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Detect database file changes using a 128-bit segment of the file header

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that includes the change counter.  Ticket #2303. (CVS 3844)

FossilOrigin-Name: e44995debf2456e55b502783849e93a045a527c8
This commit is contained in:
drh
2007-04-16 15:02:19 +00:00
parent 9e4e6e502f
commit 86a88114fa
5 changed files with 65 additions and 55 deletions
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58
src/pager.c
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@@ -18,7 +18,7 @@
** file simultaneously, or one process from reading the database while
** another is writing.
**
** @(#) $Id: pager.c,v 1.328 2007/04/13 04:01:59 drh Exp $
** @(#) $Id: pager.c,v 1.329 2007/04/16 15:02:19 drh Exp $
*/
#ifndef SQLITE_OMIT_DISKIO
#include "sqliteInt.h"
@@ -294,7 +294,7 @@ struct Pager {
Pager *pNext; /* Linked list of pagers in this thread */
#endif
char *pTmpSpace; /* Pager.pageSize bytes of space for tmp use */
u32 iChangeCount; /* Db change-counter for which cache is valid */
char dbFileVers[16]; /* Changes whenever database file changes */
};
/*
@@ -1131,12 +1131,14 @@ static int pager_playback_one_page(Pager *pPager, OsFile *jfd, int useCksum){
#ifdef SQLITE_CHECK_PAGES
pPg->pageHash = pager_pagehash(pPg);
#endif
CODEC1(pPager, pData, pPg->pgno, 3);
/* If this was page 1, then restore the value of Pager.iChangeCount */
/* If this was page 1, then restore the value of Pager.dbFileVers.
** Do this before any decoding. */
if( pgno==1 ){
pPager->iChangeCount = retrieve32bits(pPg, 24);
memcpy(&pPager->dbFileVers, &((u8*)pData)[24],sizeof(pPager->dbFileVers));
}
/* Decode the page just read from disk */
CODEC1(pPager, pData, pPg->pgno, 3);
}
return rc;
}
@@ -2441,6 +2443,9 @@ static int pager_write_pagelist(PgHdr *pList){
rc = sqlite3OsWrite(pPager->fd, pData, pPager->pageSize);
PAGER_INCR(sqlite3_pager_writedb_count);
PAGER_INCR(pPager->nWrite);
if( pList->pgno==1 ){
memcpy(&pPager->dbFileVers, &pData[24], sizeof(pPager->dbFileVers));
}
}
#ifndef NDEBUG
else{
@@ -2680,6 +2685,10 @@ static int readDbPage(Pager *pPager, PgHdr *pPg, Pgno pgno){
PAGER_INCR(pPager->nRead);
IOTRACE(("PGIN %p %d\n", pPager, pgno));
PAGERTRACE3("FETCH %d page %d\n", PAGERID(pPager), pPg->pgno);
if( pgno==1 ){
memcpy(&pPager->dbFileVers, &((u8*)PGHDR_TO_DATA(pPg))[24],
sizeof(pPager->dbFileVers));
}
CODEC1(pPager, PGHDR_TO_DATA(pPg), pPg->pgno, 3);
return rc;
}
@@ -2780,16 +2789,21 @@ static int pagerSharedLock(Pager *pPager){
if( pPager->pAll ){
/* The shared-lock has just been acquired on the database file
** and there are already pages in the cache (from a previous
** read or write transaction). If the value of the change-counter
** stored in Pager.iChangeCount matches that found on page 1 of
** the database file, then no database changes have occured since
** the cache was last valid and it is safe to retain the cached
** pages. Otherwise, if Pager.iChangeCount does not match the
** change-counter on page 1 of the file, the current cache contents
** must be discarded.
** read or write transaction). Check to see if the database
** has been modified. If the database has changed, flush the
** cache.
**
** Database changes is detected by looking at 15 bytes beginning
** at offset 24 into the file. The first 4 of these 16 bytes are
** a 32-bit counter that is incremented with each change. The
** other bytes change randomly with each file change when
** a codec is in use.
**
** There is a vanishingly small chance that a change will not be
** deteched. The chance of an undetected change is so small that
** it can be neglected.
*/
u8 zC[4];
u32 iChangeCounter = 0;
char dbFileVers[sizeof(pPager->dbFileVers)];
sqlite3PagerPagecount(pPager);
if( pPager->errCode ){
@@ -2797,21 +2811,20 @@ static int pagerSharedLock(Pager *pPager){
}
if( pPager->dbSize>0 ){
/* Read the 4-byte change counter directly from the file. */
rc = sqlite3OsSeek(pPager->fd, 24);
if( rc!=SQLITE_OK ){
return rc;
}
rc = sqlite3OsRead(pPager->fd, zC, 4);
rc = sqlite3OsRead(pPager->fd, &dbFileVers, sizeof(dbFileVers));
if( rc!=SQLITE_OK ){
return rc;
}
iChangeCounter = (zC[0]<<24) + (zC[1]<<16) + (zC[2]<<8) + zC[3];
}else{
memset(dbFileVers, 0, sizeof(dbFileVers));
}
if( iChangeCounter!=pPager->iChangeCount ){
if( memcmp(pPager->dbFileVers, dbFileVers, sizeof(dbFileVers))!=0 ){
pager_reset(pPager);
pPager->iChangeCount = iChangeCounter;
}
}
}
@@ -3029,10 +3042,6 @@ int sqlite3PagerAcquire(
return rc;
}
}
/* If this was page 1, then restore the value of Pager.iChangeCount */
if( pgno==1 ){
pPager->iChangeCount = retrieve32bits(pPg, 24);
}
/* Link the page into the page hash table */
h = pgno & (pPager->nHash-1);
@@ -3722,7 +3731,6 @@ static int pager_incr_changecounter(Pager *pPager){
/* Increment the value just read and write it back to byte 24. */
change_counter++;
put32bits(((char*)PGHDR_TO_DATA(pPgHdr))+24, change_counter);
pPager->iChangeCount = change_counter;
/* Release the page reference. */
sqlite3PagerUnref(pPgHdr);