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A minor logic correction in the previous check-in. Also added a lengthy

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comment describing the meanings of various flags in the 
{quote: PgHdr} structure. (CVS 4080)

FossilOrigin-Name: 57bf8204cde47dfeb31c064f2b128b9a8d94189f
This commit is contained in:
drh
2007-06-16 11:17:45 +00:00
parent 5e3853112a
commit 732c817f8b
3 changed files with 81 additions and 11 deletions
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78
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 1.345 2007/06/16 04:42:12 drh Exp $
** @(#) $Id: pager.c,v 1.346 2007/06/16 11:17:46 drh Exp $
*/
#ifndef SQLITE_OMIT_DISKIO
#include "sqliteInt.h"
@@ -150,6 +150,78 @@
** The PgHdr.dirty flag is set when sqlite3PagerWrite() is called and
** is cleared again when the page content is written back to the original
** database file.
**
** Details of important structure elements:
**
** needSync
**
** If this is true, this means that it is not safe to write the page
** content to the database because the original content needed
** for rollback has not by synced to the main rollback journal.
** The original content may have been written to the rollback journal
** but it has not yet been synced. So we cannot write to the database
** file because power failure might cause the page in the journal file
** to never reach the disk. It is as if the write to the journal file
** does not occur until the journal file is synced.
**
** This flag is false if the page content exactly matches what
** currently exists in the database file. The needSync flag is also
** false if the original content has been written to the main rollback
** journal and synced. If the page represents a new page that has
** been added onto the end of the database during the current
** transaction, the needSync flag is true until the original database
** size in the journal header has been synced to disk.
**
** inJournal
**
** This is true if the original page has been written into the main
** rollback journal. This is always false for new pages added to
** the end of the database file during the current transaction.
** And this flag says nothing about whether or not the journal
** has been synced to disk. For pages that are in the original
** database file, the following expression should always be true:
**
** inJournal = (pPager->aInJournal[(pgno-1)/8] & (1<<((pgno-1)%8))!=0
**
** The pPager->aInJournal[] array is only valid for the original
** pages of the database, not new pages that are added to the end
** of the database, so obviously the above expression cannot be
** valid for new pages. For new pages inJournal is always 0.
**
** dirty
**
** When true, this means that the content of the page has been
** modified and needs to be written back to the database file.
** If false, it means that either the content of the page is
** unchanged or else the content is unimportant and we do not
** care whether or not it is preserved.
**
** alwaysRollback
**
** This means that the sqlite3PagerDontRollback() API should be
** ignored for this page. The DontRollback() API attempts to say
** that the content of the page on disk is unimportant (it is an
** unused page on the freelist) so that it is unnecessary to
** rollback changes to this page because the content of the page
** can change without changing the meaning of the database. This
** flag overrides any DontRollback() attempt. This flag is set
** when a page that originally contained valid data is added to
** the freelist. Later in the same transaction, this page might
** be pulled from the freelist and reused for something different
** and at that point the DontRollback() API will be called because
** pages taken from the freelist do not need to be protected by
** the rollback journal. But this flag says that the page was
** not originally part of the freelist so that it still needs to
** be rolled back in spite of any subsequent DontRollback() calls.
**
** needRead
**
** This flag means (when true) that the content of the page has
** not yet been loaded from disk. The in-memory content is just
** garbage. (Actually, we zero the content, but you should not
** make any assumptions about the content nevertheless.) If the
** content is needed in the future, it should be read from the
** original database file.
*/
typedef struct PgHdr PgHdr;
struct PgHdr {
@@ -4299,11 +4371,9 @@ int sqlite3PagerMovepage(Pager *pPager, DbPage *pPg, Pgno pgno){
}
if( pPager->aInJournal && (int)pgno<=pPager->origDbSize ){
pPg->inJournal = (pPager->aInJournal[pgno/8] & (1<<(pgno&7)))!=0;
}else if( (int)pgno>=pPager->origDbSize ){
pPg->inJournal = 1;
}else{
pPg->inJournal = 0;
assert( pPg->needSync==0 );
assert( pPg->needSync==0 || (int)pgno>pPager->origDbSize );
}
/* Change the page number for pPg and insert it into the new hash-chain. */