SQLite

** The pager is used to access a database disk file.  It implements
** atomic commit and rollback through the use of a journal file that
** is separate from the database file.  The pager also implements file
** locking to prevent two processes from writing the same database
** file simultaneously, or one process from reading the database while
** another is writing.
**
** @(#) $Id: pager.c,v 1.625 2009/07/25 17:39:14 danielk1977 Exp $
*/
#ifndef SQLITE_OMIT_DISKIO
#include "sqliteInt.h"

/*
** Macros for troubleshooting.  Normally turned off
*/

    /* Decode the page just read from disk */
    CODEC1(pPager, pData, pPg->pgno, 3, rc=SQLITE_NOMEM);
    sqlite3PcacheRelease(pPg);
  }
  return rc;
}

#if !defined(NDEBUG) || defined(SQLITE_COVERAGE_TEST)
/*
** This routine looks ahead into the main journal file and determines
** whether or not the next record (the record that begins at file
** offset pPager->journalOff) is a well-formed page record consisting
** of a valid page number, pPage->pageSize bytes of content, followed
** by a valid checksum.
**
** The pager never needs to know this in order to do its job.   This
** routine is only used from within assert() and testcase() macros.
*/
static int pagerNextJournalPageIsValid(Pager *pPager){
  Pgno pgno;           /* The page number of the page */
  u32 cksum;           /* The page checksum */
  int rc;              /* Return code from read operations */
  sqlite3_file *fd;    /* The file descriptor from which we are reading */
  u8 *aData;           /* Content of the page */

  /* Read the page number header */
  fd = pPager->jfd;
  rc = read32bits(fd, pPager->journalOff, &pgno);
  if( rc!=SQLITE_OK ){ return 0; }                                  /*NO_TEST*/
  if( pgno==0 || pgno==PAGER_MJ_PGNO(pPager) ){ return 0; }         /*NO_TEST*/
  if( pgno>(Pgno)pPager->dbSize ){ return 0; }                      /*NO_TEST*/

  /* Read the checksum */
  rc = read32bits(fd, pPager->journalOff+pPager->pageSize+4, &cksum);
  if( rc!=SQLITE_OK ){ return 0; }                                  /*NO_TEST*/

  /* Read the data and verify the checksum */
  aData = (u8*)pPager->pTmpSpace;
  rc = sqlite3OsRead(fd, aData, pPager->pageSize, pPager->journalOff+4);
  if( rc!=SQLITE_OK ){ return 0; }                                  /*NO_TEST*/
  if( pager_cksum(pPager, aData)!=cksum ){ return 0; }              /*NO_TEST*/

  /* Reach this point only if the page is valid */
  return 1;
}
#endif /* !defined(NDEBUG) || defined(SQLITE_COVERAGE_TEST) */

/*
** Parameter zMaster is the name of a master journal file. A single journal
** 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.
**
** Argument zMaster may point to Pager.pTmpSpace. So that buffer is not 

    ** When rolling back a hot journal, nRec==0 always means that the next
    ** chunk of the journal contains zero pages to be rolled back.  But
    ** when doing a ROLLBACK and the nRec==0 chunk is the last chunk in
    ** the journal, it means that the journal might contain additional
    ** pages that need to be rolled back and that the number of pages 
    ** should be computed based on the journal file size.
    */
    testcase( nRec==0 && !isHot
         && pPager->journalHdr+JOURNAL_HDR_SZ(pPager)!=pPager->journalOff
         && ((szJ - pPager->journalOff) / JOURNAL_PG_SZ(pPager))>0
         && pagerNextJournalPageIsValid(pPager)
    );
    if( nRec==0 && !isHot &&
        pPager->journalHdr+JOURNAL_HDR_SZ(pPager)==pPager->journalOff ){
      nRec = (int)((szJ - pPager->journalOff) / JOURNAL_PG_SZ(pPager));
      isUnsync = 1;
    }

    /* If this is the first header read from the journal, truncate the

    assert( rc!=SQLITE_DONE );

    /*
    ** The "pPager->journalHdr+JOURNAL_HDR_SZ(pPager)==pPager->journalOff"
    ** test is related to ticket #2565.  See the discussion in the
    ** pager_playback() function for additional information.
    */
    assert( !(nJRec==0
         && pPager->journalHdr+JOURNAL_HDR_SZ(pPager)!=pPager->journalOff
         && ((szJ - pPager->journalOff) / JOURNAL_PG_SZ(pPager))>0
         && pagerNextJournalPageIsValid(pPager))
    );
    if( nJRec==0 
     && pPager->journalHdr+JOURNAL_HDR_SZ(pPager)==pPager->journalOff
    ){
      nJRec = (u32)((szJ - pPager->journalOff)/JOURNAL_PG_SZ(pPager));
    }
    for(ii=0; rc==SQLITE_OK && ii<nJRec && pPager->journalOff<szJ; ii++){
      rc = pager_playback_one_page(pPager, 1, 0, &pPager->journalOff, 1, pDone);