tdbb->tdbb_attachment->att_flags & ATT_cancel_raise)
			if (JRD_reschedule(tdbb, 0, FALSE)) {
				tdbb->tdbb_attachment->att_flags &=
					~(ATT_exclusive_pending | ATT_attach_pending);
				ERR_punt();
			}
#endif
	}

	tdbb->tdbb_attachment->att_flags &=
		~(ATT_exclusive_pending | ATT_attach_pending);
	return FALSE;
}


void CCH_expand(TDBB tdbb, ULONG number)
{
/**************************************
 *
 *	C C H _ e x p a n d
 *
 **************************************
 *
 * Functional description
 *	Expand the cache to at least a given number of buffers.  If
 *	it's already that big, don't do anything.
 *
 **************************************/

	SET_TDBB(tdbb);

	BCB_MUTEX_ACQUIRE;
	expand_buffers(tdbb, number);
	BCB_MUTEX_RELEASE;
}


PAG CCH_fake(TDBB tdbb, WIN * window, SSHORT latch_wait)
{
/**************************************
 *
 *	C C H _ f a k e
 *
 **************************************
 *
 * Functional description
 *	Fake a fetch to a page.  Rather than reading it, however,
 *	zero it in memory.  This is used when allocating a new page.
 *
 * input
 *	latch_wait:	1 => Wait as long as necessary to get the latch.
 *				This can cause deadlocks of course.
 *			0 => If the latch can't be acquired immediately,
 *				or an IO would be necessary, then give
 *				up and return 0.
 *	      		<negative number> => Latch timeout interval in seconds.
 *
 * return
 *	pag pointer if successful.
 *	NULL pointer if timeout occurred (only possible if latch_wait <> 1).
 *	NULL pointer if latch_wait=0 and the faked page would have to be
 *			before reuse.
 *
 **************************************/
	DBB dbb;
	BDB bdb;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

/* if there has been a shadow added recently, go out and
   find it before we grant any more write locks */

	if (dbb->dbb_ast_flags & DBB_get_shadows)
		SDW_get_shadows();

	bdb = get_buffer(tdbb, window->win_page, LATCH_exclusive, latch_wait);
	if (!bdb)
		return NULL;			/* latch timeout occurred */

/* If a dirty orphaned page is being reused - better write it first
   to clear current precedences and checkpoint state. This would also
   update the bcb_free_pages field appropriately. */

	if (bdb->bdb_flags & (BDB_dirty | BDB_db_dirty)) {
		/* If the caller didn't want to wait at all, then
		   return 'try to fake an other page' to the caller. */

		if (!latch_wait) {
			release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);
			return NULL;
		}

		if (!write_buffer
			(tdbb, bdb, bdb->bdb_page, TRUE, tdbb->tdbb_status_vector, TRUE))
			CCH_unwind(tdbb, TRUE);
	}
	else if (QUE_NOT_EMPTY(bdb->bdb_lower)) {
		/* Clear residual precedence left over from AST-level I/O. */

		clear_precedence(dbb, bdb);
	}

	bdb->bdb_flags = (BDB_writer | BDB_faked);
	bdb->bdb_scan_count = 0;

	lock_buffer(tdbb, bdb, LCK_WAIT, pag_undefined);

	MOVE_CLEAR(bdb->bdb_buffer, (SLONG) dbb->dbb_page_size);
	window->win_buffer = bdb->bdb_buffer;
	window->win_expanded_buffer = NULL;
	window->win_bdb = bdb;
	window->win_flags = 0;
	CCH_MARK(tdbb, window);

	return bdb->bdb_buffer;
}


PAG CCH_fetch(TDBB tdbb,
			  WIN * window,
			  USHORT lock_type,
			  SSHORT page_type,
			  SSHORT checksum, SSHORT latch_wait, BOOLEAN read_shadow)
{
/**************************************
 *
 *	C C H _ f e t c h
 *
 **************************************
 *
 * Functional description
 *	Fetch a specific page.  If it's already in cache,
 *	so much the better.
 *
 * input
 *	latch_wait:	1 => Wait as long as necessary to get the latch.
 *				This can cause deadlocks of course.
 *			0 => If the latch can't be acquired immediately,
 *				give up and return 0.
 *	      		<negative number> => Latch timeout interval in seconds.
 *
 * return
 *	PAG if successful.
 *	NULL pointer if timeout occurred (only possible if latch_wait <> 1).
 *
 **************************************/
	SSHORT fetch_lock_return;
	BDB bdb;

	SET_TDBB(tdbb);

/* FETCH_LOCK will return 0, 1, -1 or -2 */

	fetch_lock_return =
		CCH_FETCH_LOCK(tdbb, window, lock_type, LCK_WAIT, latch_wait,
					   page_type);

	if (fetch_lock_return == 1)
		CCH_FETCH_PAGE(tdbb, window, checksum, read_shadow);	/* must read page from disk */
	else if (fetch_lock_return == -2 || fetch_lock_return == -1)
		return NULL;			/* latch or lock timeout */

	bdb = window->win_bdb;

/* If a page was read or prefetched on behalf of a large scan
   then load the window scan count into the buffer descriptor.
   This buffer scan count is decremented by releasing a buffer
   with CCH_RELEASE_TAIL.

   Otherwise zero the buffer scan count to prevent the buffer
   from being queued to the LRU tail. */

	if (window->win_flags & WIN_large_scan) {
		if (fetch_lock_return == 1 ||
			bdb->bdb_flags & BDB_prefetch || bdb->bdb_scan_count < 0)
			bdb->bdb_scan_count = window->win_scans;
	}
	else if (window->win_flags & WIN_garbage_collector) {
		if (fetch_lock_return == 1)
			bdb->bdb_scan_count = -1;
		if (bdb->bdb_flags & BDB_garbage_collect)
			window->win_flags |= WIN_garbage_collect;
	}
	else if (window->win_flags & WIN_secondary) {
		if (fetch_lock_return == 1)
			bdb->bdb_scan_count = -1;
	}
	else {
		bdb->bdb_scan_count = 0;
		if (bdb->bdb_flags & BDB_garbage_collect)
			bdb->bdb_flags &= ~BDB_garbage_collect;
	}

/* Validate the fetched page matches the expected type */

	if (bdb->bdb_buffer->pag_type != (SCHAR) page_type &&
		page_type != pag_undefined)
			page_validation_error(tdbb, window, page_type);

	return window->win_buffer;
}


SSHORT CCH_fetch_lock(TDBB tdbb,
					  WIN * window,
					  USHORT lock_type,
					  SSHORT wait, SSHORT latch_wait, SSHORT page_type)
{
/**************************************
 *
 *	C C H _ f e t c h _ l o c k
 *
 **************************************
 *
 * Functional description
 *	Fetch a lock for a specific page.
 *	Return TRUE if the page needs to be
 *	read and FALSE if not. If a timeout
 *	was passed (wait < 0) and the lock
 *	could not be granted return wait.
 *
 * input
 *
 *	wait: LCK_WAIT = TRUE = 1	=> Wait as long a necessary to get the lock.
 *	      LCK_NO_WAIT = FALSE = 0	=> If the lock can't be acquired immediately,
 *						give up and return -1.
 *	      <negative number>		=> Lock timeout interval in seconds.
 *
 *	latch_wait:	1 => Wait as long as necessary to get the latch.
 *				This can cause deadlocks of course.
 *			0 => If the latch can't be acquired immediately,
 *				give up and return -2.
 *	      		<negative number> => Latch timeout interval in seconds.
 *
 * return
 *	0:	fetch & lock were successful, page doesn't need to be read.
 *	1:	fetch & lock were successful, page needs to be read from disk.
 *	-1:	lock timed out, fetch failed.
 *	-2:	latch timed out, fetch failed, lock not attempted.
 *
 **************************************/
	DBB dbb;
	BDB bdb;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

/* if there has been a shadow added recently, go out and
   find it before we grant any more write locks */

	if (dbb->dbb_ast_flags & DBB_get_shadows)
		SDW_get_shadows();

/* Look for the page in the cache. */

	bdb = get_buffer(tdbb, window->win_page,
					 ((lock_type >= LCK_write) ? LATCH_exclusive :
					  LATCH_shared), latch_wait);
	if ((latch_wait != 1) && (bdb == 0))
		return -2;				/* latch timeout */

	if (lock_type >= LCK_write)
		bdb->bdb_flags |= BDB_writer;

/* the expanded index buffer is only good when the page is
   fetched for read; if it is ever fetched for write, it must
   be discarded */

	if (bdb->bdb_expanded_buffer && (lock_type > LCK_read)) {
		delete bdb->bdb_expanded_buffer;
		bdb->bdb_expanded_buffer = NULL;
	}

	window->win_bdb = bdb;
	window->win_buffer = bdb->bdb_buffer;
	window->win_expanded_buffer = bdb->bdb_expanded_buffer;

/* lock_buffer returns 0 or 1 or -1. */
	return lock_buffer(tdbb, bdb, wait, page_type);
}


void CCH_fetch_page(
					TDBB tdbb,
					WIN * window,
					SSHORT compute_checksum, BOOLEAN read_shadow)
{
/**************************************
 *
 *	C C H _ f e t c h _ p a g e
 *
 **************************************
 *
 * Functional description
 *	Fetch a specific page.  If it's already in cache,
 *	so much the better.  When "compute_checksum" is 1, compute
 * 	the checksum of the page.  When it is 2, compute
 *	the checksum only when the page type is nonzero.
 *
 **************************************/
	DBB dbb;
	BDB bdb;
	PAG page;
	ISC_STATUS *status;
	FIL file;
	SSHORT retryCount;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;
	bdb = window->win_bdb;

	status = tdbb->tdbb_status_vector;

	page = bdb->bdb_buffer;
	bdb->bdb_incarnation = ++dbb->dbb_page_incarnation;

	AST_CHECK;
	++dbb->dbb_reads;
#ifdef SUPERSERVER
	THREAD_EXIT;
#endif
	page = bdb->bdb_buffer;
	file = dbb->dbb_file;
	retryCount = 0;

/* We will read a page, and if there is an I/O error we will try to
   use the shadow file, and try reading again, for a maximum of
   3 tries, before it gives up.

   The read_shadow flag is set to false only in the call to
   FETCH_NO_SHADOW, which is only called from validate
   code.

   read_shadow = FALSE -> IF an I/O error occurs give up (exit
   the loop, clean up, and return). So the caller,
   validate in most cases, can know about it and attempt
   to remedy the situation.

   read_shadow = TRUE -> IF an I/O error occurs attempt
   to rollover to the shadow file.  If the I/O error is
   persistant (more than 3 times) error out of the routine by
   calling CCH_unwind, and eventually punting out. */
   
	if (!dbb->backup_manager->lock_state(false)) 
	{
		PAGE_LOCK_RELEASE(bdb->bdb_lock);
		CCH_unwind(tdbb, TRUE);
	}
	int bak_state = dbb->backup_manager->get_state();
	ULONG diff_page = 0;
	if (bak_state == nbak_state_stalled || bak_state == nbak_state_merge) {
		if (!dbb->backup_manager->lock_alloc(false)) 
		{
			PAGE_LOCK_RELEASE(bdb->bdb_lock);
			dbb->backup_manager->unlock_state();
			CCH_unwind(tdbb, TRUE);
		}
		diff_page = dbb->backup_manager->get_page_index(bdb->bdb_page);
		dbb->backup_manager->unlock_alloc();
		NBAK_TRACE(("Reading page %d, state=%d, diff page=%d", bdb->bdb_page, bak_state, diff_page));
	}

	if (bak_state == nbak_state_normal || 
		(bak_state == nbak_state_stalled && !diff_page) ||
		// In merge mode, if we are reading past beyond old end of file and page is in .delta file
		// then we maintain actual page in difference file. Always read it from there.
		(bak_state == nbak_state_merge && 
			(!diff_page || (bdb->bdb_page < dbb->backup_manager->get_backup_pages())))
	   ) 
	{
		NBAK_TRACE(("Reading page %d, state=%d, diff page=%d from DISK",
			bdb->bdb_page, bak_state, diff_page));
		// Read page from disk as normal
		while (!PIO_read(file, bdb, page, status)) {
			if (!read_shadow) {
				break;
			}
#ifdef SUPERSERVER
			THREAD_ENTER;
#endif
			if (!CCH_rollover_to_shadow(dbb, file, FALSE)) {
				PAGE_LOCK_RELEASE(bdb->bdb_lock);
				dbb->backup_manager->unlock_state();
				CCH_unwind(tdbb, TRUE);
			}
			if (file != dbb->dbb_file)
				file = dbb->dbb_file;
			else {
				if (retryCount++ == 3) {
					ib_fprintf(ib_stderr,
							   "IO error loop Unwind to avoid a hang\n");
					PAGE_LOCK_RELEASE(bdb->bdb_lock);
					dbb->backup_manager->unlock_state();
					CCH_unwind(tdbb, TRUE);
				}
			}
#ifdef SUPERSERVER
			THREAD_EXIT;
#endif
		}
	}
	
	if (diff_page && (
		bak_state == nbak_state_stalled || 
	    (bak_state == nbak_state_merge && 
		 (bdb->bdb_page >= dbb->backup_manager->get_backup_pages() ||
		  page->pag_scn() < dbb->backup_manager->get_current_scn())
		)))
	{		
		NBAK_TRACE(("Reading page %d, state=%d, diff page=%d from DIFFERENCE", 
			bdb->bdb_page, bak_state, diff_page));
		if (!dbb->backup_manager->read_difference(diff_page, page)) {
			PAGE_LOCK_RELEASE(bdb->bdb_lock);
			dbb->backup_manager->unlock_state();
			CCH_unwind(tdbb, TRUE);
		}
		if (page->pag_type == pag_undefined) {
			// Page was marked as allocated inside the difference file, but not really used
			// this is very rare, but possible case (after certain errors). 
			// Read (or re-read) page from database
			NBAK_TRACE(("Re-reading page %d, state=%d, diff page=%d from DISK", 
				bdb->bdb_page, bak_state, diff_page));
			while (!PIO_read(file, bdb, page, status)) {
				if (!read_shadow) {
					break;
				}
#ifdef SUPERSERVER
				THREAD_ENTER;
#endif
				if (!CCH_rollover_to_shadow(dbb, file, FALSE)) {
					PAGE_LOCK_RELEASE(bdb->bdb_lock);
					dbb->backup_manager->unlock_state();
					CCH_unwind(tdbb, TRUE);
				}
				if (file != dbb->dbb_file)
					file = dbb->dbb_file;
				else {
					if (retryCount++ == 3) {
						ib_fprintf(ib_stderr,
								   "IO error loop Unwind to avoid a hang\n");
						PAGE_LOCK_RELEASE(bdb->bdb_lock);
						dbb->backup_manager->unlock_state();
						CCH_unwind(tdbb, TRUE);
					}
				}
#ifdef SUPERSERVER
				THREAD_EXIT;
#endif
			}
		}
	}
	
	dbb->backup_manager->unlock_state();

#ifdef DEBUG_SAVE_BDB_PAGE
/* This debug option will not work with WAL or existing databases.
   Verify that page number saved in pag_offset by write_page(). */

	if (bdb->bdb_page != page->pag_offset)
		DEBUG_PRINTF
			("Incorrect page number in pag_offset (CCH_fetch_page)\n");
#endif

#ifndef NO_CHECKSUM
	if (((compute_checksum == 1)
		 || ((compute_checksum == 2) && page->pag_type))
		&& ((page->pag_checksum != CCH_checksum(bdb))
			&& !(dbb->dbb_flags & DBB_damaged))) {
#ifdef SUPERSERVER
		THREAD_ENTER;
#endif
		IBERR_build_status(tdbb->tdbb_status_vector,
						   gds_db_corrupt,
						   gds_arg_string, "",
						   gds_arg_gds, gds_bad_checksum,
						   gds_arg_gds, gds_badpage,
						   gds_arg_number, (SLONG) bdb->bdb_page, 0);
		/* We should invalidate this bad buffer. */

		PAGE_LOCK_RELEASE(bdb->bdb_lock);
		CCH_unwind(tdbb, TRUE);
	}
#endif /* NO_CHECKSUM */

#ifdef SUPERSERVER
	THREAD_ENTER;
#endif
	AST_CHECK;

	bdb->bdb_flags &= ~(BDB_not_valid | BDB_read_pending);
	window->win_buffer = bdb->bdb_buffer;
}


void CCH_fini(TDBB tdbb)
{
/**************************************
 *
 *	C C H _ f i n i
 *
 **************************************
 *
 * Functional description
 *	Shut down buffer operation.
 *
 **************************************/
	DBB dbb;
	BCB bcb;
	BDB bdb;
	BOOLEAN flush_error;
	bcb_repeat *tail, *end;
#ifdef CACHE_WRITER
	QUE que;
	LWT lwt_;
#endif

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;
	flush_error = FALSE;

	try {

/* If we've been initialized, either flush buffers
   or release locks, depending on where we've been
   bug-checked; as a defensive programming measure,
   make sure that the buffers were actually allocated */

	if ((bcb = dbb->dbb_bcb) &&
            (tail = bcb->bcb_rpt) && (tail->bcb_bdb))
		if (dbb->dbb_flags & DBB_bugcheck || flush_error)
			for (end = bcb->bcb_rpt + bcb->bcb_count;
                               tail < end; tail++) {
				bdb = tail->bcb_bdb;
				if (bdb->bdb_length)
					continue;
				if (bdb->bdb_expanded_buffer) {
					delete bdb->bdb_expanded_buffer;
					bdb->bdb_expanded_buffer = NULL;
				}
				PAGE_LOCK_RELEASE(bdb->bdb_lock);
			}
		else
			CCH_flush(tdbb, (USHORT) FLUSH_FINI, (SLONG) 0);


/* If this is the last user, cleanup the log page. */

	if (dbb->dbb_wal) {
		if (CCH_exclusive(tdbb, LCK_EX, LCK_NO_WAIT))
			CCH_do_log_shutdown(tdbb, 0);
	}

#ifdef CACHE_READER

/* Shutdown the dedicated cache reader for this database. */

	if ((bcb = dbb->dbb_bcb) && (bcb->bcb_flags & BCB_cache_reader)) {
		SLONG count;
		EVENT event = dbb->dbb_reader_event;

		bcb->bcb_flags &= ~BCB_cache_reader;
		ISC_event_post(event);
		count = ISC_event_clear(event);
		THREAD_EXIT;
		ISC_event_wait(1, &event, &count, 0, NULL, 0);
		THREAD_ENTER;
		/* Now dispose off the cache reader associated semaphore */
		ISC_event_fini(event);
	}
#endif

#ifdef CACHE_WRITER

/* Shutdown the dedicated cache writer for this database. */

	if ((bcb = dbb->dbb_bcb) && (bcb->bcb_flags & BCB_cache_writer)) {
		SLONG count;
		EVENT event = dbb->dbb_writer_event_fini;
		/* initialize initialization event */
		ISC_event_init(event, 0, 0);
		count = ISC_event_clear(event);

		bcb->bcb_flags &= ~BCB_cache_writer;
		ISC_event_post(dbb->dbb_writer_event); /* Wake up running thread */
		THREAD_EXIT;
		ISC_event_wait(1, &event, &count, 0, NULL, 0);
		THREAD_ENTER;
		/* Cleanup initialization event */
		ISC_event_fini(event);
	}
#endif

/* close the database file and all associated shadow files */

	PIO_close(dbb->dbb_file);
	SDW_close();

	if ( (bcb = dbb->dbb_bcb) ) {
		while (bcb->bcb_memory)
			gds__free(LLS_POP(&bcb->bcb_memory));
#ifdef CACHE_WRITER
		/* Dispose off any associated latching semaphores */
		while (QUE_NOT_EMPTY(bcb->bcb_free_lwt)) {
			que = bcb->bcb_free_lwt.que_forward;
			QUE_DELETE((*que));
			lwt_ = (LWT) BLOCK(que, LWT, lwt_waiters);
			ISC_event_fini(&lwt_->lwt_event);
		}
#endif
	}

	}	// try
	catch (const std::exception&)
	{
		if (!flush_error) {
			flush_error = TRUE;
		} else {
			ERR_punt();
		}
	}

}


void CCH_flush(TDBB tdbb, USHORT flush_flag, SLONG tra_number)
{
/**************************************
 *
 *	C C H _ f l u s h
 *
 **************************************
 *
 * Functional description
 *	Flush all buffers.  If the release flag is set,
 *	release all locks.
 *
 **************************************/
	DBB dbb;
	BCB bcb;
	BDB bdb;
	ULONG i;
	SLONG transaction_mask;
	ISC_STATUS *status;
	LATCH latch;
	BOOLEAN all_flag, release_flag, sweep_flag, sys_only, write_thru;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

	bcb = dbb->dbb_bcb;
	status = tdbb->tdbb_status_vector;

/* note that some of the code for btc_flush()
   replicates code in the for loop
   to minimize call overhead -- changes should
   be made in both places */

	if (flush_flag & (FLUSH_TRAN | FLUSH_SYSTEM)) {
		transaction_mask =
			(tra_number) ? 1L << (tra_number & (BITS_PER_LONG - 1)) : 0;
		if (!transaction_mask && (flush_flag & FLUSH_SYSTEM))
			sys_only = TRUE;
		else
			sys_only = FALSE;

#ifdef SUPERSERVER_V2
		if (!dbb->dbb_wal && !(dbb->dbb_flags & DBB_force_write)
			&& transaction_mask) {
			dbb->dbb_flush_cycle |= transaction_mask;
			if (!(bcb->bcb_flags & BCB_writer_active))
				ISC_event_post(dbb->dbb_writer_event);
		}
		else
#endif
			btc_flush(tdbb, transaction_mask, sys_only, status);
	}
	else {
		all_flag = (flush_flag & FLUSH_ALL) ? TRUE : FALSE;
		release_flag = (flush_flag & FLUSH_RLSE) ? TRUE : FALSE;
		write_thru = release_flag;
		sweep_flag = (flush_flag & FLUSH_SWEEP) ? TRUE : FALSE;
		latch = (release_flag) ? LATCH_exclusive : LATCH_none;

		for (i = 0; (bcb = dbb->dbb_bcb) && i < bcb->bcb_count; i++) {
			bdb = bcb->bcb_rpt[i].bcb_bdb;
			if (bdb->bdb_length)
				continue;
			if (!release_flag
				&& !(bdb->bdb_flags & (BDB_dirty | BDB_db_dirty))) continue;
			if (latch == LATCH_exclusive
				&& latch_bdb(tdbb, latch, bdb, bdb->bdb_page, 1) == -1)
				cache_bugcheck(302);	/* msg 302 unexpected page change */
			if (latch == LATCH_exclusive && bdb->bdb_use_count > 1)
				cache_bugcheck(210);	/* msg 210 page in use during flush */
#ifdef SUPERSERVER
			if (bdb->bdb_flags & BDB_db_dirty)
				if (all_flag
					|| (sweep_flag
						&& (!bdb->bdb_parent && bdb != bcb->bcb_btree))) {
					if (!write_buffer
						(tdbb, bdb, bdb->bdb_page, write_thru, status, TRUE))
						CCH_unwind(tdbb, TRUE);
				}
#else
			if (bdb->bdb_flags & BDB_dirty)
				if (!write_buffer
					(tdbb, bdb, bdb->bdb_page, FALSE, status,
					 TRUE)) CCH_unwind(tdbb, TRUE);
#endif
			if (release_flag)
			{
				PAGE_LOCK_RELEASE(bdb->bdb_lock);
			}
			if (latch == LATCH_exclusive)
				release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);
		}
	}

	// 
	// Check if flush needed
	//
	int max_unflushed_writes = Config::getMaxUnflushedWrites();
	time_t max_unflushed_write_time = Config::getMaxUnflushedWriteTime();
	bool max_num = (max_unflushed_writes >= 0);
	bool max_time = (max_unflushed_write_time >= 0);

	bool doFlush = false;

	if (!(dbb->dbb_file->fil_flags & FIL_force_write) && (max_num || max_time))
	{
		time_t now = time(0);

		THD_MUTEX_LOCK(dbb->dbb_mutexes + DBB_MUTX_flush_count);
		
		// If this is the first commit set last_flushed_write to now
		if (!dbb->last_flushed_write)
		{
			dbb->last_flushed_write = now;
		}

		// test max_num condition and max_time condition
		max_num = max_num 
			&& (dbb->unflushed_writes == max_unflushed_writes);
		max_time = max_time
			&& (now - dbb->last_flushed_write > max_unflushed_write_time);

		if (max_num || max_time)
		{
			doFlush = true;
			dbb->unflushed_writes = 0;
			dbb->last_flushed_write = now;
		}
		else
		{
			dbb->unflushed_writes++;
		}

		THD_MUTEX_UNLOCK(dbb->dbb_mutexes + DBB_MUTX_flush_count);
	}

	if (doFlush)
	{
		PIO_flush(dbb->dbb_file);
		if (dbb->dbb_shadow)
		{
			PIO_flush(dbb->dbb_shadow->sdw_file);
		}
	}

/* take the opportunity when we know there are no pages
   in cache to check that the shadow(s) have not been
   scheduled for shutdown or deletion */

	SDW_check();
}


BOOLEAN CCH_free_page(TDBB tdbb)
{
/**************************************
 *
 *	C C H _ f r e e _ p a g e
 *
 **************************************
 *
 * Functional description
 *	Check if the cache is below its free pages
 *	threshold and write a page on the LRU tail.
 *
 **************************************/
	DBB dbb;
	BCB bcb;
	BDB bdb;

/* Called by VIO/garbage_collector() when it is idle to
   help quench the thirst for free pages. */

	dbb = tdbb->tdbb_database;
	bcb = dbb->dbb_bcb;

	if (dbb->dbb_flags & DBB_read_only)
		return FALSE;

	if (bcb->bcb_flags & BCB_free_pending &&
		(bdb = get_buffer(tdbb, FREE_PAGE, LATCH_none, 1))) {
		if (!write_buffer
			(tdbb, bdb, bdb->bdb_page, TRUE, tdbb->tdbb_status_vector, TRUE))
			CCH_unwind(tdbb, FALSE);
		else
			return TRUE;
	}

	return FALSE;
}


SLONG CCH_get_incarnation(WIN * window)
{
/**************************************
 *
 *	C C H _ g e t _ i n c a r n a t i o n
 *
 **************************************
 *
 * Functional description
 *	Get page incarnation associated with buffer.
 *
 **************************************/

	return window->win_bdb->bdb_incarnation;
}


PAG CCH_handoff(TDBB	tdbb,
				WIN* window,
				SLONG	page,
				SSHORT	lock,
				SSHORT	page_type,
				SSHORT	latch_wait,
				SSHORT	release_tail)
{
/**************************************
 *
 *	C C H _ h a n d o f f
 *
 **************************************
 *
 * Functional description
 *	Follow a pointer handing off the lock.  Fetch the new page
 *	before retiring the old page lock.
 *
 * input
 *	latch_wait:	1 => Wait as long as necessary to get the latch.
 *				This can cause deadlocks of course.
 *			0 => If the latch can't be acquired immediately,
 *				give up and return 0.
 *	      		<negative number> => Latch timeout interval in seconds.
 *
 * return
 *	PAG if successful.
 *	0 if a latch timeout occurred (only possible if latch_wait <> 1).
 *		The latch on the fetched page is downgraded to shared.
 *		The fetched page is unmarked.
 *
 **************************************/
	WIN temp;
	SSHORT must_read;
	BDB bdb;

/* The update, if there was one, of the input page is complete.
   The cache buffer can be 'unmarked'.  It is important to
   unmark before CCH_unwind is (might be) called. */

	SET_TDBB(tdbb);

	unmark(tdbb, window);

/* If the 'from-page' and 'to-page' of the handoff are the
   same and the latch requested is shared then downgrade it. */

	if ((window->win_page == page) && (lock == LCK_read)) {
		release_bdb(tdbb, window->win_bdb, FALSE, TRUE, FALSE);
		return window->win_buffer;
	}

	temp = *window;
	window->win_page = page;
	must_read =
		CCH_FETCH_LOCK(tdbb, window, lock, LCK_WAIT, latch_wait, page_type);

/* Latch or lock timeout, return failure. */

	if (must_read == -2 || must_read == -1) {
		*window = temp;
		CCH_RELEASE(tdbb, window);
		return NULL;
	}

	if (release_tail)
		CCH_RELEASE_TAIL(tdbb, &temp);
	else
		CCH_RELEASE(tdbb, &temp);

	if (must_read)
		CCH_FETCH_PAGE(tdbb, window, TRUE, TRUE);

	bdb = window->win_bdb;

/* If a page was read or prefetched on behalf of a large scan
   then load the window scan count into the buffer descriptor.
   This buffer scan count is decremented by releasing a buffer
   with CCH_RELEASE_TAIL.

   Otherwise zero the buffer scan count to prevent the buffer
   from being queued to the LRU tail. */

	if (window->win_flags & WIN_large_scan) {
		if (must_read == 1 ||
			bdb->bdb_flags & BDB_prefetch || bdb->bdb_scan_count < 0)
			bdb->bdb_scan_count = window->win_scans;
	}
	else if (window->win_flags & WIN_garbage_collector) {
		if (must_read == 1)
			bdb->bdb_scan_count = -1;
		if (bdb->bdb_flags & BDB_garbage_collect)
			window->win_flags |= WIN_garbage_collect;
	}
	else if (window->win_flags & WIN_secondary) {
		if (must_read == 1)
			bdb->bdb_scan_count = -1;
	}
	else {
		bdb->bdb_scan_count = 0;
		if (bdb->bdb_flags & BDB_garbage_collect)
			bdb->bdb_flags &= ~BDB_garbage_collect;
	}

/* Validate the fetched page matches the expected type */

	if (bdb->bdb_buffer->pag_type != (SCHAR) page_type &&
		page_type != pag_undefined)
			page_validation_error(tdbb, window, page_type);

	return window->win_buffer;
}


void CCH_init(TDBB tdbb, ULONG number)
{
/**************************************
 *
 *	C C H _ i n i t
 *
 **************************************
 *
 * Functional description
 *	Initialize the cache.  Allocate buffers control block,
 *	buffer descriptors, and actual buffers.
 *
 **************************************/
	DBB dbb;
	BCB bcb_ = 0;
	SLONG count;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

/* Check for database-specific page buffers */

	if (dbb->dbb_page_buffers)
		number = dbb->dbb_page_buffers;

/* Enforce page buffer cache constraints */

	if (number < MIN_PAGE_BUFFERS)
		number = MIN_PAGE_BUFFERS;
	if (number > MAX_PAGE_BUFFERS)
		number = MAX_PAGE_BUFFERS;

	count = number;

/* Allocate and initialize buffers control block */

	while (!bcb_) {
		try {
			bcb_ = FB_NEW_RPT(*dbb->dbb_bufferpool, number) bcb;
		} catch(const std::exception&) {
			/* If the buffer control block can't be allocated, memory is
			very low. Recalculate the number of buffers to account for
			page buffer overhead and reduce that number by a 25% fudge
			factor. */

			number = (sizeof(bcb_repeat) * number) / PAGE_OVERHEAD;
			number -= number >> 2;

			if (number < MIN_PAGE_BUFFERS)
				ERR_post(isc_cache_too_small, 0);
		}
	}

	dbb->dbb_bcb = bcb_;
	QUE_INIT(bcb_->bcb_in_use);
	QUE_INIT(bcb_->bcb_empty);
	QUE_INIT(bcb_->bcb_free_lwt);

/* initialization of memory is system-specific */

	bcb_->bcb_count = memory_init(tdbb, bcb_, number);
	bcb_->bcb_free_minimum = (SSHORT) MIN(bcb_->bcb_count / 4, 128);

	if (bcb_->bcb_count < MIN_PAGE_BUFFERS)
		ERR_post(isc_cache_too_small, 0);

/* Log if requested number of page buffers could not be allocated. */

	if (count != (SLONG) bcb_->bcb_count)
		gds__log
			("Database: %s\n\tAllocated %ld page buffers of %ld requested",
			 tdbb->tdbb_attachment->att_filename->str_data, bcb_->bcb_count, count);

	if (dbb->dbb_lock->lck_logical != LCK_EX)
		dbb->dbb_ast_flags |= DBB_assert_locks;

#ifdef CACHE_READER
	EVENT event = dbb->dbb_reader_event;
	ISC_event_init(event, 0, 0);
	count = ISC_event_clear(event);
	if (gds__thread_start
		(reinterpret_cast < FPTR_INT_VOID_PTR > (cache_reader), dbb,
		 THREAD_high, 0, 0))
		ERR_bugcheck_msg("cannot start thread");

	THREAD_EXIT;
	ISC_event_wait(1, &event, &count, 5 * 1000000, NULL, 0);
	THREAD_ENTER;
#endif

#ifdef CACHE_WRITER
	if (!(dbb->dbb_flags & DBB_read_only)) {
		EVENT event = dbb->dbb_writer_event_init;
		/* Initialize initialization event */
		ISC_event_init(event, 0, 0);
		count = ISC_event_clear(event);

		if (gds__thread_start
			(reinterpret_cast < FPTR_INT_VOID_PTR > (cache_writer), dbb,
			THREAD_high, 0, 0)) {
			ERR_bugcheck_msg("cannot start thread");
		}
		THREAD_EXIT;
		ISC_event_wait(1, &event, &count, 5 * 1000000, NULL, 0);
		THREAD_ENTER;
		/* Clean up initialization event */
		ISC_event_fini(event);
	}
#endif
}


void CCH_journal_page(TDBB tdbb, WIN * window)
{
/**************************************
 *
 *	C C H _ j o u r n a l _ p a g e
 *
 **************************************
 *
 * Functional description
 *	Mark a page to be journalled.
 *
 **************************************/
	DBB dbb;
	BCB bcb;
	BDB bdb, journal;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

/* If the database isn't journalled, don't bother */

	if (!dbb->dbb_wal)
		return;

/* If there is a journal buffer allocated, release it */

	bdb = window->win_bdb;

	if ( (journal = bdb->bdb_jrn_bdb) ) {
		bdb->bdb_jrn_bdb = NULL;
		journal->bdb_length = 0;
		BCB_MUTEX_ACQUIRE;
		bcb = dbb->dbb_bcb;
		QUE_INSERT(bcb->bcb_empty, journal->bdb_que);
		BCB_MUTEX_RELEASE;
	}

	bdb->bdb_flags |= BDB_journal;
}


void CCH_journal_record(TDBB	tdbb,
						WIN*	window,
						UCHAR*	header,
						USHORT	h_length,
						UCHAR*	data,
						USHORT	d_length)
{
/**************************************
 *
 *	C C H _ j o u r n a l _ r e c o r d
 *
 **************************************
 *
 * Functional description
 *	Add a record to the journal for a page.  If the journal buffer
 *	for the page overflows, prepare to journal the entire page.
 *
 **************************************/
	DBB dbb;
	BDB bdb, journal;
	UCHAR *p;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

/* If the database isn't journalled, don't bother */

	if (!dbb->dbb_wal)
		return;

	bdb = window->win_bdb;

/* If the entire page is marked for journalling, don't bother, either */

	if (bdb->bdb_flags & BDB_journal) {
		if (!(journal = bdb->bdb_jrn_bdb))
			return;
		else {
#ifdef DEV_BUILD
			if (journal->bdb_sequence !=
				MISC_checksum_log_rec((UCHAR *) journal->bdb_buffer,
									  journal->bdb_length, 0, 0))
				DEBUG_PRINTF("Checksum error in journal buffer\n");
#endif
			/* If we are journaling a b-tree segment, this new journal
			 * segment replaces all the previous records for this page.
			 * So get rid of previous records.
			 */
			if ((h_length)
				&& (((JRNH *) header)->jrnh_type == JRNP_BTREE_SEGMENT))
				journal->bdb_length = 0;
		}
	}
	else {
		/* Get the journal bdb in exclusive mode and wait for any other thread
		   interested in this bdb (for a different page number) to loose interest
		   in this bdb */
		journal = bdb->bdb_jrn_bdb =
			get_buffer(tdbb, (SLONG) JOURNAL_PAGE, LATCH_exclusive, 1);
		if (latch_bdb(tdbb, LATCH_mark, journal, journal->bdb_page, 1) == -1)
			cache_bugcheck(302);	/* msg 302 unexpected page change */

		/* Now we can safely release this bdb to clear its bdb_io, bdb_exclusive and
		   bdb_use_count fields.  */
		release_bdb(tdbb, journal, FALSE, FALSE, FALSE);

		journal->bdb_length = 0;
#ifdef DEV_BUILD
		journal->bdb_sequence = 0;
#endif
		bdb->bdb_flags |= BDB_journal;
	}

/* If we're about to overflow the journal buffer, punt and journal the
   whole page instead. */

	p = (UCHAR *) journal->bdb_buffer + journal->bdb_length;
	journal->bdb_length += h_length + d_length;

	if (journal->bdb_length >= dbb->dbb_page_size) {
		CCH_journal_page(tdbb, window);
		return;
	}

/* Move pad byte, header, and data into journal buffer */

	if (h_length) {
		do {
			*p++ = *header++;
		} while (--h_length);
	}

	if (d_length) {
		do {
			*p++ = *data++;
		} while (--d_length);
	}

	if (journal->bdb_length & 1) {
		++journal->bdb_length;
		*p = JRNP_NULL;
	}

#ifdef DEV_BUILD
	journal->bdb_sequence =
		MISC_checksum_log_rec((UCHAR *) journal->bdb_buffer,
							  journal->bdb_length, 0, 0);
#endif
}


void CCH_mark(TDBB tdbb, WIN * window, USHORT mark_system)
{
/**************************************
 *
 *	C C H _ m a r k
 *
 **************************************
 *
 * Functional description
 *	Mark a window as dirty.
 *
 **************************************/
	DBB dbb;
	JRD_TRA transaction;
	ULONG trans_bucket;
	SLONG number;
	BDB bdb;
	BCB bcb;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

	dbb->dbb_marks++;
	bcb = dbb->dbb_bcb;
	bdb = window->win_bdb;
	BLKCHK(bdb, type_bdb);

/* A LATCH_mark is needed before the bdb can be marked.
   This prevents a write while the page is being modified. */

	if (latch_bdb(tdbb, LATCH_mark, bdb, bdb->bdb_page, 1) == -1)
		cache_bugcheck(302);	/* msg 302 unexpected page change */

	bdb->bdb_incarnation = ++dbb->dbb_page_incarnation;

	if (!(bdb->bdb_flags & BDB_writer))
		BUGCHECK(208);			/* msg 208 page not accessed for write */

/* mark the dirty bit vector for this specific transaction,
   if it exists; otherwise mark that the system transaction
   has updated this page */

	if ((transaction = tdbb->tdbb_transaction) &&
		(number = transaction->tra_number)) {
		if (!(tdbb->tdbb_flags & TDBB_sweeper)) {
			trans_bucket = number & (BITS_PER_LONG - 1);
			bdb->bdb_transactions |= (1L << trans_bucket);
			if (number > bdb->bdb_mark_transaction)
				bdb->bdb_mark_transaction = number;
		}
	}
	else
		bdb->bdb_flags |= BDB_system_dirty;

	if (mark_system)
		bdb->bdb_flags |= BDB_system_dirty;

	if (!(tdbb->tdbb_flags & TDBB_sweeper) ||
		bdb->bdb_flags & BDB_system_dirty) {
		if (!bdb->bdb_parent && bdb != bcb->bcb_btree)
			btc_insert(dbb, bdb);
	}

#ifdef SUPERSERVER
	bdb->bdb_flags |= BDB_db_dirty;
#endif

	bdb->bdb_flags |= (BDB_dirty | BDB_marked);
}


void CCH_mark_must_write(TDBB tdbb, WIN * window)
{
/**************************************
 *
 *	C C H _ m a r k _ m u s t _ w r i t e
 *
 **************************************
 *
 * Functional description
 *	Mark a window as dirty and must_write.
 *	This will prevent the page from being
 *	inserted in the dirty binary tree when
 *	the intention is to write the page
 *	immediately any way.
 *
 **************************************/
	BDB bdb;

	SET_TDBB(tdbb);
	bdb = window->win_bdb;
	BLKCHK(bdb, type_bdb);

	if (!(bdb->bdb_flags & BDB_writer))
		BUGCHECK(208);			/* msg 208 page not accessed for write */

	bdb->bdb_flags |= (BDB_dirty | BDB_must_write);

	CCH_MARK(tdbb, window);
}


void CCH_must_write(WIN * window)
{
/**************************************
 *
 *	C C H _ m u s t _ w r i t e
 *
 **************************************
 *
 * Functional description
 *	Mark a window as "must write".
 *
 **************************************/
	BDB bdb;

	bdb = window->win_bdb;
	BLKCHK(bdb, type_bdb);

	if (!(bdb->bdb_flags & BDB_marked))
		BUGCHECK(208);			/* msg 208 page not accessed for write */

	bdb->bdb_flags |= (BDB_dirty | BDB_must_write);
}



LCK CCH_page_lock(TDBB tdbb)
{
/**************************************
 *
 *	C C H _ p a g e _ l o c k
 *
 **************************************
 *
 * Functional description
 *	Allocate a page-type lock.
 *
 **************************************/
	DBB dbb;
	LCK lock;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

	lock = FB_NEW_RPT(*dbb->dbb_bufferpool, sizeof(SLONG)) lck;
	lock->lck_type = LCK_bdb;
	lock->lck_owner_handle = LCK_get_owner_handle(tdbb, lock->lck_type);
	lock->lck_length = sizeof(SLONG);

	lock->lck_dbb = dbb;
	lock->lck_parent = dbb->dbb_lock;

	return lock;
}


void CCH_precedence(TDBB tdbb, WIN * window, SLONG page)
{
/**************************************
 *
 *	C C H _ p r e c e d e n c e
 *
 **************************************
 *
 * Functional description
 *	Given a window accessed for write and a page number,
 *	establish a precedence relationship such that the
 *	specified page will always be written before the page
 *	associated with the window.
 *
 *	If the "page number" is negative, it is really a transaction
 *	id.  In this case, the precedence relationship is to the
 *	database header page from which the transaction id was
 *	obtained.  If the header page has been written since the
 *	transaction id was assigned, no precedence relationship
 *	is required.
 *
 **************************************/
/* If the page is zero, the caller isn't really serious */

	if (page == 0)
		return;

	check_precedence(tdbb, window, page);
}


void CCH_prefetch(TDBB tdbb, SLONG * pages, SSHORT count)
{
/**************************************
 *
 *	C C H _ p r e f e t c h
 *
 **************************************
 *
 * Functional description
 *	Given a vector of pages, set corresponding bits
 *	in global prefetch bitmap. Initiate an asynchronous
 *	I/O and get the cache reader reading in our behalf
 *	as well.
 *
 **************************************/
#ifdef CACHE_READER
	DBB dbb;
	BCB bcb;
	SLONG page, first_page, *end;
	JrdMemoryPool *old_pool;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;
	bcb = dbb->dbb_bcb;

	if (!count || !(bcb->bcb_flags & BCB_cache_reader))	/* Caller isn't really serious. */
		return;

/* Switch default pool to permanent pool for setting bits in
   prefetch bitmap. */

	old_pool = tdbb->tdbb_default;
	tdbb->tdbb_default = dbb->dbb_bufferpool;

/* The global prefetch bitmap is the key to the I/O coalescense
   mechanism which dovetails all thread prefetch requests to
   minimize sequential I/O requests. It also enables multipage
   I/O by implicitly sorting page vector requests. */

	first_page = 0;
	for (end = pages + count; pages < end;)
		if (page = *pages++) {
			SBM_set(tdbb, &bcb->bcb_prefetch, page);
			if (!first_page)
				first_page = page;
		}

/* Not likely that the caller's page vector was
   empty but check anyway. */

	if (first_page) {
		struct prf prefetch;

		prefetch_init(&prefetch, tdbb);
		prefetch_prologue(&prefetch, &first_page);
		prefetch_io(&prefetch, tdbb->tdbb_status_vector);
		prefetch_epilogue(&prefetch, tdbb->tdbb_status_vector);
	}

	tdbb->tdbb_default = old_pool;
#endif
}


BOOLEAN CCH_prefetch_pages(TDBB tdbb)
{
/**************************************
 *
 *	C C H _ p r e f e t c h _ p a g e s
 *
 **************************************
 *
 * Functional description
 *	Check the prefetch bitmap for a set
 *	of pages and read them into the cache.
 *
 **************************************/

/* Place holder to be implemented when predictive prefetch is
   enabled. This is called by VIO/garbage_collector() when it
   is idle to help satisfy the prefetch demand. */

	return FALSE;
}


void CCH_recover_shadow(TDBB tdbb, SBM sbm_rec)
{
/**************************************
 *
 *	C C H _ r e c o v e r _ s h a d o w
 *
 **************************************
 *
 * Functional description
 *	Walk through the sparse bit map created during recovery and
 *	write all changed pages to all the shadows.
 *
 **************************************/
	SLONG page_no = -1;
	WIN window;
	DBB dbb;
	ISC_STATUS *status;
	int result;
	SLONG seqno, offset, p_offset;
	SCHAR walname[257];

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;
	status = tdbb->tdbb_status_vector;

	if (!sbm_rec) {
		/* Now that shadows are initialized after WAL, write the header
		   page with the recover bit to shadow. */

		if (dbb->dbb_wal) {
			window.win_page = LOG_PAGE;
			CCH_FETCH(tdbb, &window, LCK_write, pag_log);
			CCH_write_all_shadows(tdbb, 0, window.win_bdb, status, 1, FALSE);
			CCH_RELEASE(tdbb, &window);
		}
		return;
	}

	result = TRUE;

	if (dbb->dbb_shadow) {
		while (SBM_next(sbm_rec, &page_no, RSE_get_forward)) {
			window.win_page = page_no;
			CCH_FETCH(tdbb, &window, LCK_write, pag_undefined);

			result =
				CCH_write_all_shadows(tdbb, 0, window.win_bdb, status, 1,
									  FALSE);
			CCH_RELEASE(tdbb, &window);
		}
	}

	if (result == FALSE)
		ERR_punt();
/*
 * do 2 control points after a recovery to flush all the pages to the
 * database and shadow. Note that this has to be doen after the shadows
 * are updated.
 */

	if (dbb->dbb_wal) {
		/* Flush cache to write all changes to disk */

		CCH_flush(tdbb, (USHORT) FLUSH_ALL, 0);

		/* Inform wal writer about check point (actually a fake check point) */

		if (WAL_checkpoint_force
			(tdbb->tdbb_status_vector, dbb->dbb_wal, &seqno, walname,
			 &p_offset, &offset) != FB_SUCCESS)
			ERR_punt();

		/* Update the log page */

		AIL_upd_cntrl_pt(walname, (USHORT) strlen(walname), seqno, offset,
						 p_offset);
		AIL_upd_cntrl_pt(walname, (USHORT) strlen(walname), seqno, offset,
						 p_offset);

		/* Get the log page changes to disk */

		CCH_flush(tdbb, (USHORT) FLUSH_ALL, 0);

		/* Inform wal writer that older log files are no longer required */

		if (WAL_checkpoint_recorded(tdbb->tdbb_status_vector, dbb->dbb_wal)
			!= FB_SUCCESS)
			ERR_punt();
	}
	else
		CCH_flush(tdbb, (USHORT) FLUSH_ALL, 0);

/* release the bit map */

	SBM_release(sbm_rec);
}


void invalidate_and_release_buffer(TDBB tdbb, BDB bdb) {
	// This function should be called before difference processing is done.
	// So there should be no need to no need to release difference locks though
	DBB dbb = tdbb->tdbb_database;
	bdb->bdb_flags |= BDB_not_valid;
	bdb->bdb_flags &= ~BDB_dirty;
	set_write_direction(dbb, bdb, BDB_write_undefined);
	TRA_invalidate(dbb, bdb->bdb_transactions);
	bdb->bdb_transactions = 0;
	release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);
}

void CCH_release(TDBB tdbb, WIN * window, BOOLEAN release_tail)
{
/**************************************
 *
 *	C C H _ r e l e a s e
 *
 **************************************
 *
 * Functional description
 *	Release a window. If the release_tail
 *	flag is TRUE then make the buffer
 *	least-recently-used.
 *
 **************************************/
	DBB dbb;
	BDB bdb;
	SSHORT use_count;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

	bdb = window->win_bdb;
	BLKCHK(bdb, type_bdb);

/* if an expanded buffer has been created, retain it
   for possible future use */

	bdb->bdb_expanded_buffer = window->win_expanded_buffer;
	window->win_expanded_buffer = NULL;

/* A large sequential scan has requested that the garbage
   collector garbage collect. Mark the buffer so that the
   page isn't released to the LRU tail before the garbage
   collector can process the page. */

	if (window->win_flags & WIN_large_scan &&
		window->win_flags & WIN_garbage_collect)
	{
		bdb->bdb_flags |= BDB_garbage_collect;
		window->win_flags &= ~WIN_garbage_collect;
	}

	if (bdb->bdb_use_count == 1)
	{
		if (bdb->bdb_flags & BDB_dirty) {
			SSHORT write_direction;
			// We are going to release a page that is dirty. Lets determine 
			// location of the page in difference file and write destination 
			// so BDB AST handlers can safely use this information
			if (!dbb->backup_manager->lock_state(true)) {
				invalidate_and_release_buffer(tdbb, bdb);
				CCH_unwind(tdbb, TRUE);
			}
#ifndef SUPERSERVER
			bdb->bdb_diff_generation = dbb->backup_manager->get_current_generation();
#endif
			if (bdb->bdb_page != HEADER_PAGE) // SCN of header page is adjusted in nbak.cpp
				bdb->bdb_buffer->pag_scn() = dbb->backup_manager->get_current_scn(); // Set SCN for the page
			int backup_state = dbb->backup_manager->get_state();
			switch (backup_state) {
			case nbak_state_normal:
				write_direction = BDB_write_normal; 
				break;
			case nbak_state_stalled:
				write_direction = BDB_write_diff;
				break;
			case nbak_state_merge:
				if (tdbb->tdbb_flags & TDBB_backup_merge || 
					bdb->bdb_page < dbb->backup_manager->get_backup_pages())
				{
					write_direction = BDB_write_normal;
				} else
					write_direction = BDB_write_both;
				break;
			}
			switch (write_direction) {
			case BDB_write_diff:
				if (!dbb->backup_manager->lock_alloc(true)) {
					dbb->backup_manager->unlock_state();
					invalidate_and_release_buffer(tdbb, bdb);
					CCH_unwind(tdbb, TRUE);
				}
				bdb->bdb_difference_page = dbb->backup_manager->get_page_index(bdb->bdb_page);
				dbb->backup_manager->unlock_alloc();
				if (!bdb->bdb_difference_page) {
					if (!dbb->backup_manager->lock_alloc_write(true)) {
						dbb->backup_manager->unlock_state();
						invalidate_and_release_buffer(tdbb, bdb);
						CCH_unwind(tdbb, TRUE);
					}
					bdb->bdb_difference_page = dbb->backup_manager->allocate_difference_page(bdb->bdb_page);
					dbb->backup_manager->unlock_alloc_write();
					if (!bdb->bdb_difference_page) {
						dbb->backup_manager->unlock_state();
						invalidate_and_release_buffer(tdbb, bdb);
						CCH_unwind(tdbb, TRUE);
					}
					NBAK_TRACE(("Allocate difference page %d for database page %d", 
						bdb->bdb_difference_page, bdb->bdb_page));
				} else {
					NBAK_TRACE(("Map existing difference page %d to database page %d", 
						bdb->bdb_difference_page, bdb->bdb_page));
				}
				break;
			case BDB_write_both:
				if (!dbb->backup_manager->lock_alloc(true)) {
					dbb->backup_manager->unlock_state();
					invalidate_and_release_buffer(tdbb, bdb);
					CCH_unwind(tdbb, TRUE);
				}
				bdb->bdb_difference_page = dbb->backup_manager->get_page_index(bdb->bdb_page);
				dbb->backup_manager->unlock_alloc();
				if (bdb->bdb_difference_page) {
					NBAK_TRACE(("Map existing difference page %d to database page %d (write_both)", 
						bdb->bdb_difference_page, bdb->bdb_page));
				} else {
					// This may really happen. Database file can grow while in merge mode
					write_direction = BDB_write_normal;
				}
				break;
			}
			if (!set_write_direction(dbb, bdb, write_direction)) {
				dbb->backup_manager->unlock_state();
				invalidate_and_release_buffer(tdbb, bdb);
				CCH_unwind(tdbb, TRUE);
			}
			dbb->backup_manager->unlock_state();
		}
		bool marked = bdb->bdb_flags & BDB_marked;
		bdb->bdb_flags &= ~(BDB_writer | BDB_marked | BDB_faked);		
		if (marked)
		{
			release_bdb(tdbb, bdb, FALSE, FALSE, TRUE);
		}
		if (bdb->bdb_flags & BDB_must_write)
		{
			/* Downgrade exclusive latch to shared to allow concurrent share access
			   to page during I/O. */

			release_bdb(tdbb, bdb, FALSE, TRUE, FALSE);
			if (!write_buffer(	tdbb,
								bdb,
								bdb->bdb_page,
								FALSE,
								tdbb->tdbb_status_vector,
								TRUE))
			{
				btc_insert(dbb, bdb);	/* Don't lose track of must_write */
				CCH_unwind(tdbb, TRUE);
			}
		}
#ifndef PAGE_LATCHING
		if (bdb->bdb_flags & BDB_no_blocking_ast)
		{
			if (bdb->bdb_flags & (BDB_db_dirty | BDB_dirty))
			{
				if (!write_buffer(	tdbb,
									bdb,
									bdb->bdb_page,
									FALSE,
									tdbb->tdbb_status_vector,
									TRUE))
				{
					/* Reassert blocking AST after write failure with dummy lock convert
					   to same level. This will re-enable blocking AST notification. */

					LCK_convert_opt(tdbb, bdb->bdb_lock,
									bdb->bdb_lock->lck_logical);
					CCH_unwind(tdbb, TRUE);
				}
			}

			PAGE_LOCK_RELEASE(bdb->bdb_lock);
			bdb->bdb_flags &= ~BDB_no_blocking_ast;
			bdb->bdb_ast_flags &= ~BDB_blocking;
		}
#endif

		/* Make buffer the least-recently-used by queueing it
		   to the LRU tail. */

		if (release_tail)
		{
			if ((window->win_flags & WIN_large_scan &&
				 bdb->bdb_scan_count > 0 &&
				 !(--bdb->bdb_scan_count) &&
				 !(bdb->bdb_flags & BDB_garbage_collect)) ||
				(window->win_flags & WIN_garbage_collector &&
				 bdb->bdb_flags & BDB_garbage_collect &&
				 !(bdb->bdb_scan_count)))
			{
				BCB bcb;

				if (window->win_flags & WIN_garbage_collector)
				{
					bdb->bdb_flags &= ~BDB_garbage_collect;
				}
				bcb = dbb->dbb_bcb;
				QUE_LEAST_RECENTLY_USED(bdb->bdb_in_use);
				bdb->bdb_sequence = 0;
#ifdef CACHE_WRITER
				if (bdb->bdb_flags & (BDB_dirty | BDB_db_dirty))
				{
					bcb->bcb_flags |= BCB_free_pending;
					if (bcb->bcb_flags & BCB_cache_writer &&
						!(bcb->bcb_flags & BCB_writer_active))
					{
						ISC_event_post(dbb->dbb_writer_event);
					}
				}
#endif
			}
		}
	}

	release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);
	use_count = bdb->bdb_use_count;

	if (use_count < 0)
		BUGCHECK(209);			/* msg 209 attempt to release page not acquired */

	if (!use_count && (bdb->bdb_ast_flags & BDB_blocking))
	{
		PAGE_LOCK_RE_POST(bdb->bdb_lock);
	}

	window->win_bdb = NULL;
}


void CCH_release_exclusive(TDBB tdbb)
{
/**************************************
 *
 *	C C H _ r e l e a s e _ e x c l u s i v e
 *
 **************************************
 *
 * Functional description
 *	Release exclusive access to database.
 *
 **************************************/
	DBB dbb;
	ATT attachment;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;
	dbb->dbb_flags &= ~DBB_exclusive;

	if ( (attachment = tdbb->tdbb_attachment) )
		attachment->att_flags &= ~ATT_exclusive;

	if (dbb->dbb_ast_flags & DBB_blocking)
		LCK_re_post(dbb->dbb_lock);
}


void CCH_release_journal(TDBB tdbb, SLONG number)
{
/**************************************
 *
 *	C C H _ r e l e a s e _ j o u r n a l
 *
 **************************************
 *
 * Functional description
 *	Get rid of journal buffer for a page which has been released.
 *
 **************************************/
	WIN window;
	BDB bdb, journal;
	BCB bcb;
	DBB dbb;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;
	if (!dbb->dbb_wal)
		return;

	window.win_page = number;
	CCH_FETCH(tdbb, &window, LCK_write, pag_undefined);

/* if there is still a journal buffer, zero it out */

	bdb = window.win_bdb;
	if (!(bdb->bdb_flags & BDB_journal)) {
		CCH_RELEASE(tdbb, &window);
		return;
	}

	bdb->bdb_flags &= ~BDB_journal;

	if (!(journal = bdb->bdb_jrn_bdb)) {
		CCH_RELEASE(tdbb, &window);
		return;
	}

	journal->bdb_length = 0;

	BCB_MUTEX_ACQUIRE;
	bcb = dbb->dbb_bcb;
	QUE_INSERT(bcb->bcb_empty, journal->bdb_que);
	BCB_MUTEX_RELEASE;

	bdb->bdb_jrn_bdb = NULL;
	CCH_RELEASE(tdbb, &window);
}


BOOLEAN CCH_rollover_to_shadow(DBB dbb, FIL file, BOOLEAN inAst)
{
/**************************************
 *
 *	C C H _ r o l l o v e r _ t o _ s h a d o w
 *
 **************************************
 *
 * Functional description
 *	An I/O error has been detected on the
 *	main database file.  Roll over to use
 *	the shadow file.
 *
 **************************************/
/* Is the shadow subsystem yet initialized */
	if (!dbb->dbb_shadow_lock)
		return FALSE;
/* notify other process immediately to ensure all read from sdw
   file instead of db file */
	return (SDW_rollover_to_shadow(file, inAst));
}


void DLL_EXPORT CCH_shutdown_database(DBB dbb)
{
/**************************************
 *
 *	C C H _ s h u t d o w n _ d a t a b a s e
 *
 **************************************
 *
 * Functional description
 *	Shutdown database physical page locks.
 *
 **************************************/
	BCB bcb;
	BDB bdb;
	bcb_repeat * tail, *end;
	TDBB tdbb;

	tdbb = GET_THREAD_DATA;

	if ((bcb = dbb->dbb_bcb) && (tail = bcb->bcb_rpt) && (tail->bcb_bdb))
		for (end = tail + bcb->bcb_count; tail < end; tail++) {
			bdb = tail->bcb_bdb;
			if (bdb->bdb_length)
				continue;
			bdb->bdb_flags &= ~(BDB_dirty | BDB_db_dirty);
			PAGE_LOCK_RELEASE(bdb->bdb_lock);
		}

#ifndef SUPERSERVER
	PIO_close(dbb->dbb_file);
	SDW_close();
#endif
}

void CCH_unwind(TDBB tdbb, BOOLEAN punt)
{
/**************************************
 *
 *	C C H _ u n w i n d
 *
 **************************************
 *
 * Functional description
 *	Synchronously unwind cache after I/O or lock error.
 *
 **************************************/
	DBB dbb;
	BCB bcb;
	BDB bdb;
	bcb_repeat * tail, *end;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

/* CCH_unwind is called when any of the following occurs:
	- IO error
	- journaling error
	- bad page checksum
	- wrong page type
	- page locking (not latching) deadlock (doesn't happen on Netware) */

	if (!(bcb = dbb->dbb_bcb) || (tdbb->tdbb_flags & TDBB_no_cache_unwind)) {
		if (punt) {
			ERR_punt();
		} else {
			return;
		}
	}

/* A cache error has occurred. Scan the cache for buffers
   which may be in use and release them. */

	for (tail = bcb->bcb_rpt, end = tail + bcb->bcb_count; tail < end; tail++)
	{
		bdb = tail->bcb_bdb;
#ifndef SUPERSERVER
		if (bdb->bdb_length || !bdb->bdb_use_count) {
			continue;
		}
		if (bdb->bdb_flags & BDB_marked) {
			cache_bugcheck(268);	/* msg 268 buffer marked during cache unwind */
		}
		bdb->bdb_flags &= ~BDB_writer;
		while (bdb->bdb_use_count) {
			release_bdb(tdbb, bdb, TRUE, FALSE, FALSE);
		}
		PAG page = bdb->bdb_buffer;
		if ((page->pag_type == pag_header) ||
			(page->pag_type == pag_transactions))
		{
			++bdb->bdb_use_count;
			// Adjust backup page locks
			if (bdb->bdb_flags & BDB_dirty)
				set_write_direction(dbb, bdb, BDB_write_undefined);
			bdb->bdb_flags &= ~(BDB_dirty | 
				BDB_writer | BDB_marked | BDB_faked | BDB_db_dirty);
			PAGE_LOCK_RELEASE(bdb->bdb_lock);
			--bdb->bdb_use_count;
		}
#else
		if (!bdb->bdb_use_count) {
			continue;
		}
		if (bdb->bdb_io == tdbb) {
			release_bdb(tdbb, bdb, TRUE, FALSE, FALSE);
		}
		if (bdb->bdb_exclusive == tdbb) {
			/* The sanity check below can't be enforced for the log page because
			   the ail.c code does IO while having the log page marked (should be
			   fixed in the future). */
			if ((bdb->bdb_flags & BDB_marked) && (bdb->bdb_page != LOG_PAGE))
				cache_bugcheck(268);	/* msg 268 buffer marked during cache unwind */
			bdb->bdb_flags &= ~(BDB_writer | BDB_faked | BDB_must_write);
			release_bdb(tdbb, bdb, TRUE, FALSE, FALSE);
		}
		for (int i = 0; i < BDB_max_shared; ++i) {
			if (bdb->bdb_shared[i] == tdbb) {
				release_bdb(tdbb, bdb, TRUE, FALSE, FALSE);
			}
		}
#endif
	}

	if (punt)
		ERR_punt();
}


BOOLEAN CCH_validate(WIN * window)
{
/**************************************
 *
 *	C C H _ v a l i d a t e
 *
 **************************************
 *
 * Functional description
 *	Give a page a quick once over looking for unhealthyness.
 *
 **************************************/
	BDB bdb;
	PAG page;
	USHORT sum;

	bdb = window->win_bdb;

/* If page is marked for write, checksum is questionable */

	if ((bdb->bdb_flags & (BDB_dirty | BDB_db_dirty)))
		return TRUE;

	page = window->win_buffer;
	sum = CCH_checksum(bdb);

	if (sum == page->pag_checksum)
		return TRUE;

	return FALSE;
}


BOOLEAN CCH_write_all_shadows(TDBB tdbb,
							  SDW shadow,
							  BDB bdb,
							  ISC_STATUS * status, USHORT checksum, BOOLEAN inAst)
{
/**************************************
 *
 *	C C H _ w r i t e _ a l l _ s h a d o w s
 *
 **************************************
 *
 * Functional description
 *	Compute a checksum and write a page out to all shadows
 *	detecting failure on write.
 *	If shadow is null, write to all shadows, otherwise only to specified
 *	shadow.
 *
 **************************************/
	BOOLEAN result = TRUE;
	SLONG last, *spare_buffer = NULL;
	FIL next_file, shadow_file;
	HDR header;
	PAG old_buffer, page;
	DBB dbb;
	UCHAR *q;
	SDW sdw;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

	sdw = shadow ? shadow : dbb->dbb_shadow;

	if (!sdw) {
		return TRUE;
	}

	try {

	if (bdb->bdb_page == HEADER_PAGE) {
		/* allocate a spare buffer which is large enough,
		   and set up to release it in case of error */

		spare_buffer =
			(SLONG *) dbb->dbb_bufferpool->allocate((SLONG) dbb->dbb_page_size,0
#ifdef DEBUG_GDS_ALLOC
			  ,__FILE__,__LINE__
#endif
			);

		page = (PAG) spare_buffer;
		MOVE_FAST((UCHAR *) bdb->bdb_buffer, (UCHAR *) page, HDR_SIZE);
		old_buffer = bdb->bdb_buffer;
		bdb->bdb_buffer = page;
	}
	else {
		page = bdb->bdb_buffer;
		if (checksum)
			page->pag_checksum = CCH_checksum(bdb);
	}

	for (; sdw; sdw = sdw->sdw_next) {
		/* don't bother to write to the shadow if it is no longer viable */

		/* Fix for bug 7925. drop_gdb fails to remove secondary file if
		   the shadow is conditional. Reason being the header page not
		   being correctly initialized.

		   The following block was not being performed for a conditional
		   shadow since SDW_INVALID expanded to include conditional shadow

		   -Sudesh  07/10/95
		   old code --> if (sdw->sdw_flags & SDW_INVALID)
		 */

		if ((sdw->sdw_flags & SDW_INVALID) &&
			!(sdw->sdw_flags & SDW_conditional)) continue;

		if (bdb->bdb_page == HEADER_PAGE) {
			/* fixup header for shadow file */
			shadow_file = sdw->sdw_file;
			header = (HDR) page;
			q = (UCHAR *) dbb->dbb_file->fil_string;
			header->hdr_data[0] = HDR_end;
			header->hdr_end = HDR_SIZE;
			header->hdr_next_page = 0;

			PAG_add_header_entry(header, HDR_root_file_name,
								 (USHORT) strlen((char *) q), q);

			if ( (next_file = shadow_file->fil_next) ) {
				q = (UCHAR *) next_file->fil_string;
				last = next_file->fil_min_page - 1;
				PAG_add_header_entry(header, HDR_file,
									 (USHORT) strlen((char *) q), q);
				PAG_add_header_entry(header, HDR_last_page, sizeof(last),
									 (UCHAR *) & last);
			}

			header->hdr_flags |= hdr_active_shadow;
			header->hdr_header.pag_checksum = CCH_checksum(bdb);
		}

/* This condition makes sure that PIO_write is performed in case of
   conditional shadow only if the page is Header page

	-Sudesh 07/10/95
*/
		if ((sdw->sdw_flags & SDW_conditional) &&
			(bdb->bdb_page != HEADER_PAGE)) continue;

		/* if a write failure happens on an AUTO shadow, mark the
		   shadow to be deleted at the next available opportunity when we
		   know we don't have a page fetched */

		if (!PIO_write(sdw->sdw_file, bdb, page, status))
			if (sdw->sdw_flags & SDW_manual)
				result = FALSE;
			else {
				sdw->sdw_flags |= SDW_delete;
				if (!inAst && SDW_check_conditional()) {
					if (SDW_lck_update((SLONG) 0)) {
						SDW_notify();
						CCH_unwind(tdbb, FALSE);
						SDW_dump_pages();
						ERR_post(gds_deadlock, 0);
					}
				}
			}
		/* If shadow was specified, break out of loop */

		if (shadow)
			break;
	}

	if (bdb->bdb_page == HEADER_PAGE)
		bdb->bdb_buffer = old_buffer;

	if (spare_buffer)
		dbb->dbb_bufferpool->deallocate(spare_buffer);

	}	// try
	catch (const std::exception&) {
		if (spare_buffer) {
			dbb->dbb_bufferpool->deallocate(spare_buffer);
		}
		ERR_punt();
	}

	return result;
}


static BDB alloc_bdb(TDBB tdbb, BCB bcb, UCHAR ** memory)
{
/**************************************
 *
 *	a l l o c _ b d b
 *
 **************************************
 *
 * Functional description
 *	Allocate buffer descriptor block.
 *
 **************************************/
	BDB bdb_;
#ifndef PAGE_LATCHING
	LCK lock;
#endif
	DBB dbb;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

	bdb_ = FB_NEW(*dbb->dbb_bufferpool) bdb;
	bdb_->bdb_dbb = dbb;

#ifndef PAGE_LATCHING
	try {
		bdb_->bdb_lock = lock = CCH_page_lock(tdbb);
	}
	catch (const std::exception&) {
		delete bdb_;
		throw;
	}
	lock->lck_ast = blocking_ast_bdb;
	lock->lck_object = reinterpret_cast<blk*>(bdb_);
#endif

	bdb_->bdb_buffer = (PAG) * memory;
	*memory += dbb->dbb_page_size;

	QUE_INIT(bdb_->bdb_higher);
	QUE_INIT(bdb_->bdb_lower);
	QUE_INIT(bdb_->bdb_waiters);
	QUE_INSERT(bcb->bcb_empty, bdb_->bdb_que);

	return bdb_;
}


#ifndef PAGE_LATCHING
static int blocking_ast_bdb(void* ast_object)
{
/**************************************
 *
 *	b l o c k i n g _ a s t _ b d b
 *
 **************************************
 *
 * Functional description
 *	Blocking AST for buffer control blocks.  This is called at
 *	AST (signal) level to indicate that a lock is blocking another
 *	process.  If the BDB is in use, set a flag indicating that the
 *	lock is blocking and return.  When the bdb is released at normal
 *	level the lock will be down graded.  If the BDB is not in use,
 *	write the page if dirty then downgrade lock.  Things would be
 *	much hairier if UNIX supported asynchronous IO, but it doesn't.
 *	WHEW!
 *
 **************************************/
	BDB bdb = reinterpret_cast<BDB>(ast_object);
	DBB dbb;
	BOOLEAN keep_pages;
	ISC_STATUS_ARRAY ast_status;
	struct tdbb thd_context, *tdbb;

	ISC_ast_enter();

/* Since this routine will be called asynchronously, we must establish
   a thread context. */

	SET_THREAD_DATA;

	BLKCHK(bdb, type_bdb);
	dbb = bdb->bdb_dbb;

	tdbb->tdbb_database = dbb;
	tdbb->tdbb_attachment = NULL;
	tdbb->tdbb_quantum = QUANTUM;
	tdbb->tdbb_request = NULL;
	tdbb->tdbb_transaction = NULL;
	tdbb->tdbb_status_vector = ast_status;

/* Do some fancy footwork to make sure that pages are
   not removed from the btc tree at AST level.  Then
   restore the flag to whatever it was before. */

	keep_pages = dbb->dbb_bcb->bcb_flags & BCB_keep_pages;
	dbb->dbb_bcb->bcb_flags |= BCB_keep_pages;
	ast_status[1] = 0;

	down_grade(tdbb, bdb);

	if (!keep_pages)
		dbb->dbb_bcb->bcb_flags &= ~BCB_keep_pages;

	if (ast_status[1])
		gds__log_status(dbb->dbb_file->fil_string, ast_status);

/* Restore the prior thread context */

	RESTORE_THREAD_DATA;

	ISC_ast_exit();

    return 0;
}
#endif


static void btc_flush(
					  TDBB tdbb,
					  SLONG transaction_mask,
					  BOOLEAN sys_only, ISC_STATUS * status)
{
/**************************************
 *
 *	b t c _ f l u s h
 *
 **************************************
 *
 * Functional description
 *	Walk the dirty page binary tree, flushing all buffers
 *	that could have been modified by this transaction.
 *	The pages are flushed in page order to roughly
 *	emulate an elevator-type disk controller. Iteration
 *	is used to minimize call overhead.
 *
 **************************************/
	BDB bdb, next;
	SLONG max_seen, page, next_page;
	DBB dbb;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

/* traverse the tree, flagging to prevent pages
   from being removed from the tree during write_page() --
   this simplifies worrying about random pages dropping
   out when dependencies have been set up */

	max_seen = MIN_PAGE_NUMBER;

/* Pick starting place at leftmost node */

	BTC_MUTEX_ACQUIRE;
	for (next = dbb->dbb_bcb->bcb_btree; next && next->bdb_left;
		 next = next->bdb_left);

	if (next)
		next_page = next->bdb_page;

/* Walk tree.  If we get lost, reposition and continue */

	while ( (bdb = next) ) {
		/* If we're lost, reposition */

		if ((bdb->bdb_page != next_page) ||
			(!bdb->bdb_parent && (bdb != dbb->dbb_bcb->bcb_btree))) {
			for (bdb = dbb->dbb_bcb->bcb_btree; bdb;)
				if (bdb->bdb_left && (max_seen < bdb->bdb_page))
					bdb = bdb->bdb_left;
				else if (bdb->bdb_right && (max_seen > bdb->bdb_page))
					bdb = bdb->bdb_right;
				else
					break;
			if (!bdb)
				break;
		}

		/* Decide where to go next.  The options are (right, then down to the left)
		   or up */

		if (bdb->bdb_right && (max_seen < bdb->bdb_right->bdb_page))
			for (next = bdb->bdb_right; next->bdb_left;
				 next = next->bdb_left);
		else
			next = bdb->bdb_parent;

		if (next)
			next_page = next->bdb_page;

		if (max_seen >= bdb->bdb_page)
			continue;

		max_seen = bdb->bdb_page;

		/* forget about this page if it was written out
		   as a dependency while we were walking the tree */

		if (!(bdb->bdb_flags & BDB_dirty)) {
			BTC_MUTEX_RELEASE;
			btc_remove(bdb);
			BTC_MUTEX_ACQUIRE;
			continue;
		}

		/* this code replicates code in CCH_flush() --
		   changes should be made in both places */

		if (bdb->bdb_length)
			continue;

		page = bdb->bdb_page;
		BTC_MUTEX_RELEASE;

#ifndef SUPERSERVER
		if (bdb->bdb_use_count)
			cache_bugcheck(210);	/* msg 210 page in use during flush */
#endif
		/* if any transaction has dirtied this page,
		   check to see if it could have been this one */

		if ((transaction_mask & bdb->bdb_transactions) ||
			(bdb->bdb_flags & BDB_system_dirty) ||
			(!transaction_mask && !sys_only) || (!bdb->bdb_transactions)) {
			if (!write_buffer(tdbb, bdb, page, FALSE, status, TRUE))
				CCH_unwind(tdbb, TRUE);
		}

		/* re-post the lock only if it was really written */

		if ((bdb->bdb_ast_flags & BDB_blocking) &&
			!(bdb->bdb_flags & BDB_dirty)) {
			PAGE_LOCK_RE_POST(bdb->bdb_lock);
		}
		BTC_MUTEX_ACQUIRE;
	}

	BTC_MUTEX_RELEASE;
}


static void btc_insert(DBB dbb, BDB bdb)
{
/**************************************
 *
 *	b t c _ i n s e r t
 *
 **************************************
 *
 * Functional description
 *	Insert a buffer into the dirty page
 *	binary tree.
 *
 **************************************/
	BDB node;
	SLONG page;

/* if the page is already in the tree (as in when it is
   written out as a dependency while walking the tree),
   just leave well enough alone -- this won't check if
   it's at the root but who cares then */

	if (bdb->bdb_parent)
		return;

	SET_DBB(dbb);

/* if the tree is empty, this is now the tree */

	BTC_MUTEX_ACQUIRE;
	if (!(node = dbb->dbb_bcb->bcb_btree)) {
		dbb->dbb_bcb->bcb_btree = bdb;
		BTC_MUTEX_RELEASE;
		return;
	}

/* insert the page sorted by page number;
   do this iteratively to minimize call overhead */

	page = bdb->bdb_page;

	while (TRUE) {
		if (page == node->bdb_page)
			break;

		if (page < node->bdb_page) {
			if (!node->bdb_left) {
				node->bdb_left = bdb;
				bdb->bdb_parent = node;
				break;
			}
			else
				node = node->bdb_left;
		}
		else {
			if (!node->bdb_right) {
				node->bdb_right = bdb;
				bdb->bdb_parent = node;
				break;
			}
			else
				node = node->bdb_right;
		}
	}

	BTC_MUTEX_RELEASE;
}


static void btc_remove(BDB bdb)
{
/**************************************
 *
 *	b t c _ r e m o v e
 *
 **************************************
 *
 * Functional description
 * 	Remove a page from the dirty page binary tree.
 *	The idea is to place the left child of this
 *	page in this page's place, then make the
 *	right child of this page a child of the left
 *	child -- this removal mechanism won't promote
 *	a balanced tree but that isn't of primary
 *	importance.
 *
 **************************************/
	BDB ptr;
	BDB new_child, bdb_parent;
	DBB dbb;
	BCB bcb;

	dbb = bdb->bdb_dbb;

/* engage in a little defensive programming to make
   sure the node is actually in the tree */

	BTC_MUTEX_ACQUIRE;
	bcb = dbb->dbb_bcb;
	if ((!bcb->bcb_btree) ||
		(!bdb->bdb_parent &&
		 !bdb->bdb_left && !bdb->bdb_right && (bcb->bcb_btree != bdb))) {
		if ((bdb->bdb_flags & BDB_must_write) || !(bdb->bdb_flags & BDB_dirty)) {	/* Must writes aren't worth the effort */
			BTC_MUTEX_RELEASE;
			return;
		}
		else
			cache_bugcheck(211);	/* msg 211 attempt to remove page from dirty page list when not there */
	}

/* make a new child out of the left and right children */

	if ( (new_child = bdb->bdb_left) ) {
		for (ptr = new_child; ptr->bdb_right; ptr = ptr->bdb_right);
		if ( (ptr->bdb_right = bdb->bdb_right) )
			ptr->bdb_right->bdb_parent = ptr;
	}
	else
		new_child = bdb->bdb_right;

/* link the parent with the child node --
   if no parent place it at the root */

	if (!(bdb_parent = bdb->bdb_parent))
		bcb->bcb_btree = new_child;
	else if (bdb_parent->bdb_left == bdb)
		bdb_parent->bdb_left = new_child;
	else
		bdb_parent->bdb_right = new_child;

	if (new_child)
		new_child->bdb_parent = bdb_parent;

/* initialize the node for next usage */

	bdb->bdb_left = bdb->bdb_right = bdb->bdb_parent = NULL;
	BTC_MUTEX_RELEASE;
}


static void cache_bugcheck(int number)
{
/**************************************
 *
 *	c a c h e _ b u g c h e c k
 *
 **************************************
 *
 * Functional description
 *	There has been a bugcheck during a cache operation.  Release
 *	the cache mutex and post the bugcheck.
 *
 **************************************/

	BUGCHECK(number);
}


#ifdef CACHE_READER
static void THREAD_ROUTINE cache_reader(DBB dbb)
{
/**************************************
 *
 *	c a c h e _ r e a d e r
 *
 **************************************
 *
 * Functional description
 *	Prefetch pages into cache for sequential scans.
 *	Use asynchronous I/O to keep two prefetch requests
 *	busy at a time.
 *
 **************************************/
	struct tdbb thd_context, *tdbb;
	BCB bcb;
	BDB bdb;
	SLONG count, starting_page;
	ISC_STATUS_ARRAY status_vector;
	EVENT reader_event;
	BOOLEAN found;
	struct prf prefetch1, prefetch2;
	PRF post_prefetch, next_prefetch;

	THREAD_ENTER;

/* Establish a thread context. */
/* Note: Since this function operates as its own thread,
   we have no need to restore the THREAD CONTEXT on exit.
   Once we reach the end, the thread will die, thus implicitly
   killing all its contexts. */
	SET_THREAD_DATA;

/* Dummy attachment needed for lock owner identification. */

	tdbb->tdbb_database = dbb;
	tdbb->tdbb_default = dbb->dbb_bufferpool;
	tdbb->tdbb_status_vector = status_vector;
	tdbb->tdbb_quantum = QUANTUM;
	tdbb->tdbb_attachment = FB_NEW(*dbb->dbb_bufferpool) att();
	tdbb->tdbb_attachment->att_database = dbb;
	tdbb->tdbb_attachment->att_filename = dbb->dbb_filename;

/* This try block is specifically to protect the LCK_init call: if
   LCK_init fails we won't be able to accomplish anything anyway, so
   return, unlike the other try blocks further down the page. */

	try {

		LCK_init(tdbb, LCK_OWNER_attachment);
		reader_event = dbb->dbb_reader_event;
		bcb = dbb->dbb_bcb;
		bcb->bcb_flags |= BCB_cache_reader;
		ISC_event_post(reader_event);	/* Notify our creator that we have started  */
	}
	catch (const std::exception&) {
		gds__log_status(dbb->dbb_file->fil_string, status_vector);
		THREAD_EXIT;
		return;
	}

	try {

/* Set up dual prefetch control blocks to keep multiple prefetch
   requests active at a time. Also helps to prevent the cache reader
   from becoming dedicated or locked into a single reqeust's prefetch
   demands. */

	prefetch_init(&prefetch1, tdbb);
	prefetch_init(&prefetch2, tdbb);

	while (bcb->bcb_flags & BCB_cache_reader) {
		count = ISC_event_clear(reader_event);
		bcb->bcb_flags |= BCB_reader_active;
		found = FALSE;
		starting_page = -1;
		next_prefetch = &prefetch1;

		if (dbb->dbb_flags & DBB_suspend_bgio) {
			THREAD_EXIT;
			ISC_event_wait(1, &reader_event, &count, 10 * 1000000, NULL, 0);
			THREAD_ENTER;
			continue;
		}

		/* Make a pass thru the global prefetch bitmap looking for something
		   to read. When the entire bitmap has been scanned and found to be
		   empty then wait for new requests. */

		do {
			if (!(prefetch1.prf_flags & PRF_active) &&
				SBM_next(bcb->bcb_prefetch, &starting_page, RSE_get_forward)) {
				found = TRUE;
				prefetch_prologue(&prefetch1, &starting_page);
				prefetch_io(&prefetch1, status_vector);
			}

			if (!(prefetch2.prf_flags & PRF_active) &&
				SBM_next(bcb->bcb_prefetch, &starting_page, RSE_get_forward)) {
				found = TRUE;
				prefetch_prologue(&prefetch2, &starting_page);
				prefetch_io(&prefetch2, status_vector);
			}

			post_prefetch = next_prefetch;
			next_prefetch =
				(post_prefetch == &prefetch1) ? &prefetch2 : &prefetch1;
			if (post_prefetch->prf_flags & PRF_active)
				prefetch_epilogue(post_prefetch, status_vector);

			if (found) {
				/* If the cache writer or garbage collector is idle, put
				   them to work prefetching pages. */
#ifdef CACHE_WRITER
				if (bcb->bcb_flags & BCB_cache_writer &&
					!(bcb->bcb_flags & BCB_writer_active))
						ISC_event_post(dbb->dbb_writer_event);
#endif
#ifdef GARBAGE_THREAD
				if (dbb->dbb_flags & DBB_garbage_collector &&
					!(dbb->dbb_flags & DBB_gc_active))
					ISC_event_post(dbb->dbb_gc_event);
#endif
			}
		} while (prefetch1.prf_flags & PRF_active
				 || prefetch2.prf_flags & PRF_active);

		/* If there's more work to do voluntarily ask to be rescheduled.
		   Otherwise, wait for event notification. */

		if (found)
			JRD_reschedule(tdbb, 0, TRUE);
		else if (bcb->bcb_flags & BCB_free_pending &&
				 (bdb = get_buffer(tdbb, FREE_PAGE, LATCH_none, 1))) {
			/* In our spare time, help writer clean the cache. */

			write_buffer(tdbb, bdb, bdb->bdb_page, TRUE, status_vector, TRUE);
		}
		else {
			bcb->bcb_flags &= ~BCB_reader_active;
			THREAD_EXIT;
			ISC_event_wait(1, &reader_event, &count, 10 * 1000000, NULL, 0);
			THREAD_ENTER;
		}
		bcb = dbb->dbb_bcb;
	}

	LCK_fini(tdbb, LCK_OWNER_attachment);
	delete tdbb->tdbb_attachment;
	bcb->bcb_flags &= ~BCB_cache_reader;
	ISC_event_post(reader_event);
	THREAD_EXIT;

	}	// try
	catch (const std::exception&) {
		bcb = dbb->dbb_bcb;
		gds__log_status(dbb->dbb_file->fil_string, status_vector);
	}
}
#endif


#ifdef CACHE_WRITER
static void THREAD_ROUTINE cache_writer(DBB dbb)
{
/**************************************
 *
 *	c a c h e _ w r i t e r
 *
 **************************************
 *
 * Functional description
 *	Write dirty pages to database to maintain an
 *	adequate supply of free pages. If WAL is enabled,
 *	perform database checkpoint when WAL subsystem
 *	deems it necessary.
 *
 **************************************/
	struct tdbb thd_context, *tdbb;
	bcb_repeat * tail, *end;
	BCB bcb;
	BDB bdb;
	SLONG count, seq, p_off, off, starting_page;
#ifdef SUPERSERVER_V2
	SLONG commit_mask;
#endif
	bool start_chkpt;
	ISC_STATUS_ARRAY status_vector;
	SCHAR walname[256];
	EVENT writer_event;

	THREAD_ENTER;

/* Establish a thread context. */
/* Note: Since this function operates as its own thread,
   we have no need to restore the THREAD CONTEXT on exit.
   Once we reach the end, the thread will die, thus implicitly
   killing all its contexts. */
	SET_THREAD_DATA;

/* Dummy attachment needed for lock owner identification. */

	tdbb->tdbb_database = dbb;
	tdbb->tdbb_default = dbb->dbb_bufferpool;
	tdbb->tdbb_status_vector = status_vector;
	tdbb->tdbb_quantum = QUANTUM;
	tdbb->tdbb_attachment = FB_NEW(*dbb->dbb_bufferpool) att;
	tdbb->tdbb_attachment->att_database = dbb;
	tdbb->tdbb_attachment->att_filename = dbb->dbb_filename;

/* This try block is specifically to protect the LCK_init call: if
   LCK_init fails we won't be able to accomplish anything anyway, so
   return, unlike the other try blocks further down the page. */

	try {
		writer_event = dbb->dbb_writer_event;
		ISC_event_init(writer_event, 0, 0);
		LCK_init(tdbb, LCK_OWNER_attachment);
		bcb = dbb->dbb_bcb;
		bcb->bcb_flags |= BCB_cache_writer;

		/* Notify our creator that we have started */
		ISC_event_post(dbb->dbb_writer_event_init);
	}
	catch (const std::exception&) {
		gds__log_status(dbb->dbb_file->fil_string, status_vector);
		ISC_event_fini(writer_event);
		THREAD_EXIT;
		return;
	}

	try {

		while (bcb->bcb_flags & BCB_cache_writer)
		{
			count = ISC_event_clear(writer_event);
			bcb->bcb_flags |= BCB_writer_active;
			starting_page = -1;	

			if (dbb->dbb_flags & DBB_suspend_bgio) {
				THREAD_EXIT;
				ISC_event_wait(1, &writer_event, &count, 10 * 1000000, NULL, 0);
				THREAD_ENTER;
				continue;
			}
	
#ifdef SUPERSERVER_V2
			/* Flush buffers for lazy commit */

			if (!(dbb->dbb_flags & DBB_force_write) &&
				(commit_mask = dbb->dbb_flush_cycle)) {
				dbb->dbb_flush_cycle = 0;
				btc_flush(tdbb, commit_mask, FALSE, status_vector);
			}
#endif

			if (bcb->bcb_flags & BCB_free_pending) {
				if (bdb = get_buffer(tdbb, FREE_PAGE, LATCH_none, 1)) {
					write_buffer(tdbb, bdb, bdb->bdb_page, TRUE, status_vector, TRUE);
					bcb = dbb->dbb_bcb;
				}

				/* If the cache reader or garbage collector is idle, put
				   them to work freeing pages. */
#ifdef CACHE_READER
				if (bcb->bcb_flags & BCB_cache_reader &&
					!(bcb->bcb_flags & BCB_reader_active)) {
						ISC_event_post(dbb->dbb_reader_event);
				}
#endif
#ifdef GARBAGE_THREAD
				if (dbb->dbb_flags & DBB_garbage_collector &&
					!(dbb->dbb_flags & DBB_gc_active)) {
						ISC_event_post(dbb->dbb_gc_event);
				}
#endif
			}

			if (dbb->dbb_wal) {
				if (bcb->bcb_checkpoint) {
					if (bdb = get_buffer(tdbb, CHECKPOINT_PAGE, LATCH_none, 1)) {
						write_buffer(tdbb, bdb, bdb->bdb_page, TRUE, status_vector, TRUE);
						bcb = dbb->dbb_bcb;
					}
					else {
						bcb->bcb_checkpoint = 0;
					}
				}

				if (!bcb->bcb_checkpoint && !(bcb->bcb_flags & BCB_checkpoint_db)) {
					if (WAL_checkpoint_start(status_vector, dbb->dbb_wal, 
						&start_chkpt) != FB_SUCCESS) {
						gds__log_status(dbb->dbb_file->fil_string, status_vector);
					}
					else {
						if (start_chkpt) {
							bcb->bcb_flags |= BCB_checkpoint_db;
						}
					}
				}

				if ((bcb->bcb_flags & BCB_checkpoint_db) && !bcb->bcb_checkpoint) {
					if (WAL_checkpoint_finish(status_vector, dbb->dbb_wal,
						&seq, walname, &p_off, &off) != FB_SUCCESS) {
						gds__log_status(dbb->dbb_file->fil_string, status_vector);
					}
					else {
						AIL_checkpoint_finish(status_vector, dbb, seq, walname, p_off, off);
						bcb = dbb->dbb_bcb;
						bcb->bcb_flags &= ~BCB_checkpoint_db;
						for (tail = bcb->bcb_rpt, end = tail + bcb->bcb_count;
							 tail < end; tail++) {
							bdb = tail->bcb_bdb;
							if (bdb->bdb_length) {
								continue;
							}
							if (bdb->bdb_flags & BDB_db_dirty) {
								bdb->bdb_flags |= BDB_checkpoint;
								++bcb->bcb_checkpoint;
							}
						}
					}
				}
			}

			/* If there's more work to do voluntarily ask to be rescheduled.
			   Otherwise, wait for event notification. */

			if ((bcb->bcb_flags & BCB_free_pending) ||
				bcb->bcb_checkpoint || dbb->dbb_flush_cycle) {
				JRD_reschedule(tdbb, 0, TRUE);
			}
#ifdef CACHE_READER
			else if (SBM_next(bcb->bcb_prefetch, &starting_page, RSE_get_forward)) {
				/* Prefetch some pages in our spare time and in the process
				   garbage collect the prefetch bitmap. */
				struct prf prefetch;

				prefetch_init(&prefetch, tdbb);
				prefetch_prologue(&prefetch, &starting_page);
				prefetch_io(&prefetch, status_vector);
				prefetch_epilogue(&prefetch, status_vector);
			}
#endif
			else {
				bcb->bcb_flags &= ~BCB_writer_active;
				THREAD_EXIT;
				ISC_event_wait(1, &writer_event, &count, 10 * 1000000, NULL, 0);
				THREAD_ENTER;
			}
			bcb = dbb->dbb_bcb;
		}

		LCK_fini(tdbb, LCK_OWNER_attachment);
		delete tdbb->tdbb_attachment;
		bcb->bcb_flags &= ~BCB_cache_writer;
		/* Notify the finalization caller that we're finishing. */
		ISC_event_post(dbb->dbb_writer_event_fini);
		ISC_event_fini(writer_event);
		THREAD_EXIT;

	}	// try
	catch (const std::exception&) {
		bcb = dbb->dbb_bcb;
		gds__log_status(dbb->dbb_file->fil_string, status_vector);
	}
}
#endif


static void check_precedence(TDBB tdbb, WIN * window, SLONG page)
{
/**************************************
 *
 *	c h e c k _ p r e c e d e n c e
 *
 **************************************
 *
 * Functional description
 *	Given a window accessed for write and a page number,
 *	establish a precedence relationship such that the
 *	specified page will always be written before the page
 *	associated with the window.
 *
 *	If the "page number" is negative, it is really a transaction
 *	id.  In this case, the precedence relationship is to the
 *	database header page from which the transaction id was
 *	obtained.  If the header page has been written since the
 *	transaction id was assigned, no precedence relationship
 *	is required.
 *
 **************************************/
	DBB dbb;
	BDB high = (BDB)0, low = (BDB)0;
	PRE precedence;
	QUE que, mod_que;
	BCB bcb;
	SLONG low_page, high_page;
	SSHORT relationship;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

/* If this is really a transaction id, sort things out */

	if (page < 0)
		if (-page <= dbb->dbb_last_header_write)
			return;
		else
			page = 0;

/* Start by finding the buffer containing the high priority page */

	BCB_MUTEX_ACQUIRE;
	PRE_MUTEX_ACQUIRE;
	bcb = dbb->dbb_bcb;
	mod_que = &bcb->bcb_rpt[page % bcb->bcb_count].bcb_page_mod;

	for (que = mod_que->que_forward; que != mod_que; que = que->que_forward)
		if ((high = BLOCK(que, BDB, bdb_que))->bdb_page == page)
			break;

	BCB_MUTEX_RELEASE;
	if (que == mod_que) {
		PRE_MUTEX_RELEASE;
		return;
	}

/* Found the higher precedence buffer.  If it's not dirty, don't sweat it.
   If it's the same page, ditto.  */

	if (!(high->bdb_flags & BDB_dirty)
		|| (high->bdb_page == window->win_page)) {
		PRE_MUTEX_RELEASE;
		return;
	}

	low = window->win_bdb;

	if ((low->bdb_flags & BDB_marked) && !(low->bdb_flags & BDB_faked))
		cache_bugcheck(212);	/* msg 212 CCH_precedence: block marked */

/* If already related, there's nothing more to do. If the precedence
   search was too complex to complete, just write the high page and
   forget about about establishing the relationship. */

	if (QUE_NOT_EMPTY(high->bdb_lower)) {
		relationship = related(low, high, PRE_SEARCH_LIMIT);
		if (relationship == PRE_EXISTS) {
			PRE_MUTEX_RELEASE;
			return;
		}
		else if (relationship == PRE_UNKNOWN) {
			high_page = high->bdb_page;
			PRE_MUTEX_RELEASE;
			if (!write_buffer
				(tdbb, high, high_page, FALSE, tdbb->tdbb_status_vector,
				 TRUE)) CCH_unwind(tdbb, TRUE);
			return;
		}
	}

/* Check to see if we're going to create a cycle or the precedence search
   was too complex to complete.  If so, force a write of the "after"
   (currently fetched) page.  Assuming everyone obeys the rules and calls
   precedence before marking the buffer, everything should be ok */

	if (QUE_NOT_EMPTY(low->bdb_lower)) {
		relationship = related(high, low, PRE_SEARCH_LIMIT);
		if (relationship == PRE_EXISTS || relationship == PRE_UNKNOWN) {
			low_page = low->bdb_page;
			PRE_MUTEX_RELEASE;
			if (!write_buffer
				(tdbb, low, low_page, FALSE, tdbb->tdbb_status_vector, TRUE))
				CCH_unwind(tdbb, TRUE);
			PRE_MUTEX_ACQUIRE;
		}
	}

/* We're going to establish a new precedence relationship.  Get a block,
   fill in the appropriate fields, and insert it into the various ques */

	bcb = dbb->dbb_bcb;			/* Re-initialize */

	if ( (precedence = bcb->bcb_free) )
		bcb->bcb_free = (PRE) precedence->pre_hi;
	else
		precedence = FB_NEW(*dbb->dbb_bufferpool) pre;

	precedence->pre_low = low;
	precedence->pre_hi = high;
	precedence->pre_flags = 0;
	QUE_INSERT(low->bdb_higher, precedence->pre_higher);
	QUE_INSERT(high->bdb_lower, precedence->pre_lower);
	PRE_MUTEX_RELEASE;
}



static void clear_precedence(DBB dbb, BDB bdb)
{
/**************************************
 *
 *	c l e a r _ p r e c e d e n c e
 *
 **************************************
 *
 * Functional description
 *	Clear precedence relationships to lower precedence block.
 *
 **************************************/
	QUE que;
	PRE precedence;
	BDB low_bdb;
	BCB bcb;

	SET_DBB(dbb);

	PRE_MUTEX_ACQUIRE;
	bcb = dbb->dbb_bcb;

/* Loop thru lower precedence buffers.  If any can be downgraded,
   by all means down grade them. */

	while (QUE_NOT_EMPTY(bdb->bdb_lower)) {
		que = bdb->bdb_lower.que_forward;
		precedence = BLOCK(que, PRE, pre_lower);
		low_bdb = precedence->pre_low;
		QUE_DELETE(precedence->pre_higher);
		QUE_DELETE(precedence->pre_lower);
		precedence->pre_hi = (BDB) bcb->bcb_free;
		bcb->bcb_free = precedence;
		if (!(precedence->pre_flags & PRE_cleared)) {
			if (low_bdb->bdb_ast_flags & BDB_blocking)
			{
				PAGE_LOCK_RE_POST(low_bdb->bdb_lock);
			}
		}
	}

	PRE_MUTEX_RELEASE;
}


static BDB dealloc_bdb(BDB bdb)
{
/**************************************
 *
 *	d e a l l o c _ b d b
 *
 **************************************
 *
 * Functional description
 *	Deallocate buffer descriptor block.
 *
 **************************************/

	if (bdb) {
#ifndef PAGE_LATCHING
		if (bdb->bdb_lock)
			delete bdb->bdb_lock;
#endif
		QUE_DELETE(bdb->bdb_que);
		delete bdb;
	}

	return 0;
}


#ifndef PAGE_LATCHING
static BOOLEAN down_grade(TDBB tdbb, BDB bdb)
{
/**************************************
 *
 *	d o w n _ g r a d e
 *
 **************************************
 *
 * Functional description
 *	A lock on a page is blocking another process.  If possible, downgrade
 *	the lock on the buffer.  This may be called from either AST or
 *	regular level.  Return TRUE if the down grade was successful.  If the
 *	down grade was deferred for any reason, return FALSE.
 *
 **************************************/
	LCK lock;
	QUE que;
	PRE precedence;
	BDB blocking_bdb;
	DBB dbb;
	USHORT in_use, invalid;

	SET_TDBB(tdbb);

	bdb->bdb_ast_flags |= BDB_blocking;
	lock = bdb->bdb_lock;
	dbb = bdb->bdb_dbb;

	if (dbb->dbb_flags & DBB_bugcheck) {
		PAGE_LOCK_RELEASE(bdb->bdb_lock);
		bdb->bdb_ast_flags &= ~BDB_blocking;
		// Release backup pages lock as buffer is no longer dirty
		if (bdb->bdb_flags & BDB_dirty) {
			bdb->bdb_flags &= ~BDB_dirty;
			set_write_direction(dbb, bdb, BDB_write_undefined);
		}
		return TRUE;
	}

/* If the BDB is in use and, being written or already
   downgraded to read, mark it as blocking and exit. */

	if (bdb->bdb_use_count)
		return FALSE;

	latch_bdb(tdbb, LATCH_io, bdb, bdb->bdb_page, 1);

/* If the page isn't dirty, the lock can be quietly downgraded. */

	if (!(bdb->bdb_flags & BDB_dirty)) {
		bdb->bdb_ast_flags &= ~BDB_blocking;
#ifdef VMS
		if (lock->lck_logical == LCK_write)
			LCK_convert(tdbb, lock, LCK_read, LCK_WAIT);
		else
			PAGE_LOCK_RELEASE(bdb->bdb_lock);
#else
		LCK_downgrade(tdbb, lock);
#endif
		release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);
		return TRUE;
	}

	in_use = invalid = FALSE;

	if (bdb->bdb_flags & BDB_not_valid)
		invalid = TRUE;

/* If there are higher precedence guys, see if they can be written. */

	for (que = bdb->bdb_higher.que_forward; que != &bdb->bdb_higher;
		 que = que->que_forward) {
		precedence = BLOCK(que, PRE, pre_higher);
		if (precedence->pre_flags & PRE_cleared)
			continue;
		if (invalid) {
			precedence->pre_flags |= PRE_cleared;
			continue;
		}
		blocking_bdb = precedence->pre_hi;
		if (blocking_bdb->bdb_flags & BDB_dirty) {
			down_grade(tdbb, blocking_bdb);
			if (blocking_bdb->bdb_flags & BDB_dirty)
				in_use = TRUE;
			if (blocking_bdb->bdb_flags & BDB_not_valid) {
				invalid = TRUE;
				in_use = FALSE;
				que = bdb->bdb_higher.que_forward;
			}
		}
	}

/* If any higher precedence buffer can't be written, mark this buffer
   as blocking and exit. */

	if (in_use) {
		release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);
		return FALSE;
	}

/* Everything is clear to write this buffer.  Do so and reduce the lock */

	if (invalid
		|| !write_page(tdbb, bdb, FALSE, tdbb->tdbb_status_vector, TRUE)) {
		bdb->bdb_flags |= BDB_not_valid;
		// Release backup pages lock
		bdb->bdb_flags &= ~BDB_dirty;
		set_write_direction(dbb, bdb, BDB_write_undefined);
		bdb->bdb_ast_flags &= ~BDB_blocking;
		TRA_invalidate(dbb, bdb->bdb_transactions);
		bdb->bdb_transactions = 0;
		PAGE_LOCK_RELEASE(bdb->bdb_lock);
	}
	else {
		bdb->bdb_ast_flags &= ~BDB_blocking;
#ifdef VMS
		LCK_convert(tdbb, lock, LCK_read, LCK_WAIT);
#else
		LCK_downgrade(tdbb, lock);
#endif
	}

/* Clear precedence relationships to lower precedence buffers.  Since it
   isn't safe to tweak the que pointers from AST level, just mark the
   precedence links as cleared.  Somebody else will clean up the precedence
   blocks. */

	for (que = bdb->bdb_lower.que_forward; que != &bdb->bdb_lower;
		 que = que->que_forward) {
		precedence = BLOCK(que, PRE, pre_lower);
		blocking_bdb = precedence->pre_low;
		if (bdb->bdb_flags & BDB_not_valid)
			blocking_bdb->bdb_flags |= BDB_not_valid;
		precedence->pre_flags |= PRE_cleared;
		if (blocking_bdb->bdb_flags & BDB_not_valid ||
			blocking_bdb->bdb_ast_flags & BDB_blocking)
			down_grade(tdbb, blocking_bdb);
	}

	bdb->bdb_flags &= ~BDB_not_valid;
	release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);

	return TRUE;
}
#endif


static void expand_buffers(TDBB tdbb, ULONG number)
{
/**************************************
 *
 *	e x p a n d _ b u f f e r s
 *
 **************************************
 *
 * Functional description
 *	Expand the cache to at least a given number of buffers.  If
 *	it's already that big, don't do anything.
 *
 **************************************/
	DBB dbb;
	BDB bdb;
	BCB old;
	LLS stack;
	QUE que, mod_que;
	UCHAR *memory;
	JrdMemoryPool* old_pool;
	bcb_repeat * new_tail, *old_tail, *new_end, *old_end;
	ULONG num_per_seg, num_in_seg, left_to_do;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;
	old = dbb->dbb_bcb;

	if (number <= old->bcb_count || number > MAX_PAGE_BUFFERS)
		return;

/* for Win16 platform, we want to ensure that no cache buffer ever
   ends on a segment boundary */

	num_in_seg = 0;
	num_per_seg = number - old->bcb_count;
	left_to_do = num_per_seg;

/* Allocate and initialize buffers control block */

	old_pool = tdbb->tdbb_default;
	tdbb->tdbb_default = dbb->dbb_bufferpool;

	old = dbb->dbb_bcb;
	old_end = old->bcb_rpt + old->bcb_count;

	BCB new_ = FB_NEW_RPT(*dbb->dbb_bufferpool, number) bcb;
	new_->bcb_count = number;
	new_->bcb_free_minimum = (SSHORT) MIN(number / 4, 128);	/* 25% clean page reserve */
	new_->bcb_checkpoint = old->bcb_checkpoint;
	new_->bcb_flags = old->bcb_flags;
	new_end = new_->bcb_rpt + number;

/* point at the dirty page binary tree */

	new_->bcb_btree = old->bcb_btree;

/* point at the free precedence blocks */

	new_->bcb_free = old->bcb_free;

/* position the new bcb in the in use, empty and latch queues */

	QUE_INSERT(old->bcb_in_use, new_->bcb_in_use);
	QUE_DELETE(old->bcb_in_use);
	QUE_INSERT(old->bcb_empty, new_->bcb_empty);
	QUE_DELETE(old->bcb_empty);
	QUE_INSERT(old->bcb_free_lwt, new_->bcb_free_lwt);
	QUE_DELETE(old->bcb_free_lwt);

/* Copy addresses of previously allocated buffer space to new block */

	for (stack = old->bcb_memory; stack; stack = stack->lls_next)
		LLS_PUSH(stack->lls_object, &new_->bcb_memory);

/* Initialize tail of new buffer control block */

	for (new_tail = new_->bcb_rpt; new_tail < new_end; new_tail++)
		QUE_INIT(new_tail->bcb_page_mod);

/* Move any active buffers from old block to new */

	new_tail = new_->bcb_rpt;

	for (old_tail = old->bcb_rpt; old_tail < old_end; old_tail++, new_tail++) {
		new_tail->bcb_bdb = old_tail->bcb_bdb;
		while (QUE_NOT_EMPTY(old_tail->bcb_page_mod)) {
			que = old_tail->bcb_page_mod.que_forward;
			bdb = BLOCK(que, BDB, bdb_que);
			QUE_DELETE((*que));
			mod_que =
				&new_->bcb_rpt[bdb->bdb_page % new_->bcb_count].bcb_page_mod;
			QUE_INSERT((*mod_que), (*que));
		}
	}

/* Allocate new buffer descriptor blocks */

	for (; new_tail < new_end; new_tail++) {
		/* if current segment is exhausted, allocate another */

		if (!num_in_seg) {
			memory = (UCHAR *)gds__alloc((SLONG) dbb->dbb_page_size *
										 (num_per_seg + 1));
			LLS_PUSH(memory, &new_->bcb_memory);
			memory = (UCHAR *) (((U_IPTR) memory + dbb->dbb_page_size - 1) &
								~((int) dbb->dbb_page_size - 1));
			num_in_seg = num_per_seg;
			left_to_do -= num_per_seg;
			if (num_per_seg > left_to_do)
				num_per_seg = left_to_do;
		}
		new_tail->bcb_bdb = alloc_bdb(tdbb, new_, &memory);
		num_in_seg--;
	}

/* Set up new buffer control, release old buffer control, and clean up */

	dbb->dbb_bcb = new_;

	delete old;
	tdbb->tdbb_default = old_pool;
}


static BDB get_buffer(TDBB tdbb, SLONG page, LATCH latch, SSHORT latch_wait)
{
/**************************************
 *
 *	g e t _ b u f f e r
 *
 **************************************
 *
 * Functional description
 *	Get a buffer.  If possible, get a buffer already assigned
 *	to the page.  Otherwise get one from the free list or pick
 *	the least recently used buffer to be reused.
 *	Note the following special page numbers:
 *	     -1 indicates that a buffer is required for journaling
 *	     -2 indicates a special scratch buffer for shadowing
 *
 * input
 *	page:		page to get
 *	latch:		type of latch to acquire on the page.
 *	latch_wait:	1 => Wait as long as necessary to get the latch.
 *				This can cause deadlocks of course.
 *			0 => If the latch can't be acquired immediately,
 *				give up and return 0;
 *	      		<negative number> => Latch timeout interval in seconds.
 *
 * return
 *	bdb pointer if successful.
 *	NULL pointer if timeout occurred (only possible is latch_wait <> 1).
 *		     if cache manager doesn't have any pages to write anymore.
 *
 **************************************/
	DBB dbb;
	QUE mod_que;
	QUE que;
	BDB oldest, bdb;
	BCB bcb;
	PRE precedence;
	SSHORT latch_return, walk;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;
	walk = dbb->dbb_bcb->bcb_free_minimum;

	BCB_MUTEX_ACQUIRE;

	while (TRUE) {
	  find_page:

		bcb = dbb->dbb_bcb;
		if (page >= 0) {
			/* Check to see if buffer has already been assigned to page */

			mod_que = &bcb->bcb_rpt[page % bcb->bcb_count].bcb_page_mod;
			for (que = mod_que->que_forward; que != mod_que;
				 que = que->que_forward) 
				if ((bdb = BLOCK(que, BDB, bdb_que))->bdb_page == page) {
#ifdef SUPERSERVER_V2
					if (page != HEADER_PAGE)
#endif
						QUE_MOST_RECENTLY_USED(bdb->bdb_in_use);
					BCB_MUTEX_RELEASE;
					if ( (latch_return =
						latch_bdb(tdbb, latch, bdb, page, latch_wait)) ) {
						if (latch_return == 1)
							return NULL;	/* permitted timeout happened */
						BCB_MUTEX_ACQUIRE;
						goto find_page;
					}
					else {
						bdb->bdb_flags &= ~(BDB_faked | BDB_prefetch);
						bdb->bdb_sequence = dbb->dbb_fetches++;
						return bdb;
					}
				}
		}
#ifdef CACHE_WRITER
		else if ((page == FREE_PAGE) || (page == CHECKPOINT_PAGE)) {
			/* This code is only used by the background I/O threads:
			   cache writer, cache reader and garbage collector. */

			for (que = bcb->bcb_in_use.que_backward;
				 que != &bcb->bcb_in_use; que = que->que_backward) {
				bdb = BLOCK(que, BDB, bdb_in_use);
				if (page == FREE_PAGE) {
					if (bdb->bdb_use_count ||
						bdb->bdb_flags & BDB_free_pending) continue;
					if (bdb->bdb_flags & BDB_db_dirty) {
						BCB_MUTEX_RELEASE;
						return bdb;
					}
					if (!--walk) {
						bcb->bcb_flags &= ~BCB_free_pending;
						break;
					}
				}
				else {			/* if (page == CHECKPOINT_PAGE) */

					if (bdb->bdb_flags & BDB_checkpoint) {
						BCB_MUTEX_RELEASE;
						return bdb;
					}
				}
			}

			BCB_MUTEX_RELEASE;
			return NULL;
		}
#endif

		for (que = bcb->bcb_in_use.que_backward;
			 que != &bcb->bcb_in_use || QUE_NOT_EMPTY(bcb->bcb_empty);
			 que = que->que_backward) {
			bcb = dbb->dbb_bcb;	/* Re-initialize in the loop */
			mod_que = &bcb->bcb_rpt[page % bcb->bcb_count].bcb_page_mod;

			/* If there is an empty buffer sitting around, allocate it */

			if (QUE_NOT_EMPTY(bcb->bcb_empty)) {
				que = bcb->bcb_empty.que_forward;
				QUE_DELETE((*que));
				bdb = BLOCK(que, BDB, bdb_que);
				if (page >= 0) {
					QUE_INSERT((*mod_que), (*que));
#ifdef SUPERSERVER_V2
					/* Reserve a buffer for header page with deferred header
					   page write mechanism. Otherwise, a deadlock will occur
					   if all dirty pages in the cache must force header page
					   to disk before they can be written but there is no free
					   buffer to read the header page into. */

					if (page != HEADER_PAGE)
#endif
						QUE_INSERT(bcb->bcb_in_use, bdb->bdb_in_use);
				}

				/* This correction for bdb_use_count below is needed to
				   avoid a deadlock situation in latching code.  It's not
				   clear though how the bdb_use_count can get < 0 for a bdb
				   in bcb_empty queue */
				if (bdb->bdb_use_count < 0)
					cache_bugcheck(301);	/* msg 301 Non-zero use_count of a buffer in the empty que */

				bdb->bdb_page = page;
				bdb->bdb_flags = BDB_read_pending;
				bdb->bdb_scan_count = 0;
				/* The following latch should never fail because the buffer is 'empty'
				   and the page is not in cache. */
				if (latch_bdb(tdbb, latch, bdb, page, -100) == -1)
					cache_bugcheck(302);	/* msg 302 unexpected page change */
#ifndef PAGE_LATCHING
				if (page >= 0)
					bdb->bdb_lock->lck_logical = LCK_none;
				else
					PAGE_LOCK_RELEASE(bdb->bdb_lock);
#endif
				bdb->bdb_sequence = dbb->dbb_fetches++;
				BCB_MUTEX_RELEASE;
				return bdb;
			}

			/* get the oldest buffer as the least recently used -- note
			   that since there are no empty buffers this queue cannot be empty */

			if (bcb->bcb_in_use.que_forward == &bcb->bcb_in_use)
				cache_bugcheck(213);	/* msg 213 insufficient cache size */

			oldest = BLOCK(que, BDB, bdb_in_use);
			LATCH_MUTEX_ACQUIRE;
			if (oldest->bdb_use_count
				|| (oldest->bdb_flags & BDB_free_pending)
				|| !writeable(oldest)) {
				LATCH_MUTEX_RELEASE;
				continue;
			}

#ifdef SUPERSERVER_V2
			/* If page has been prefetched but not yet fetched, let
			   it cycle once more thru LRU queue before re-using it. */

			if (oldest->bdb_flags & BDB_prefetch) {
				oldest->bdb_flags &= ~BDB_prefetch;
				que = que->que_forward;
				QUE_MOST_RECENTLY_USED(oldest->bdb_in_use);
				LATCH_MUTEX_RELEASE;
				continue;
			}
#endif
			LATCH_MUTEX_RELEASE;
#ifdef CACHE_WRITER
			if (oldest->bdb_flags & (BDB_dirty | BDB_db_dirty)) {
				bcb->bcb_flags |= BCB_free_pending;
				if (bcb->bcb_flags & BCB_cache_writer &&
					!(bcb->bcb_flags & BCB_writer_active))
						ISC_event_post(dbb->dbb_writer_event);
				if (walk) {
					if (!--walk)
						break;
					else
						continue;
				}
			}
#endif
			bdb = oldest;
			if (latch_bdb(tdbb, LATCH_exclusive, bdb, bdb->bdb_page, 0))
				continue;		/* bdb changed, continue looking for a buffer */
			QUE_MOST_RECENTLY_USED(bdb->bdb_in_use);
			bdb->bdb_flags |= BDB_free_pending;

			/* If the buffer selected is dirty, arrange to have it written. */

			if (bdb->bdb_flags & (BDB_dirty | BDB_db_dirty)) {
				BCB_MUTEX_RELEASE;
#ifdef SUPERSERVER
				if (!write_buffer
					(tdbb, bdb, bdb->bdb_page, TRUE, tdbb->tdbb_status_vector,
					 TRUE))
#else
				if (!write_buffer
					(tdbb, bdb, bdb->bdb_page, FALSE,
					 tdbb->tdbb_status_vector, TRUE))
#endif
				{
					bdb->bdb_flags &= ~BDB_free_pending;
					release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);
					CCH_unwind(tdbb, TRUE);
				}
			}

			bcb = dbb->dbb_bcb;	/* Re-initialize */

			/* If the buffer is still in the dirty tree, remove it.
			   In any case, release any lock it may have. */

			if (bdb->bdb_parent || (bdb == bcb->bcb_btree))
				btc_remove(bdb);

			/* if the page has an expanded index buffer, release it */

			if (bdb->bdb_expanded_buffer) {
				delete bdb->bdb_expanded_buffer;
				bdb->bdb_expanded_buffer = NULL;
			}

			/* Cleanup any residual precedence blocks.  Unless something is
			   screwed up, the only precedence blocks that can still be hanging
			   around are ones cleared at AST level. */

			PRE_MUTEX_ACQUIRE;
			while (QUE_NOT_EMPTY(bdb->bdb_higher)) {
				QUE que2;

				que2 = bdb->bdb_higher.que_forward;
				precedence = BLOCK(que2, PRE, pre_higher);
				QUE_DELETE(precedence->pre_higher);
				QUE_DELETE(precedence->pre_lower);
				precedence->pre_hi = (BDB) bcb->bcb_free;
				bcb->bcb_free = precedence;
			}
			PRE_MUTEX_RELEASE;

			clear_precedence(dbb, bdb);

			/* remove the buffer from the "mod" queue and place it
			   in it's new spot, provided it's not a negative (scratch) page */

			BCB_MUTEX_ACQUIRE;
			if (bdb->bdb_page >= 0)
				QUE_DELETE(bdb->bdb_que);
			QUE_INSERT(bcb->bcb_empty, bdb->bdb_que);
			QUE_DELETE(bdb->bdb_in_use)

				bdb->bdb_page = JOURNAL_PAGE;
			release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);
			break;
		}

		if (que == &bcb->bcb_in_use)
#ifdef SUPERSERVER
			expand_buffers(tdbb, bcb->bcb_count + 75);
#else
			cache_bugcheck(214);	/* msg 214 no cache buffers available for reuse */
#endif
	}
}


static void journal_buffer(ISC_STATUS * status, BDB bdb)
{
/**************************************
 *
 *	j o u r n a l _ b u f f e r
 *
 **************************************
 *
 * Functional description
 *	Write buffer (or associated record buffer) to journal pending
 *	write of page.
 *
 **************************************/
	BDB journal;
	BCB bcb;
	DBB dbb;
	ULONG seqno, offset;
	JRND header;

	dbb = bdb->bdb_dbb;
	bdb->bdb_flags &= ~BDB_journal;

	seqno = offset = 0;

/* If the whole page is to go, let somebody else do the work */

	if (!(journal = bdb->bdb_jrn_bdb)) {
		bdb->bdb_buffer->pag_seqno = seqno;
		bdb->bdb_buffer->pag_offset = offset;
		return;
	}

/* Format header and write journal record */

#ifdef DEV_BUILD
	if (journal->bdb_sequence !=
		MISC_checksum_log_rec((UCHAR *) journal->bdb_buffer,
							  journal->bdb_length, 0, 0))
		DEBUG_PRINTF("Checksum error in journal buffer during AIL_put()\n");
#endif

	header.jrnd_header.jrnh_type = JRN_PAGE;
	header.jrnd_page = bdb->bdb_page;
	header.jrnd_length = journal->bdb_length;

	AIL_put(dbb, status, &header.jrnd_header, JRND_SIZE,
			(UCHAR *) journal->bdb_buffer,
			journal->bdb_length, bdb->bdb_buffer->pag_seqno,
			bdb->bdb_buffer->pag_offset, &seqno, &offset);

	bdb->bdb_buffer->pag_seqno = seqno;
	bdb->bdb_buffer->pag_offset = offset;

/* Release journal record buffer */

	journal->bdb_length = 0;

	BCB_MUTEX_ACQUIRE;
	bcb = dbb->dbb_bcb;
	QUE_INSERT(bcb->bcb_empty, journal->bdb_que);
	BCB_MUTEX_RELEASE;
	bdb->bdb_jrn_bdb = NULL;
}


#ifdef PAGE_LATCHING
static SSHORT latch_bdb(
						TDBB tdbb,
						LATCH type, BDB bdb, SLONG page, SSHORT latch_wait)
{
/**************************************
 *
 *	l a t c h _ b d b
 *
 **************************************
 *
 * Functional description
 *
 * input
 *	type:		LATCH_none, LATCH_exclusive, LATCH_io, LATCH_shared, or LATCH_mark.
 *	bdb:		object to acquire latch on.
 *	page:		page of bdb, for verification.
 *	latch_wait:	1 => Wait as long as necessary to get the latch.
 *				This can cause deadlocks of course.
 *			0 => If the latch can't be acquired immediately,
 *				give up and return 1.
 *	      		<negative number> => Latch timeout interval in seconds.
 *
 * return
 *	0:	latch successfully acquired.
 *	1:	latch not acquired due to a (permitted) timeout.
 *	-1:	latch not acquired, bdb doesn't corresponds to page.
 *
 **************************************/
	LWT lwt_;
	SLONG count;
	EVENT event;
	QUE que;
	BCB bcb;
	DBB dbb;
	SSHORT timeout_occurred = FALSE;
	SSHORT i;

/* If the buffer has been reassigned to another page
   make the caller deal with it. */

	if (bdb->bdb_page != page)
		return -1;

	LATCH_MUTEX_ACQUIRE;

/* Handle the easy case first, no users of the buffer. */

	if (!bdb->bdb_use_count) {
		switch (type) {
		case LATCH_shared:
			++bdb->bdb_use_count;
			bdb->bdb_shared[0] = tdbb;
			break;
		case LATCH_exclusive:
			++bdb->bdb_use_count;
			bdb->bdb_exclusive = tdbb;
			break;
		case LATCH_io:
			++bdb->bdb_use_count;
			bdb->bdb_io = tdbb;
			break;
		case LATCH_mark:
			cache_bugcheck(295);	/* inconsistent LATCH_mark call */
			break;
		case LATCH_none:
			break;
		}
		LATCH_MUTEX_RELEASE;
		return 0;
	}

/* Grant the latch request if it is compatible with existing
   latch holders.  Pending latches are queued in the order in
   which they are requested except that io/mark latches are queued
   ahead of all other latch requests (to avoid deadlocks and
   this does not cause starvation).  Also, shared latches are granted
   immediately if a disk write is in progress.
   Note that asking for a higher mode latch when already holding a
   share latch results in deadlock.  CCH_handoff routinely acquires a
   shared latch while owning already a shared latch on the page
   (the case of handing off to the same page).  If the BDB_must_write
   flag is set, then an exclusive latch request will be followed by
   an io latch request. */

	switch (type) {

	case LATCH_none:
		LATCH_MUTEX_RELEASE;
		return 0;

	case LATCH_shared:
		if (bdb->bdb_flags & BDB_read_pending)
			break;
		if (bdb->bdb_exclusive) {
			if (bdb->bdb_exclusive != tdbb)
				break;			/* someone else owns exclusive latch */
		}
		else {
			/* Note that InterBase often 'hands-off' to the same page, for both
			   shared and exlusive latches. */
			/* Check is we own already an exclusive latch. */
			for (i = 0; (i < BDB_max_shared) && (bdb->bdb_shared[i] != tdbb);
				 i++);
			if (i >= BDB_max_shared) {	/* we don't own a shared latch yet */
				/* If there are latch-waiters, and they are not waiting for an
				   io_latch, then we have to wait also (there must be a exclusive
				   latch waiter).  If there is an IO in progress, the violate the
				   fairness and sneak ahead of the exclusive (or io) waiters. */

				if ((QUE_NOT_EMPTY(bdb->bdb_waiters)) && !bdb->bdb_io)
					break;		/* be fair and wait behind exclusive latch requests */
			}
		}
		/* Nobody owns an exlusive latch, or sneak ahead of exclusive latch
		   waiters while an io is in progress. */

		for (i = 0; (i < BDB_max_shared) && bdb->bdb_shared[i]; i++);
		if (i >= BDB_max_shared)
			break;
		++bdb->bdb_use_count;
		bdb->bdb_shared[i] = tdbb;
		LATCH_MUTEX_RELEASE;
		return 0;

	case LATCH_io:
		if (bdb->bdb_flags & BDB_read_pending)
			break;
		if (bdb->bdb_io)
			break;				/* someone else owns the io latch */
		++bdb->bdb_use_count;
		bdb->bdb_io = tdbb;
		LATCH_MUTEX_RELEASE;
		return 0;

	case LATCH_exclusive:
		/* Exclusive latches wait for existing shared latches and
		   (unfortunately) for existing io latches.  This is not as
		   bad as it sounds because an exclusive latch is typically followed
		   by a mark latch, which then would wait behind the io latch. */
		/* Note that the ail-code latches the same buffer multiple times
		   in shared and exclusive */
		/* Note that InterBase often 'hands-off' to the same page, for both
		   shared and exlusive latches. */
		if (bdb->bdb_use_count && (bdb->bdb_exclusive != tdbb))
			break;
		++bdb->bdb_use_count;
		bdb->bdb_exclusive = tdbb;
		LATCH_MUTEX_RELEASE;
		return 0;

	case LATCH_mark:
		if (bdb->bdb_exclusive != tdbb)
			cache_bugcheck(295);	/* inconsistent LATCH_mark call */
		/* Some InterBase code marks a buffer more than once. */
		if (bdb->bdb_io && (bdb->bdb_io != tdbb))
			break;
		bdb->bdb_io = tdbb;
		LATCH_MUTEX_RELEASE;
		return 0;

	default:
		break;
	}

/* If the caller doesn't want to wait for this latch, then return now. */
	if (latch_wait == 0) {
		LATCH_MUTEX_RELEASE;
		return 1;
	}

/* Get or create a latch wait block and wait for someone to grant
   the latch. */

	dbb = tdbb->tdbb_database;
	bcb = dbb->dbb_bcb;

	if (QUE_NOT_EMPTY(bcb->bcb_free_lwt)) {
		que = bcb->bcb_free_lwt.que_forward;
		QUE_DELETE((*que));
		lwt_ = (LWT) BLOCK(que, LWT, lwt_waiters);
	}
	else {
		lwt_ = FB_NEW(*dbb->dbb_bufferpool) lwt;
		QUE_INIT(lwt_->lwt_waiters);
		ISC_event_init(&lwt_->lwt_event, 0, 0);
	}

	lwt_->lwt_flags |= LWT_pending;
	lwt_->lwt_latch = type;
	lwt_->lwt_tdbb = tdbb;

/* Give priority to IO.  This might prevent deadlocks while performing
   precedence writes.  This does not cause starvation because an
   exclusive latch is needed to dirty the page again. */
	if ((type == LATCH_io) || (type == LATCH_mark))
		QUE_INSERT(bdb->bdb_waiters, lwt_->lwt_waiters)
			else
		QUE_APPEND(bdb->bdb_waiters, lwt_->lwt_waiters);

	event = &lwt_->lwt_event;

/* Loop until the latch is granted or until a timeout occurrs. */
	for (count = ISC_event_clear(event);
		 ((lwt_->lwt_flags & LWT_pending) && !timeout_occurred);
		 count = ISC_event_clear(event)) {
		LATCH_MUTEX_RELEASE;
		THREAD_EXIT;
		if (latch_wait == 1)
			timeout_occurred =
				ISC_event_wait(1, &event, &count, 120 * 1000000, NULL, event);
		else
			timeout_occurred =
				ISC_event_wait(1, &event, &count, -latch_wait * 1000000,
							   NULL, event);
		THREAD_ENTER;
		LATCH_MUTEX_ACQUIRE;
	}

	bcb = dbb->dbb_bcb;			/* Re-initialize */
	QUE_DELETE(lwt_->lwt_waiters);
	QUE_INSERT(bcb->bcb_free_lwt, lwt_->lwt_waiters);

/* If the latch is not granted then a timeout must have occurred. */
	if ((lwt_->lwt_flags & LWT_pending) && timeout_occurred) {
		LATCH_MUTEX_RELEASE;
		if (latch_wait == 1) {
			IBERR_build_status(tdbb->tdbb_status_vector, gds_deadlock, 0);
			CCH_unwind(tdbb, TRUE);
		}
		else
			return 1;
	}

	if (bdb->bdb_page != page) {
		LATCH_MUTEX_RELEASE;
		release_bdb(tdbb, bdb, TRUE, FALSE, FALSE);
		return -1;
	}

	LATCH_MUTEX_RELEASE;
	return 0;
}


#else
static SSHORT latch_bdb(
						TDBB tdbb,
						LATCH type, BDB bdb, SLONG page, SSHORT latch_wait)
{
/**************************************
 *
 *	l a t c h _ b d b
 *
 **************************************
 *
 * Functional description
 *	Simple optimized latching for single-threaded
 *	non-SUPERSERVER platforms.
 *
 **************************************/

	++bdb->bdb_use_count;

	switch (type) {
	case LATCH_shared:
		break;
	case LATCH_exclusive:
		bdb->bdb_exclusive = tdbb;
		break;
	case LATCH_io:
		bdb->bdb_io = tdbb;
		break;
	case LATCH_mark:
		bdb->bdb_io = tdbb;
	case LATCH_none:
		--bdb->bdb_use_count;
		break;
	}

	return 0;
}
#endif


static SSHORT lock_buffer(
						  TDBB tdbb,
						  BDB bdb, SSHORT wait, SSHORT page_type)
{
/**************************************
 *
 *	l o c k _ b u f f e r
 *
 **************************************
 *
 * Functional description
 *	Get a lock on page for a buffer.  If the lock ever slipped
 *	below READ, indicate that the page must be read.
 *
 * input:
 *	wait: LCK_WAIT = TRUE = 1	=> Wait as long a necessary to get the lock.
 *	      LCK_NO_WAIT = FALSE = 0	=> If the lock can't be acquired immediately,
 *						give up and return -1.
 *	      <negative number>		=> Lock timeout interval in seconds.
 *
 * return: 0  => buffer locked, page is already in memroy.
 *	   1  => buffer locked, page needs to be read from disk.
 *	   -1 => timeout on lock occurred, see input parameter 'wait'.
 *
 **************************************/
#ifdef PAGE_LATCHING
	DBB dbb;
#else
	LCK lock;
	USHORT lock_type;
	USHORT must_read;
	ISC_STATUS_ARRAY alt_status;
	ISC_STATUS *status;
	TEXT errmsg[MAX_ERRMSG_LEN + 1];
#endif

	SET_TDBB(tdbb);
#ifdef PAGE_LATCHING
	dbb = tdbb->tdbb_database;

	if (dbb->dbb_refresh_ranges && bdb->bdb_flags & BDB_writer &&
		(page_type == pag_data || page_type == pag_index)) {
		/* This gives refresh cache ranges the potential to work with page
		 * latching by taking out a temporary page lock for notification
		 * purposes.
		 * Fix cache ranges on NetWare due to latching (bug 7199)
		 * Marion change # 18270, 13-Oct-1994
		 */

		struct lck refresh;

		refresh.lck_dbb = dbb;
		refresh.lck_type = LCK_bdb;
		refresh.lck_owner_handle =
			LCK_get_owner_handle(tdbb, refresh.lck_type);
		refresh.lck_parent = dbb->dbb_lock;
		refresh.lck_length = sizeof(SLONG);
		refresh.lck_key.lck_long = bdb->bdb_page;

		if (LCK_lock_non_blocking(tdbb, &refresh, LCK_write, -1))
			LCK_release(tdbb, &refresh);
	}

	return ((bdb->bdb_flags & BDB_read_pending) ? 1 : 0);
#else

	lock_type =
		(bdb->bdb_flags & (BDB_dirty | BDB_writer)) ? LCK_write : LCK_read;
	lock = bdb->bdb_lock;

	if (lock->lck_logical >= lock_type)
		return 0;

	status = tdbb->tdbb_status_vector;

	if (lock->lck_logical == LCK_none) {
		/* Prevent header and TIP pages from generating blocking AST
		   overhead. The promise is that the lock will unconditionally
		   be released when the buffer use count indicates it is safe
		   to do so. */

		if (page_type == pag_header || page_type == pag_transactions) {
			assert(lock->lck_ast == blocking_ast_bdb);
			assert(lock->lck_object == reinterpret_cast<blk*>(bdb));
			lock->lck_ast = 0;
			lock->lck_object = NULL;
		}
		else
			assert(lock->lck_ast != NULL);

		lock->lck_key.lck_long = bdb->bdb_page;
		if (PAGE_LOCK_OPT(lock, lock_type, wait)) {
			if (!lock->lck_ast) {
				/* Restore blocking AST to lock block if it was swapped
				   out. Flag the bdb so that the lock is released when
				   the buffer is released. */

				assert(page_type == pag_header
					   || page_type == pag_transactions);
				lock->lck_ast = blocking_ast_bdb;
				lock->lck_object = reinterpret_cast<blk*>(bdb);
				bdb->bdb_flags |= BDB_no_blocking_ast;
			}
			return 1;
		}

		if (!lock->lck_ast) {
			assert(page_type == pag_header || page_type == pag_transactions);
			lock->lck_ast = blocking_ast_bdb;
			lock->lck_object = reinterpret_cast<blk*>(bdb);
		}

		/* Case: a timeout was specified, or the caller didn't want to wait,
		   return the error. */

		if ((wait == LCK_NO_WAIT)
			|| ((wait < 0) && (status[1] == gds_lock_timeout))) {
			release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);
			return -1;
		}

		/* Case: lock manager detected a deadlock, probably caused by locking the
		   bdb's in an unfortunate order.  Nothing we can do about it, return the
		   error, and log it to firebird.log. */

		gds__msg_format(0, JRD_BUGCHK, 215, sizeof(errmsg), errmsg,
						(TEXT *) bdb->bdb_page, 
						(TEXT *) (SLONG) page_type, 0, 0, 0);
		IBERR_append_status(status, gds_random, gds_arg_string,
							ERR_cstring(errmsg), 0);
		ERR_log(JRD_BUGCHK, 215, errmsg);	/* msg 215 page %ld, page type %ld lock conversion denied */

		/* CCH_unwind releases all the bdb's and calls ERR_punt()
		   ERR_punt will longjump. */

		CCH_unwind(tdbb, TRUE);
	}

/* Lock requires an upward conversion.  Sigh.  Try to get the conversion.
   If it fails, release the lock and re-seize. Save the contents of the
   status vector just in case */

	must_read = (lock->lck_logical < LCK_read) ? 1 : 0;

	memcpy(alt_status, tdbb->tdbb_status_vector, sizeof(alt_status));

	if (LCK_convert_opt(tdbb, lock, lock_type))
		return must_read;
	else if (wait == LCK_NO_WAIT) {
		release_bdb(tdbb, bdb, TRUE, FALSE, FALSE);
		return -1;
	}

	memcpy(tdbb->tdbb_status_vector, alt_status, sizeof(alt_status));

	if (PAGE_LOCK(lock, lock_type, wait))
		return 1;

/* Case: a timeout was specified, or the caller didn't want to wait,
   return the error. */

	if ((wait < 0) && (status[1] == gds_lock_timeout)) 
	{
		release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);
		return -1;
	}

/* Case: lock manager detected a deadlock, probably caused by locking the
   bdb's in an unfortunate order.  Nothing we can do about it, return the
   error, and log it to firebird.log. */

	gds__msg_format(0, JRD_BUGCHK, 216, sizeof(errmsg), errmsg,
					(TEXT*) bdb->bdb_page, 
					(TEXT*) (SLONG) page_type, 0, 0, 0);
	IBERR_append_status(status, gds_random, gds_arg_string,
						ERR_cstring(errmsg), 0);
	ERR_log(JRD_BUGCHK, 216, errmsg);	/* msg 216 page %ld, page type %ld lock denied */

	CCH_unwind(tdbb, TRUE);
	return 0;					/* Added to get rid of Compiler Warning */
#endif
}


static ULONG memory_init(TDBB tdbb, BCB bcb, ULONG number)
{
/**************************************
 *
 *	m e m o r y _ i n i t
 *
 **************************************
 *
 * Functional description
 *	Initialize memory for the cache.
 *	Return number of buffers allocated.
 *
 **************************************/
	UCHAR *memory, *memory_end;
	SLONG page_size, memory_size, buffers, old_buffers;
	bcb_repeat * tail, *end, *tail2, *old_tail;
	DBB dbb;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

	memory = 0;
	buffers = 0;
	page_size = (SLONG) dbb->dbb_page_size;
	memory_size = page_size * (number + 1);

	for (tail = bcb->bcb_rpt, end = tail + number; tail < end; tail++) {
		if (!memory) {
			/* Allocate only what is required for remaining buffers. */

			if (memory_size > (SLONG) (page_size * (number + 1)))
				memory_size = page_size * (number + 1);

			do {
				try {
					memory = (UCHAR *)gds__alloc(memory_size);
					break;
				} catch(const std::exception&) {
					/* Either there's not enough virtual memory or there is
					   but it's not virtually contiguous. Let's find out by
					   cutting the size in half to see if the buffers can be
					   scattered over the remaining virtual address space. */
					memory_size >>= 1;
					if (memory_size < MIN_BUFFER_SEGMENT)	/* Diminishing returns */
						return buffers;
				}
			} while (TRUE);

			LLS_PUSH(memory, &bcb->bcb_memory);
			memory_end = memory + memory_size;

			/* Allocate buffers on an address that is an even multiple
			   of the page size (rather the physical sector size.) This
			   is a necessary condition to support raw I/O interfaces. */
			memory = (UCHAR *) (((U_IPTR) memory + page_size - 1) &
								~((int) page_size - 1));
			old_tail = tail;
			old_buffers = buffers;
		}

		QUE_INIT(tail->bcb_page_mod);

		if (!(tail->bcb_bdb = alloc_bdb(tdbb, bcb, &memory))) {
			/* Whoops! Time to reset our expectations. Release the
			   buffer memory but use that memory size to calculate
			   a new number that takes into account the page buffer
			   overhead. Reduce this number by a 25% fudge factor to
			   leave some memory for useful work. */

			gds__free(LLS_POP(&bcb->bcb_memory));
			memory = 0;
			for (tail2 = old_tail; tail2 < tail; tail2++)
				tail2->bcb_bdb = dealloc_bdb(tail2->bcb_bdb);
			number = memory_size / PAGE_OVERHEAD;
			number -= number >> 2;
			end = old_tail + number;
			tail = --old_tail;	/* For loop continue pops tail above */
			buffers = old_buffers;
			continue;
		}

		buffers++;				/* Allocated buffers */
		number--;				/* Remaining buffers */

		/* Check if memory segment has been exhausted. */

		if (memory + page_size > memory_end)
			memory = 0;
	}

	return buffers;
}


static void page_validation_error(TDBB tdbb, WIN * window, SSHORT type)
{
/**************************************
 *
 *	p a g e _ v a l i d a t i o n _ e r r o r
 *
 **************************************
 *
 * Functional description
 *	We've detected a validation error on fetch.  Generally
 *	we've detected that the type of page fetched didn't match the
 *	type of page we were expecting.  Report an error and
 *	get out.
 *	This function will only be called rarely, as a page validation
 *	error is an indication of on-disk database corruption.
 *
 **************************************/
	BDB bdb;
	PAG page;

	SET_TDBB(tdbb);
	bdb = window->win_bdb;
	page = bdb->bdb_buffer;

	IBERR_build_status(tdbb->tdbb_status_vector,
					   gds_db_corrupt,
					   gds_arg_string, "",
					   gds_arg_gds, gds_page_type_err,
					   gds_arg_gds, gds_badpagtyp,
					   gds_arg_number, (SLONG) bdb->bdb_page,
					   gds_arg_number, (SLONG) type,
					   gds_arg_number, (SLONG) page->pag_type, 0);
/* We should invalidate this bad buffer. */
	CCH_unwind(tdbb, TRUE);
}


#ifdef CACHE_READER
static void prefetch_epilogue(PRF prefetch, ISC_STATUS * status_vector)
{
/**************************************
 *
 *	p r e f e t c h _ e p i l o g u e
 *
 **************************************
 *
 * Functional description
 *	Stall on asynchronous I/O completion.
 *	Move data from prefetch buffer to database
 *	buffers, compute the checksum, and release
 *	the latch.
 *
 **************************************/
	TDBB tdbb;
	DBB dbb;
	SCHAR *next_buffer;
	BDB *next_bdb;
	PAG page;
	ISC_STATUS status;
	USHORT i;

	if (!(prefetch->prf_flags & PRF_active))
		return;

	THREAD_EXIT;
	prefetch->prf_piob.piob_wait = TRUE;
	status = PIO_status(&prefetch->prf_piob, status_vector);
	THREAD_ENTER;

/* If there was an I/O error release all buffer latches acquired
   for the prefetch request. */

	tdbb = prefetch->prf_tdbb;
	dbb = tdbb->tdbb_database;

	if (status == FALSE) {
		next_bdb = prefetch->prf_bdbs;
		for (i = 0; i < prefetch->prf_max_prefetch; i++) {
			if (*next_bdb)
				release_bdb(tdbb, *next_bdb, TRUE, FALSE, FALSE);
			next_bdb++;
		}
		prefetch->prf_flags &= ~PRF_active;
		return;
	}

	next_buffer = prefetch->prf_io_buffer;
	next_bdb = prefetch->prf_bdbs;

	for (i = 0; i < prefetch->prf_max_prefetch; i++) {
		if (*next_bdb) {
			page = (*next_bdb)->bdb_buffer;
			if (next_buffer != reinterpret_cast<char*>(page))
				MOVE_FASTER(next_buffer, (SCHAR *) page,
							(ULONG) dbb->dbb_page_size);
			if (page->pag_checksum == CCH_checksum(*next_bdb)) {
				(*next_bdb)->bdb_flags &= ~(BDB_read_pending | BDB_not_valid);
				(*next_bdb)->bdb_flags |= BDB_prefetch;
			}
			release_bdb(tdbb, *next_bdb, TRUE, FALSE, FALSE);
		}
		next_buffer += dbb->dbb_page_size;
		next_bdb++;
	}

	prefetch->prf_flags &= ~PRF_active;
}


static void prefetch_init(PRF prefetch, TDBB tdbb)
{
/**************************************
 *
 *	p r e f e t c h _ i n i t
 *
 **************************************
 *
 * Functional description
 *	Initialize prefetch data structure.
 *	Most systems that allow access to "raw" I/O
 *	interfaces want the buffer address aligned.
 *
 **************************************/
	DBB dbb;

	dbb = tdbb->tdbb_database;

	prefetch->prf_tdbb = tdbb;
	prefetch->prf_flags = 0;
	prefetch->prf_max_prefetch = PREFETCH_MAX_TRANSFER / dbb->dbb_page_size;
	prefetch->prf_aligned_buffer =
		(SCHAR
		 *) (((U_IPTR) & prefetch->prf_unaligned_buffer + MIN_PAGE_SIZE -
			  1) & ~((U_IPTR) MIN_PAGE_SIZE - 1));
}


static void prefetch_io(PRF prefetch, ISC_STATUS * status_vector)
{
/**************************************
 *
 *	p r e f e t c h _ i o
 *
 **************************************
 *
 * Functional description
 *	Queue an asynchronous I/O to read
 *	multiple pages into prefetch buffer.
 *
 **************************************/
	TDBB tdbb;
	DBB dbb;
	BDB *next_bdb;
	USHORT i;
	ISC_STATUS status;

	tdbb = prefetch->prf_tdbb;
	dbb = tdbb->tdbb_database;

	if (!prefetch->prf_page_count)
		prefetch->prf_flags &= ~PRF_active;
	else {
		/* Get the cache reader working on our behalf too */

		if (!(dbb->dbb_bcb->bcb_flags & BCB_reader_active))
			ISC_event_post(dbb->dbb_reader_event);

		THREAD_EXIT;
		status =
			PIO_read_ahead(dbb, prefetch->prf_start_page,
						   prefetch->prf_io_buffer, prefetch->prf_page_count,
						   &prefetch->prf_piob, status_vector);
		THREAD_ENTER;
		if (status == FALSE) {
			next_bdb = prefetch->prf_bdbs;
			for (i = 0; i < prefetch->prf_max_prefetch; i++) {
				if (*next_bdb)
					release_bdb(tdbb, *next_bdb, TRUE, FALSE, FALSE);
				next_bdb++;
			}
			prefetch->prf_flags &= ~PRF_active;
		}
	}
}


static void prefetch_prologue(PRF prefetch, SLONG * start_page)
{
/**************************************
 *
 *	p r e f e t c h _ p r o l o g u e
 *
 **************************************
 *
 * Functional description
 *	Search for consecutive pages to be prefetched
 *	and latch them for I/O.
 *
 **************************************/
	TDBB tdbb;
	DBB dbb;
	BCB bcb;
	BDB *next_bdb, bdb;
	USHORT i;

	tdbb = prefetch->prf_tdbb;
	dbb = tdbb->tdbb_database;
	bcb = dbb->dbb_bcb;

	prefetch->prf_start_page = *start_page;
	prefetch->prf_page_count = 0;
	prefetch->prf_flags |= PRF_active;

	next_bdb = prefetch->prf_bdbs;
	for (i = 0; i < prefetch->prf_max_prefetch; i++) {
		*next_bdb = 0;
		if (SBM_clear(bcb->bcb_prefetch, *start_page) &&
			(*next_bdb = get_buffer(tdbb, *start_page, LATCH_shared, 0))) {
			if ((*next_bdb)->bdb_flags & BDB_read_pending)
				prefetch->prf_page_count = i + 1;
			else {
				release_bdb(tdbb, *next_bdb, TRUE, FALSE, FALSE);
				*next_bdb = 0;
			}
		}
		next_bdb++;
		(*start_page)++;
	}

/* Optimize non-sequential list prefetching to transfer
   directly to database buffers. */

	if (prefetch->prf_page_count == 1 && (bdb = prefetch->prf_bdbs[0]))
		prefetch->prf_io_buffer = reinterpret_cast<char*>(bdb->bdb_buffer);
	else
		prefetch->prf_io_buffer = prefetch->prf_aligned_buffer;

/* Reset starting page for next bitmap walk */

	--(*start_page);
}
#endif


static SSHORT related(BDB low, BDB high, SSHORT limit)
{
/**************************************
 *
 *	r e l a t e d
 *
 **************************************
 *
 * Functional description
 *	See if there are precedence relationships linking two buffers.
 *	Since precedence graphs can become very complex, limit search for
 *	precedence relationship by visiting a presribed limit of higher
 *	precedence blocks.
 *
 **************************************/
	QUE que, base;
	PRE precedence;

	base = &low->bdb_higher;

	for (que = base->que_forward; que != base; que = que->que_forward) {
		if (!--limit)
			return PRE_UNKNOWN;
		precedence = BLOCK(que, PRE, pre_higher);
		if (!(precedence->pre_flags & PRE_cleared)) {
			if (precedence->pre_hi == high)
				return PRE_EXISTS;
			limit = related(precedence->pre_hi, high, limit);
			if (limit == PRE_EXISTS || limit == PRE_UNKNOWN)
				return limit;
		}
	}

	return limit;
}


#ifdef PAGE_LATCHING
static void release_bdb(
						TDBB tdbb,
						BDB bdb,
						BOOLEAN repost,
						BOOLEAN downgrade_latch, BOOLEAN rel_mark_latch)
{
/**************************************
 *
 *	r e l e a s e _ b d b
 *
 **************************************
 *
 * Functional description
 *	Decrement the use count of a bdb, reposting
 *	blocking AST if required.
 *	If rel_mark_latch is TRUE, the the value of downgrade_latch is ignored.
 *
 **************************************/
	LWT lwt_;
	QUE wait_que, que;
	SSHORT i, granted;

	LATCH_MUTEX_ACQUIRE;

	wait_que = &bdb->bdb_waiters;

/* Releasing a LATCH_mark. */
	if (rel_mark_latch) {
		if ((bdb->bdb_io != tdbb) || (bdb->bdb_exclusive != tdbb))
			cache_bugcheck(294);	/* inconsistent LATCH_mark release */
		bdb->bdb_io = 0;
	}
	else
/* Downgrading from an exlusive to a shared latch. */
	if (downgrade_latch) {
		/* Only the transition from exclusive to shared is supported.
		   If an actual state changed, then we need to check if waiters
		   can be granted.  Otherwise, there is nothing further to do. */
		if (bdb->bdb_io == tdbb)
			cache_bugcheck(296);	/* inconsistent latch downgrade call */
		if (bdb->bdb_exclusive == tdbb) {
			bdb->bdb_exclusive = 0;
			for (i = 0; bdb->bdb_shared[i]; i++);
			bdb->bdb_shared[i] = tdbb;
		}
		else {
			LATCH_MUTEX_RELEASE;
			return;
		}
	}
	else
/* If the exclusive latch is held, then certain code does funny things:
   ail.c does: exclusive - mark - exclusive */
	if (bdb->bdb_exclusive == tdbb) {
		--bdb->bdb_use_count;
		if (!bdb->bdb_use_count) {	/* All latches are released */
			bdb->bdb_exclusive = bdb->bdb_io = 0;
			for (i = 0; i < BDB_max_shared; bdb->bdb_shared[i++] = 0);
		}
		else if (bdb->bdb_io) {	/* This is a release for an io or an exclusive latch */
			if (bdb->bdb_io == tdbb) {	/* We have an io latch */

				/* The BDB_must_write flag causes the system to latch for io, in addition
				   to the already owned latches.  Make sure not to disturb an already existing
				   exclusive latch. */
				/* ail.c does: EX => MARK => SHARED => release => EX => MARK => RELEASE => EX */
				if (!(bdb->bdb_flags & BDB_marked))
					bdb->bdb_io = 0;
			}
			else if (bdb->bdb_use_count == 1)
				/* This must be a release for our exclusive latch */
				bdb->bdb_exclusive = 0;
		}
		else {					/* This is a release for a shared latch */
			for (i = 0; (i < BDB_max_shared) && (bdb->bdb_shared[i] != tdbb);
				 i++);
			if (i < BDB_max_shared)
				bdb->bdb_shared[i] = 0;
		}
	}
	else
/* If the exclusive latch is not held, then things have to behave much nicer. */
	{
		if (bdb->bdb_flags & BDB_marked)
			cache_bugcheck(297);	/* bdb is unexpectedly marked */
		--bdb->bdb_use_count;
		if (bdb->bdb_io == tdbb)
			bdb->bdb_io = 0;
		else {
			for (i = 0; (i < BDB_max_shared) && (bdb->bdb_shared[i] != tdbb);
				 i++);
			if (i >= BDB_max_shared)
				cache_bugcheck(300);	/* can't find shared latch */
			bdb->bdb_shared[i] = 0;
		}
	}

	granted = FALSE;

	for (que = wait_que->que_forward; que != wait_que; que = que->que_forward)
		/* Note that this loop assumes that requests for LATCH_io and LATCH_mark
		   are queued before LATCH_shared and LATCH_exclusive. */
		if ((lwt_ = BLOCK(que, LWT, lwt_waiters))->lwt_flags & LWT_pending) {
			switch (lwt_->lwt_latch) {
			case (LATCH_exclusive):
				if (bdb->bdb_use_count) {
					LATCH_MUTEX_RELEASE;
					return;
				}
				else {
					++bdb->bdb_use_count;
					bdb->bdb_exclusive = lwt_->lwt_tdbb;
					lwt_->lwt_flags &= ~LWT_pending;
					ISC_event_post(&lwt_->lwt_event);
					LATCH_MUTEX_RELEASE;
					return;
				}

			case (LATCH_io):
				if (bdb->bdb_io)
					break;
				else {
					++bdb->bdb_use_count;
					bdb->bdb_io = lwt_->lwt_tdbb;
					lwt_->lwt_flags &= ~LWT_pending;
					ISC_event_post(&lwt_->lwt_event);
					granted = TRUE;
					break;
				}

			case (LATCH_mark):
				if (bdb->bdb_exclusive != lwt_->lwt_tdbb)
					cache_bugcheck(298);	/* missing exclusive latch */
				if (bdb->bdb_io)
					break;
				else {
					bdb->bdb_io = lwt_->lwt_tdbb;
					lwt_->lwt_flags &= ~LWT_pending;
					ISC_event_post(&lwt_->lwt_event);
					granted = TRUE;
					break;
				}

			case (LATCH_shared):
				if (bdb->bdb_exclusive) {
					break;		/* defensive programming */
					/* correct programming
					   LATCH_MUTEX_RELEASE;
					   return;  */
				}
				for (i = 0; (i < BDB_max_shared) && bdb->bdb_shared[i]; i++);
				if (i >= BDB_max_shared)
					break;
				++bdb->bdb_use_count;
				bdb->bdb_shared[i] = lwt_->lwt_tdbb;
				lwt_->lwt_flags &= ~LWT_pending;
				ISC_event_post(&lwt_->lwt_event);
				granted = TRUE;
				break;
			}

			if (granted && (bdb->bdb_flags & BDB_read_pending))
				/* Allow only one reader to proceed */
				break;
		}

	if (bdb->bdb_use_count || !repost) {
		LATCH_MUTEX_RELEASE;
		return;
	}

	LATCH_MUTEX_RELEASE;

	if (bdb->bdb_ast_flags & BDB_blocking)
	{
		PAGE_LOCK_RE_POST(bdb->bdb_lock);
	}
}


#else
static void release_bdb(
						TDBB tdbb,
						BDB bdb,
						BOOLEAN repost,
						BOOLEAN downgrade_latch, BOOLEAN rel_mark_latch)
{
/**************************************
 *
 *	r e l e a s e _ b d b
 *
 **************************************
 *
 * Functional description
 *	Decrement the use count of a bdb, reposting
 *	blocking AST if required.
 *	If rel_mark_latch is TRUE, the the value of downgrade_latch is ignored.
 *
 **************************************/

/* Releasing a LATCH_mark. */

	if (rel_mark_latch)
		bdb->bdb_io = 0;
	else
/* Downgrading from an exlusive to a shared latch. */
	if (downgrade_latch)
		bdb->bdb_exclusive = 0;

	else
/* If the exclusive latch is held, then certain code does funny things:
   ail.c does: exclusive - mark - exclusive */
	if (bdb->bdb_exclusive == tdbb) {
		if (!--bdb->bdb_use_count)
			bdb->bdb_exclusive = bdb->bdb_io = 0;
		else if (bdb->bdb_io) {	/* This is a release for an io or an exclusive latch */
			if (bdb->bdb_io == tdbb) {	/* We have an io latch */

				/* The BDB_must_write flag causes the system to latch for io, in addition
				   to the already owned latches.  Make sure not to disturb an already existing
				   exclusive latch. */
				/* ail.c does: EX => MARK => SHARED => release => EX => MARK => RELEASE => EX */
				if (!(bdb->bdb_flags & BDB_marked))
					bdb->bdb_io = 0;
			}
			else if (bdb->bdb_use_count == 1)
				/* This must be a release for our exclusive latch */
				bdb->bdb_exclusive = 0;
		}
	}
	else
		/* If the exclusive latch is not held, then things have to behave much nicer. */
	{
		--bdb->bdb_use_count;
		bdb->bdb_io = 0;
	}

	if (bdb->bdb_use_count || !repost)
		return;

	if (bdb->bdb_ast_flags & BDB_blocking)
		PAGE_LOCK_RE_POST(bdb->bdb_lock);
}
#endif


static BOOLEAN writeable(BDB bdb)
{
/**************************************
 *
 *	w r i t e a b l e
 *
 **************************************
 *
 * Functional description
 *	See if a buffer is writeable.  A buffer is writeable if
 *	neither it nor any of it's higher precedence cousins are
 *	marked for write.
 *
 **************************************/
	QUE que;
	PRE precedence;

	if (bdb->bdb_flags & BDB_marked)
		return FALSE;

/* If there are buffers that must be written first, check them, too. */

	for (que = bdb->bdb_higher.que_forward; que != &bdb->bdb_higher;
		 que = que->que_forward) {
		precedence = BLOCK(que, PRE, pre_higher);
		if (!(precedence->pre_flags & PRE_cleared) &&
			!writeable(precedence->pre_hi)) return FALSE;
	}

	return TRUE;
}


static int write_buffer(
						TDBB tdbb,
						BDB bdb,
						SLONG page,
						USHORT write_thru,
						ISC_STATUS * status, BOOLEAN write_this_page)
{
/**************************************
 *
 *	w r i t e _ b u f f e r
 *
 **************************************
 *
 * Functional description
 *	Write a dirty buffer.  This may recurse due to
 *	precedence problems.
 *
 * input:  write_this_page
 *		= true if the input page needs to be written
 *			before returning.  (normal case)
 *		= false if the input page is being written
 *			because of precedence.  Only write
 *			one page and return so that the caller
 *			can re-establish the need to write this
 *			page.
 *
 * return: 0 = Write failed.
 *         1 = Page is written.  Page was written by this
 *     		call, or was written by someone else, or the
 *		cache buffer is already reassigned.
 *	   2 = Only possible if write_this_page is FALSE.
 *		This input page is not written.  One
 *		page higher in precedence is written
 *		though.  Probable action: re-establich the
 * 		need to write this page and retry write.
 *
 **************************************/
	DBB dbb;
	QUE que;
	PRE precedence;
	BCB bcb;
	BOOLEAN result;
	BDB hi_bdb;
	SLONG hi_page;
	int write_status;

	SET_TDBB(tdbb);
	dbb = tdbb->tdbb_database;

	if (latch_bdb(tdbb, LATCH_io, bdb, page, 1) == -1)
		return 1;

	if ((bdb->bdb_flags & BDB_marked) && !(bdb->bdb_flags & BDB_faked))
		cache_bugcheck(217);	/* msg 217 buffer marked for update */

	if (!(bdb->bdb_flags & BDB_dirty) &&
		!(write_thru && bdb->bdb_flags & BDB_db_dirty)) {
		clear_precedence(dbb, bdb);
		release_bdb(tdbb, bdb, TRUE, FALSE, FALSE);
		return 1;
	}

/* If there are buffers that must be written first, write them now. */

	PRE_MUTEX_ACQUIRE;

	while (QUE_NOT_EMPTY(bdb->bdb_higher)) {
		bcb = dbb->dbb_bcb;		/* Re-initialize in the loop */
		que = bdb->bdb_higher.que_forward;
		precedence = BLOCK(que, PRE, pre_higher);
		if (precedence->pre_flags & PRE_cleared) {
			QUE_DELETE(precedence->pre_higher);
			QUE_DELETE(precedence->pre_lower);
			precedence->pre_hi = (BDB) bcb->bcb_free;
			bcb->bcb_free = precedence;
		}
		else {
			hi_bdb = precedence->pre_hi;
			hi_page = hi_bdb->bdb_page;
			PRE_MUTEX_RELEASE;
			release_bdb(tdbb, bdb, FALSE, FALSE, FALSE);
			write_status =
				write_buffer(tdbb, hi_bdb, hi_page, write_thru, status,
							 FALSE);
			if (write_status == 0)
				return 0;		/* return IO error */
#ifdef SUPERSERVER
			if (!write_this_page)
				return 2;		/* caller wants to re-establish the need for this write after one precedence write */
#endif
			if (latch_bdb(tdbb, LATCH_io, bdb, page, 1) == -1)
				return 1;		/* cache buffer reassigned, return 'write successful' */
			PRE_MUTEX_ACQUIRE;
		}
	}

	PRE_MUTEX_RELEASE;

#ifdef SUPERSERVER_V2
/* Header page I/O is deferred until a dirty page, which was modified by a
   transaction not updated on the header page, needs to be written. Note
   that the header page needs to be written with the target page latched to
   prevent younger transactions from modifying the target page. */

	if (page != HEADER_PAGE
		&& bdb->bdb_mark_transaction >
		dbb->dbb_last_header_write) TRA_header_write(tdbb, dbb,
													 bdb->bdb_mark_transaction);
#endif

/* Unless the buffer has been faked (recently re-allocated), write
   out the page */

	result = TRUE;
	if (
		(bdb->bdb_flags & BDB_dirty
		 || (write_thru && bdb->bdb_flags & BDB_db_dirty))
		&& !(bdb->bdb_flags & BDB_marked)) {
		if ( (result = write_page(tdbb, bdb, write_thru, status, FALSE)) )
			clear_precedence(dbb, bdb);
	}
	else
		clear_precedence(dbb, bdb);

	release_bdb(tdbb, bdb, TRUE, FALSE, FALSE);

	if (!result)
		return 0;
	else
#ifdef SUPERSERVER
	if (!write_this_page)
		return 2;
	else
#endif
		return 1;
}


static BOOLEAN write_page(
						  TDBB tdbb,
						  BDB bdb,
						  USHORT write_thru, ISC_STATUS * status, BOOLEAN inAst)
{
/**************************************
 *
 *	w r i t e _ p a g e
 *
 **************************************
 *
 * Functional description
 *	Do actions required when writing a database page,
 *	including journaling, shadowing.
 *
 **************************************/
	PAG page;
	DBB dbb;
	BOOLEAN result;
	ISC_STATUS saved_status;
	FIL file;

	if (bdb->bdb_flags & BDB_not_valid) {
		*status++ = gds_arg_gds;
		*status++ = gds_buf_invalid;
		*status++ = gds_arg_number;
		*status++ = bdb->bdb_page;
		*status++ = gds_arg_end;
		return FALSE;
	}

	result = TRUE;
	dbb = bdb->bdb_dbb;
	page = bdb->bdb_buffer;

/* Before writing db header page, make sure that the next_transaction > oldest_active
   transaction */
	if (bdb->bdb_page == HEADER_PAGE) {
		HDR header = (HDR) page;
		if (header->hdr_next_transaction) {
			if (header->hdr_oldest_active > header->hdr_next_transaction)
				BUGCHECK(266);	/*next transaction older than oldest active */

			if (header->hdr_oldest_transaction > header->hdr_next_transaction)
				BUGCHECK(267);	/* next transaction older than oldest transaction */
		}
	}

	page->pag_generation++;
	if (bdb->bdb_flags & BDB_journal) {
		saved_status = status[1];
		status[1] = 0;
		journal_buffer(status, bdb);
		if (!status[1])
			status[1] = saved_status;
		else
			return FALSE;
	}

	if (!dbb->dbb_wal || write_thru) {
		AST_CHECK;
		dbb->dbb_writes++;
		if (dbb->dbb_wal)
			WAL_flush(status, dbb->dbb_wal,
					  reinterpret_cast < long *>(&page->pag_seqno),
					  reinterpret_cast < long *>(&page->pag_offset), true);

#ifdef DEBUG_SAVE_BDB_PAGE
		/* Save page number into page->pag_offset before computing the checksum */
		page->pag_offset = bdb->bdb_page;
#endif

		/* write out page to main database file, and to any
		   shadows, making a special case of the header page */

		if (bdb->bdb_page >= 0) {
			if (bdb->bdb_write_direction == BDB_write_undefined) {
				dbb->dbb_flags |= DBB_bugcheck;
				status[0] = isc_arg_gds;
				status[1] = gds_bug_check;
				status[2] = gds_arg_string;
				status[3] = (ISC_STATUS)ERR_cstring("Undefined page write direction");
				status[4] = gds_arg_end;
				return FALSE;
			}
			page->pag_checksum = CCH_checksum(bdb);
			if (bdb->bdb_write_direction == BDB_write_diff ||
				(bdb->bdb_write_direction == BDB_write_both 
#ifndef SUPERSERVER
				 && bdb->bdb_diff_generation == dbb->backup_manager->get_current_generation()
#endif
				)) 
			{
#ifdef NBAK_DEBUG
				// We cannot call normal trace functions here as they are signal-unsafe
				char buffer[1000], *ptr = buffer;
				strcpy(ptr, "NBAK,Write page "); ptr += strlen(ptr);
				gds__ulstr(ptr, bdb->bdb_page, 0, 0); ptr += strlen(ptr);
				strcpy(ptr, " at offset "); ptr += strlen(ptr);
				gds__ulstr(ptr, bdb->bdb_difference_page, 0, 0); ptr += strlen(ptr);
				strcpy(ptr, " in difference file");
				gds__trace(buffer);
#endif
				if (!dbb->backup_manager->write_difference(
					status, bdb->bdb_difference_page, bdb->bdb_buffer)) 
				{
					bdb->bdb_flags |= BDB_io_error;
					dbb->dbb_flags |= DBB_suspend_bgio;
					return FALSE;
				}
			}
			if (bdb->bdb_write_direction == BDB_write_diff) {
				// We finished. Adjust transaction accounting and get ready for exit
				if (bdb->bdb_page == HEADER_PAGE)
					dbb->dbb_last_header_write =
						((HDR) page)->hdr_next_transaction;
				set_write_direction(dbb, bdb, BDB_write_undefined);
			} else {
				// We need to write our pages to main database files
#ifdef SUPERSERVER
				THREAD_EXIT;
#endif
				file = dbb->dbb_file;
				while (!PIO_write(file, bdb, page, status)) {
#ifdef SUPERSERVER
					THREAD_ENTER;
#endif
					if (!CCH_rollover_to_shadow(dbb, file, inAst)) {
						bdb->bdb_flags |= BDB_io_error;
						dbb->dbb_flags |= DBB_suspend_bgio;
						return FALSE;
					}
#ifdef SUPERSERVER
					THREAD_EXIT;
#endif
					file = dbb->dbb_file;
				}

#ifdef SUPERSERVER
				THREAD_ENTER;
#endif
				if (bdb->bdb_page == HEADER_PAGE)
					dbb->dbb_last_header_write =
						((HDR) page)->hdr_next_transaction;
				if (dbb->dbb_shadow)
					result =
						CCH_write_all_shadows(tdbb, 0, bdb, status, 0, inAst);
				set_write_direction(dbb, bdb, BDB_write_undefined);
			}
		}

#ifdef SUPERSERVER
		if (result) {
#ifdef CACHE_WRITER
			if (bdb->bdb_flags & BDB_checkpoint)
				--dbb->dbb_bcb->bcb_checkpoint;
#endif
			bdb->bdb_flags &= ~(BDB_db_dirty | BDB_checkpoint);
		}
#endif
		AST_CHECK;
	}

	if (!result) {
		/* If there was a write error then idle background threads
		   so that they don't spin trying to write these pages. This
		   usually results from device full errors which can be fixed
		   by someone freeing disk space. */

		bdb->bdb_flags |= BDB_io_error;
		dbb->dbb_flags |= DBB_suspend_bgio;
	}
	else {
		/* clear the dirty bit vector, since the buffer is now
		   clean regardless of which transactions have modified it */

		bdb->bdb_transactions = bdb->bdb_mark_transaction = 0;
		if (!(dbb->dbb_bcb->bcb_flags & BCB_keep_pages) &&
			(bdb->bdb_parent || bdb == dbb->dbb_bcb->bcb_btree))
			btc_remove(bdb);
		bdb->bdb_flags &= ~(BDB_dirty | BDB_must_write | BDB_system_dirty);
		if (bdb->bdb_flags & BDB_io_error) {
			/* If a write error has cleared, signal background threads
			   to resume their regular duties. If someone has freed up
			   disk space these errors will spontaneously go away. */

			bdb->bdb_flags &= ~BDB_io_error;
			dbb->dbb_flags &= ~DBB_suspend_bgio;
		}
	}

	return result;
}


static void unmark(TDBB tdbb, WIN * window)
{
/**************************************
 *
 *	u n m a r k
 *
 **************************************
 *
 * Functional description
 *	Unmark a bdb.  Called when the update of a page is
 *	complete and delaying the 'unmarking' could cause
 *	problems.
 *
 **************************************/
	BDB bdb;
	BOOLEAN marked;

	SET_TDBB(tdbb);
	bdb = window->win_bdb;
	BLKCHK(bdb, type_bdb);

	if (bdb->bdb_use_count == 1) {
		marked = (bdb->bdb_flags & BDB_marked) ? TRUE : FALSE;
		bdb->bdb_flags &= ~BDB_marked;
		if (marked)
			release_bdb(tdbb, bdb, FALSE, FALSE, TRUE);
	}
}

} // extern "C"