}
  pPager->state = PAGER_SHARED;

  return rc;
}

/*

** Check if the *-wal file that corresponds to the database opened by pPager

** exists. Assuming no error occurs, set *pExists to 1 if the file exists,





** or 0 otherwise and return SQLITE_OK. If an IO or OOM error occurs, return
** an SQLite error code.
*/
static int pagerHasWAL(Pager *pPager, int *pExists){
  int rc;                         /* Return code */
  char *zWal;                     /* Name of the WAL file */

  assert( !pPager->tempFile );
  zWal = sqlite3_mprintf("%s-wal", pPager->zFilename);
  if( !zWal ){
    rc = SQLITE_NOMEM;
  }else{

    rc = sqlite3OsAccess(pPager->pVfs, zWal, SQLITE_ACCESS_EXISTS, pExists);



    sqlite3_free(zWal);
  }
  return rc;
}

/*
** Check if the *-wal file that corresponds to the database opened by pPager
** exists. If it does, open the pager in WAL mode. Otherwise, if no error




** occurs, make sure Pager.journalMode is not set to PAGER_JOURNALMODE_WAL.
** If an IO or OOM error occurs, return an SQLite error code.



**
** If the WAL file is opened, also open a snapshot (read transaction).
**
** The caller must hold a SHARED lock on the database file to call this
** function. Because an EXCLUSIVE lock on the db file is required to delete 
** a WAL, this ensures there is no race condition between the xAccess() 
** below and an xDelete() being executed by some other connection.

*/
static int pagerOpenWalIfPresent(Pager *pPager){
  int rc = SQLITE_OK;
  if( !pPager->tempFile ){
    int isWal;                    /* True if WAL file exists */








    rc = pagerHasWAL(pPager, &isWal);

    if( rc==SQLITE_OK ){
      if( isWal ){
        pager_reset(pPager);
        rc = sqlite3PagerOpenWal(pPager, 0);
        if( rc==SQLITE_OK ){
          rc = pagerBeginReadTransaction(pPager);
        }
................................................................................
  ** it may need to be checkpointed before the connection can switch to
  ** rollback mode. Open it now so this can happen.
  */
  if( !pPager->pWal ){
    int logexists = 0;
    rc = sqlite3OsLock(pPager->fd, SQLITE_LOCK_SHARED);
    if( rc==SQLITE_OK ){
      rc = pagerHasWAL(pPager, &logexists);
    }
    if( rc==SQLITE_OK && logexists ){
      rc = sqlite3WalOpen(pPager->pVfs, pPager->fd,
                          pPager->zFilename, &pPager->pWal);
    }
  }
    

  }
  pPager->state = PAGER_SHARED;

  return rc;
}

/*
** Check for the existence of or delete the *-wal file that corresponds to
** the database opened by pPager.
**
** When pExists!=NULL, set *pExists to 1 if the *-wal file exists, or 0
** if the *-wal file does not exist.
**
** When pExists==NULL, delete the *-wal file if it exists, or the do
** nothing if the *-wal file does not exist.
**
** Return SQLITE_OK on success. If on an IO or OOM error occurs, return
** an SQLite error code.
*/
static int pagerCheckForOrDeleteWAL(Pager *pPager, int *pExists){
  int rc;                         /* Return code */
  char *zWal;                     /* Name of the WAL file */

  assert( !pPager->tempFile );
  zWal = sqlite3_mprintf("%s-wal", pPager->zFilename);
  if( !zWal ){
    rc = SQLITE_NOMEM;
  }else{
    if( pExists ){
      rc = sqlite3OsAccess(pPager->pVfs, zWal, SQLITE_ACCESS_EXISTS, pExists);
    }else{
      rc = sqlite3OsDelete(pPager->pVfs, zWal, 0);
    }
    sqlite3_free(zWal);
  }
  return rc;
}

/*
** Check if the *-wal file that corresponds to the database opened by pPager
** exists if the database is not empy, or verify that the *-wal file does
** not exist (by deleting it) if the database file is empty.
**
** If the database is not empty and the *-wal file exists, open the pager
** in WAL mode.  If the database is empty or if no *-wal file exists and
** if no error occurs, make sure Pager.journalMode is not set to

** PAGER_JOURNALMODE_WAL.
**
** Return SQLITE_OK or an error code.
**
** If the WAL file is opened, also open a snapshot (read transaction).
**
** The caller must hold a SHARED lock on the database file to call this
** function. Because an EXCLUSIVE lock on the db file is required to delete 
** a WAL on a none-empty database, this ensures there is no race condition 
** between the xAccess() below and an xDelete() being executed by some 
** other connection.
*/
static int pagerOpenWalIfPresent(Pager *pPager){
  int rc = SQLITE_OK;
  if( !pPager->tempFile ){
    int isWal;                    /* True if WAL file exists */
    int nPage;                    /* Size of the database file */
    assert( pPager->state>=SHARED_LOCK );
    rc = sqlite3PagerPagecount(pPager, &nPage);
    if( rc ) return rc;
    if( nPage==0 ){
      rc = pagerCheckForOrDeleteWAL(pPager, 0);
      isWal = 0;
    }else{
      rc = pagerCheckForOrDeleteWAL(pPager, &isWal);
    }
    if( rc==SQLITE_OK ){
      if( isWal ){
        pager_reset(pPager);
        rc = sqlite3PagerOpenWal(pPager, 0);
        if( rc==SQLITE_OK ){
          rc = pagerBeginReadTransaction(pPager);
        }
................................................................................
  ** it may need to be checkpointed before the connection can switch to
  ** rollback mode. Open it now so this can happen.
  */
  if( !pPager->pWal ){
    int logexists = 0;
    rc = sqlite3OsLock(pPager->fd, SQLITE_LOCK_SHARED);
    if( rc==SQLITE_OK ){
      rc = pagerCheckForOrDeleteWAL(pPager, &logexists);
    }
    if( rc==SQLITE_OK && logexists ){
      rc = sqlite3WalOpen(pPager->pVfs, pPager->fd,
                          pPager->zFilename, &pPager->pWal);
    }
  }
    

# 2010 July 1
#
# The author disclaims copyright to this source code.  In place of
# a legal notice, here is a blessing:
#
#    May you do good and not evil.
#    May you find forgiveness for yourself and forgive others.
#    May you share freely, never taking more than you give.
#
#***********************************************************************
# Verify that an empty database and a non-empty WAL file do not
# result in database corruption
#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
source $testdir/malloc_common.tcl
ifcapable !wal {finish_test ; return }

do_test wal4-1.1 {
  db close
  file delete -force wal4.db wal4.db-journal wal4.db-wal wal4.db-shm
  file delete -force test.db test.db-journal test.db-wal test.db-shm
  sqlite3 db2 wal4.db
  db2 eval {
    PRAGMA journal_mode=WAL;
    CREATE TABLE t1(x);
    INSERT INTO t1 VALUES(1);
    INSERT INTO t1 VALUES(2);
    SELECT x FROM t1 ORDER BY x;
  }
} {wal 1 2}
do_test wal4-1.2 {
  file delete -force test.db test.db-journal test.db-wal
  file copy wal4.db-wal test.db-wal
  sqlite3 db test.db
  catchsql {
    SELECT * FROM t1;
  }
} {1 {no such table: t1}}

do_malloc_test wal4-2.0 -tclprep {
  db close
  file delete -force test.db test.db-journal test.db-wal
  file copy wal4.db-wal test.db-wal
  sqlite3 db test.db
} -sqlbody {
  SELECT name FROM sqlite_master
}  


finish_test