SQLite

** not actually change anything.  The pager uses this to see if it
** should acquire the database exclusive lock prior to invoking
** the op==1 case.
*/
int sqlite3WalExclusiveMode(Wal *pWal, int op){
  int rc;
  assert( pWal->writeLock==0 );

  /* pWal->readLock is usually set, but might be -1 if there was a 
  ** prior error while attempting to acquire are read-lock. This cannot 
  ** happen if the connection is actually in exclusive mode (as no xShmLock
  ** locks are taken in this case). Nor should the pager attempt to
  ** upgrade to exclusive-mode following such an error.
  */
  assert( pWal->readLock>=0 || pWal->lockError );
  assert( pWal->readLock>=0 || (op<=0 && pWal->exclusiveMode==0) );

  if( op==0 ){
    if( pWal->exclusiveMode ){
      pWal->exclusiveMode = 0;

      if( walLockShared(pWal, WAL_READ_LOCK(pWal->readLock))!=SQLITE_OK ){

        pWal->exclusiveMode = 1;
      }
      rc = pWal->exclusiveMode==0;
    }else{
      /* Already in locking_mode=NORMAL */
      rc = 0;
    }

#
#   wal2-6.3.*: Changing back to rollback mode from WAL mode after setting 
#               locking_mode=exclusive.
#
#   wal2-6.4.*: Check that xShmLock calls are omitted in exclusive locking
#               mode.
#
#   wal2-6.5.*: 
#
#   wal2-6.6.*: Check that if the xShmLock() to reaquire a WAL read-lock when
#               exiting exclusive mode fails (i.e. SQLITE_IOERR), then the
#               connection silently remains in exclusive mode.
#
do_test wal2-6.1.1 {
  file delete -force test.db test.db-wal test.db-journal
  sqlite3 db test.db
  execsql {
    Pragma Journal_Mode = Wal;
    Pragma Locking_Mode = Exclusive;
  }

    SELECT * FROM t2;
  }
} {normal exclusive I II III IV}
do_test wal2-6.5.3 {
  execsql { PRAGMA wal_checkpoint }
} {}
db close

proc lock_control {method filename handle spec} {
  foreach {start n op type} $spec break
  if {$op == "lock"} { return SQLITE_IOERR }
  return SQLITE_OK
}
do_test wal2-6.6.1 {
  testvfs T
  T script lock_control
  T filter {}
  sqlite3 db test.db -vfs T
  execsql { PRAGMA locking_mode = exclusive }
  execsql { INSERT INTO t2 VALUES('V', 'VI') }
} {}
do_test wal2-6.6.2 {
  execsql { PRAGMA locking_mode = normal }
  T filter xShmLock
  execsql { INSERT INTO t2 VALUES('VII', 'VIII') }
} {}
do_test wal2-6.6.3 {
  # At this point the connection should still be in exclusive-mode, even
  # though it tried to exit exclusive-mode when committing the INSERT
  # statement above. To exit exclusive mode, SQLite has to take a read-lock 
  # on the WAL file using xShmLock(). Since that call failed, it remains
  # in exclusive mode.
  #
  sqlite3 db2 test.db -vfs T
  catchsql { SELECT * FROM t2 } db2
} {1 {database is locked}}
do_test wal2-6.6.2 {
  db2 close
  T filter {}
  execsql { INSERT INTO t2 VALUES('IX', 'X') }
} {}
do_test wal2-6.6.3 {
  # This time, we have successfully exited exclusive mode. So the second
  # connection can read the database.
  sqlite3 db2 test.db -vfs T
  catchsql { SELECT * FROM t2 } db2
} {0 {I II III IV V VI VII VIII IX X}}

db close
db2 close
T delete

#-------------------------------------------------------------------------
# Test a theory about the checksum algorithm. Theory was false and this
# test did not provoke a bug.
#
file delete -force test.db test.db-wal test.db-journal
do_test wal2-7.1.1 {