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 1.75 2003/02/12 14:09:44 drh Exp $
*/
#include "os.h"         /* Must be first to enable large file support */
#include "sqliteInt.h"
#include "pager.h"
#include <assert.h>
#include <string.h>

  char *zJournal;             /* Name of the journal file */
  OsFile fd, jfd;             /* File descriptors for database and journal */
  OsFile cpfd;                /* File descriptor for the checkpoint journal */
  int dbSize;                 /* Number of pages in the file */
  int origDbSize;             /* dbSize before the current change */
  int ckptSize;               /* Size of database (in pages) at ckpt_begin() */
  off_t ckptJSize;            /* Size of journal at ckpt_begin() */
#ifndef NDEBUG
  off_t syncJSize;            /* Size of journal at last fsync() call */
#endif
  int nRec;                   /* Number of pages written to the journal */
  u32 cksumInit;              /* Quasi-random value added to every checksum */
  int ckptNRec;               /* Number of records in the checkpoint journal */
  int nExtra;                 /* Add this many bytes to each in-memory page */
  void (*xDestructor)(void*); /* Call this routine when freeing pages */
  int nPage;                  /* Total number of in-memory pages */
  int nRef;                   /* Number of in-memory pages with PgHdr.nRef>0 */

      if( rc==SQLITE_DONE ){
        rc = SQLITE_OK;
      }
      break;
    }
  }


end_playback:
#if !defined(NDEBUG) && defined(SQLITE_TEST)
  /* For pages that were never written into the journal, restore the
  ** memory copy from the original database file.
  **
  ** This is code is used during testing only.  It is necessary to
  ** compensate for the sqliteOsTruncate() call inside 
  ** sqlitepager_rollback().
  */
  if( rc==SQLITE_OK ){
    PgHdr *pPg;
    for(pPg=pPager->pAll; pPg; pPg=pPg->pNextAll){
      char zBuf[SQLITE_PAGE_SIZE];

      if( (int)pPg->pgno <= pPager->origDbSize ){
        sqliteOsSeek(&pPager->fd, SQLITE_PAGE_SIZE*(off_t)(pPg->pgno-1));
        rc = sqliteOsRead(&pPager->fd, zBuf, SQLITE_PAGE_SIZE);
        if( rc ) break;
      }else{
        memset(zBuf, 0, SQLITE_PAGE_SIZE);
      }
      if( pPg->nRef==0 || memcmp(zBuf, PGHDR_TO_DATA(pPg), SQLITE_PAGE_SIZE) ){
        memcpy(PGHDR_TO_DATA(pPg), zBuf, SQLITE_PAGE_SIZE);
        memset(PGHDR_TO_EXTRA(pPg), 0, pPager->nExtra);
      }
      pPg->needSync = 0;
      pPg->dirty = 0;
    }
  }
#endif

  if( rc!=SQLITE_OK ){
    pager_unwritelock(pPager);
    pPager->errMask |= PAGER_ERR_CORRUPT;
    rc = SQLITE_CORRUPT;
  }else{
    rc = pager_unwritelock(pPager);
  }

  */
  if( pPager->needSync ){
    if( !pPager->tempFile ){
      assert( pPager->journalOpen );
      assert( !pPager->noSync );
#ifndef NDEBUG
      {
        off_t hdrSz, pgSz;
        hdrSz = JOURNAL_HDR_SZ(journal_format);
        pgSz = JOURNAL_PG_SZ(journal_format);
        rc = sqliteOsFileSize(&pPager->jfd, &pPager->syncJSize);
        if( rc!=0 ) return rc;
        assert( pPager->nRec*pgSz+hdrSz==pPager->syncJSize );
      }
#endif
      if( journal_format>=3 ){
        off_t szJ;
        if( pPager->fullSync ){
          TRACE1("SYNC\n");
          rc = sqliteOsSync(&pPager->jfd);

  }
  if( rc!=SQLITE_OK ){
    rc = pager_unwritelock(pPager);
    if( rc==SQLITE_OK ){
      rc = SQLITE_FULL;
    }
  }
#ifndef NDEBUG
  pPager->syncJSize = 0;
#endif
  return rc;  
}

/*
** Acquire a write-lock on the database.  The lock is removed when
** the any of the following happen:
**

  TRACE1("ROLLBACK\n");
  if( !pPager->dirtyFile || !pPager->journalOpen ){
    rc = pager_unwritelock(pPager);
    pPager->dbSize = -1;
    return rc;
  }

#if defined(SQLITE_TEST) && !defined(NDEBUG)
  /* Truncate the journal to the size it was at the conclusion of the
  ** last sqliteOsSync() call.  This is really an error check.  If the
  ** rollback still works, it means that the rollback would have also
  ** worked if it had occurred after an OS crash or unexpected power
  ** loss.
  */
  if( !pPager->noSync ){
    int m = JOURNAL_HDR_SZ(journal_format);
    assert( !pPager->tempFile );
    if( pPager->syncJSize<m ){
      pPager->syncJSize = m;
    }
    TRACE2("TRUNCATE JOURNAL %lld\n", pPager->syncJSize);
    rc =  sqliteOsTruncate(&pPager->jfd, pPager->syncJSize);
    if( rc ) return rc;
    pPager->nRec = 0;
  }
#endif

  if( pPager->errMask!=0 && pPager->errMask!=PAGER_ERR_FULL ){
    if( pPager->state>=SQLITE_WRITELOCK ){
      pager_playback(pPager);
    }
    return pager_errcode(pPager);
  }
  if( pPager->state!=SQLITE_WRITELOCK ){