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Comment changes in pager.c. (CVS 1567)

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FossilOrigin-Name: 0e420f72cd5885e32914b4d958bad811fdd9fb77
This commit is contained in:
drh
2004-06-10 23:35:50 +00:00
parent ef5ecb4126
commit 726de599bd
3 changed files with 54 additions and 31 deletions
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71
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 1.122 2004/06/10 05:59:25 danielk1977 Exp $
** @(#) $Id: pager.c,v 1.123 2004/06/10 23:35:50 drh Exp $
*/
#include "os.h" /* Must be first to enable large file support */
#include "sqliteInt.h"
@@ -59,9 +59,11 @@ static Pager *mainPager = 0;
** multiple readers accessing the same database
** file at the same time.
**
** PAGER_RESERVED Writing is permitted to the page cache only.
** The original database file has not been modified.
** Other processes may still be reading the on-disk
** PAGER_RESERVED This process has reserved the database for writing
** but has not yet made any changes. Only one process
** at a time can reserve the database. The original
** database file has not been modified so other
** processes may still be reading the on-disk
** database file.
**
** PAGER_EXCLUSIVE The page cache is writing the database.
@@ -70,16 +72,16 @@ static Pager *mainPager = 0;
** process is writing.
**
** The page cache comes up in PAGER_UNLOCK. The first time a
** sqlite_page_get() occurs, the state transitions to PAGER_SHARED.
** sqlite3pager_get() occurs, the state transitions to PAGER_SHARED.
** After all pages have been released using sqlite_page_unref(),
** the state transitions back to PAGER_UNLOCK. The first time
** that sqlite_page_write() is called, the state transitions to
** that sqlite3pager_write() is called, the state transitions to
** PAGER_RESERVED. (Note that sqlite_page_write() can only be
** called on an outstanding page which means that the pager must
** be in PAGER_SHARED before it transitions to PAGER_RESERVED.)
** The sqlite_page_rollback() and sqlite_page_commit() functions
** transition the state from PAGER_RESERVED to PAGER_EXCLUSIVE to
** PAGER_SHARED.
** The transition to PAGER_EXCLUSIVE occurs when before any changes
** are made to the database file. After an sqlite3pager_rollback()
** or sqlite_pager_commit(), the state goes back to PAGER_SHARED.
*/
#define PAGER_UNLOCK 0
#define PAGER_SHARED 1
@@ -215,13 +217,13 @@ struct Pager {
u8 memDb; /* True to inhibit all file I/O */
u8 *aInJournal; /* One bit for each page in the database file */
u8 *aInStmt; /* One bit for each page in the database */
int nMaster; /* Number of bytes to reserve for master j.p */
BusyHandler *pBusyHandler; /* Pointer to sqlite.busyHandler */
PgHdr *pFirst, *pLast; /* List of free pages */
PgHdr *pFirstSynced; /* First free page with PgHdr.needSync==0 */
PgHdr *pAll; /* List of all pages */
PgHdr *pStmt; /* List of pages in the statement subjournal */
PgHdr *aHash[N_PG_HASH]; /* Hash table to map page number of PgHdr */
int nMaster; /* Number of bytes to reserve for master j.p */
BusyHandler *pBusyHandler; /* Pointer to sqlite.busyHandler */
PgHdr *aHash[N_PG_HASH]; /* Hash table to map page number to PgHdr */
};
/*
@@ -261,16 +263,15 @@ static const unsigned char aJournalMagic[] = {
};
/*
** The size of the header and of each page in the journal varies according
** to which journal format is being used. The following macros figure out
** the sizes based on format numbers.
** The size of the header and of each page in the journal is determined
** by the following macros.
*/
#define JOURNAL_HDR_SZ(pPager) (24 + (pPager)->nMaster)
#define JOURNAL_PG_SZ(pPager) ((pPager->pageSize) + 8)
/*
** Enable reference count tracking here:
** Enable reference count tracking (for debugging) here:
*/
#ifdef SQLITE_TEST
int pager3_refinfo_enable = 0;
@@ -292,6 +293,8 @@ static const unsigned char aJournalMagic[] = {
** Read a 32-bit integer from the given file descriptor. Store the integer
** that is read in *pRes. Return SQLITE_OK if everything worked, or an
** error code is something goes wrong.
**
** All values are stored on disk as big-endian.
*/
static int read32bits(OsFile *fd, u32 *pRes){
u32 res;
@@ -469,8 +472,21 @@ static int pager_unwritelock(Pager *pPager){
** Compute and return a checksum for the page of data.
**
** This is not a real checksum. It is really just the sum of the
** random initial value and the page number. We considered do a checksum
** of the database, but that was found to be too slow.
** random initial value and the page number. We experimented with
** a checksum of the entire data, but that was found to be too slow.
**
** Note that the page number is stored at the beginning of data and
** the checksum is stored at the end. This is important. If journal
** corruption occurs due to a power failure, the most likely scenario
** is that one end or the other of the record will be changed. It is
** much less likely that the two ends of the journal record will be
** correct and the middle be corrupt. Thus, this "checksum" scheme,
** though fast and simple, catches the mostly likely kind of corruption.
**
** FIX ME: Consider adding every 200th (or so) byte of the data to the
** checksum. That way if a single page spans 3 or more disk sectors and
** only the middle sector is corrupt, we will still have a reasonable
** chance of failing the checksum and thus detecting the problem.
*/
static u32 pager_cksum(Pager *pPager, Pgno pgno, const char *aData){
u32 cksum = pPager->cksumInit + pgno;
@@ -481,10 +497,9 @@ static u32 pager_cksum(Pager *pPager, Pgno pgno, const char *aData){
** Read a single page from the journal file opened on file descriptor
** jfd. Playback this one page.
**
**
**
** There are three different journal formats. The format parameter determines
** which format is used by the journal that is played back.
** If useCksum==0 it means this journal does not use checksums. Checksums
** are not used in statement journals because statement journals do not
** need to survive power failures.
*/
static int pager_playback_one_page(Pager *pPager, OsFile *jfd, int useCksum){
int rc;
@@ -546,14 +561,13 @@ static int pager_playback_one_page(Pager *pPager, OsFile *jfd, int useCksum){
assert( pPg->nRef==0 || pPg->pgno==1 );
pData = PGHDR_TO_DATA(pPg);
memcpy(pData, aData, pPager->pageSize);
if( pPager->xDestructor ){
if( pPager->xDestructor ){ /*** FIX ME: Should this be xReinit? ***/
pPager->xDestructor(pData, pPager->pageSize);
}
if( pPager->state==PAGER_EXCLUSIVE ){
pPg->dirty = 0;
pPg->needSync = 0;
}
CODEC(pPager, pData, pPg->pgno, 3);
}
return rc;
@@ -564,6 +578,11 @@ static int pager_playback_one_page(Pager *pPager, OsFile *jfd, int useCksum){
** file that referred to the master journal file has just been rolled back.
** This routine checks if it is possible to delete the master journal file,
** and does so if it is.
**
** The master journal file contains the names of all child journals.
** To tell if a master journal can be deleted, check to each of the
** children. If all children are either missing or do not refer to
** a different master journal, then this master journal can be deleted.
*/
static int pager_delmaster(const char *zMaster){
int rc;
@@ -595,6 +614,8 @@ static int pager_delmaster(const char *zMaster){
zDb = zMasterJournal;
while( (zDb-zMasterJournal)<nMasterJournal ){
char *zJournal = 0;
/*** FIX ME: Store the full journal name in the master journal,
**** not just the base database name. ***/
sqlite3SetString(&zJournal, zDb, "-journal", 0);
if( !zJournal ){
rc = SQLITE_NOMEM;
@@ -638,6 +659,8 @@ static int pager_delmaster(const char *zMaster){
rc = read32bits(&journal, (u32*)&nMaster);
if( rc!=SQLITE_OK ) goto delmaster_out;
if( nMaster>0 && nMaster>=strlen(zMaster)+1 ){
/*** FIX ME: Consider allocating this space at the same time
**** space is allocated for holding the text of the master journal */
char *zMasterPtr = (char *)sqliteMalloc(nMaster);
if( !zMasterPtr ){
rc = SQLITE_NOMEM;