SQLite

  assert( DbMaskTest(p->btreeMask, pOp->p1) );
  assert( p->readOnly==0 );
  pDb = &db->aDb[pOp->p1];
  assert( pDb->pBt!=0 );
  assert( sqlite3SchemaMutexHeld(db, pOp->p1, 0) );
  pIn3 = &aMem[pOp->p3];
  sqlite3VdbeMemIntegerify(pIn3);
#ifdef SQLITE_ENABLE_UNLOCKED
  if( db->bUnlocked 
   && (pOp->p2==BTREE_USER_VERSION || pOp->p2==BTREE_APPLICATION_ID)
  ){
    rc = SQLITE_ERROR;
    sqlite3VdbeError(p, "cannot modify %s within UNLOCKED transaction",
        pOp->p2==BTREE_USER_VERSION ? "user_version" : "application_id"
    );
    break; 
  }
#endif
  /* See note about index shifting on OP_ReadCookie */
  rc = sqlite3BtreeUpdateMeta(pDb->pBt, pOp->p2, (int)pIn3->u.i);
  if( pOp->p2==BTREE_SCHEMA_VERSION ){






      /* When the schema cookie changes, record the new cookie internally */
    assert( db->bUnlocked==0 );
      pDb->pSchema->schema_cookie = (int)pIn3->u.i;
      db->flags |= SQLITE_InternChanges;

  }else if( pOp->p2==BTREE_FILE_FORMAT ){
    /* Record changes in the file format */
    pDb->pSchema->file_format = (u8)pIn3->u.i;
  }
  if( pOp->p1==1 ){
    /* Invalidate all prepared statements whenever the TEMP database
    ** schema is changed.  Ticket #1644 */

** P2 contains the root-page of the table to lock.
**
** P4 contains a pointer to the name of the table being locked. This is only
** used to generate an error message if the lock cannot be obtained.
*/
case OP_TableLock: {
  u8 isWriteLock = (u8)pOp->p3;
#ifdef SQLITE_ENABLE_UNLOCKED
  if( isWriteLock && db->bUnlocked && pOp->p2==1 ){
    rc = SQLITE_ERROR;
    sqlite3VdbeError(p, 
        "cannot modify database schema within UNLOCKED transaction");
    rc = SQLITE_ERROR;
    break;
  }
#endif
  if( isWriteLock || 0==(db->flags&SQLITE_ReadUncommitted) ){
    int p1 = pOp->p1; 
    assert( p1>=0 && p1<db->nDb );
    assert( DbMaskTest(p->btreeMask, p1) );
    assert( isWriteLock==0 || isWriteLock==1 );
    rc = sqlite3BtreeLockTable(db->aDb[p1].pBt, pOp->p2, isWriteLock);
    if( (rc&0xFF)==SQLITE_LOCKED ){

    }
    WALTRACE(("WAL%p: closed\n", pWal));
    sqlite3_free((void *)pWal->apWiData);
    sqlite3_free(pWal);
  }
  return rc;
}

/*
** Try to copy the wal-index header from shared-memory into (*pHdr). Return
** zero if successful or non-zero otherwise. If the header is corrupted
** (either because the two copies are inconsistent or because the checksum 
** values are incorrect), the read fails and non-zero is returned.
*/
static int walIndexLoadHdr(Wal *pWal, WalIndexHdr *pHdr){
  u32 aCksum[2];                  /* Checksum on the header content */
  WalIndexHdr h2;                 /* Second copy of the header content */
  WalIndexHdr volatile *aHdr;     /* Header in shared memory */

  /* The first page of the wal-index must be mapped at this point. */
  assert( pWal->nWiData>0 && pWal->apWiData[0] );

  /* Read the header. This might happen concurrently with a write to the
  ** same area of shared memory on a different CPU in a SMP,
  ** meaning it is possible that an inconsistent snapshot is read
  ** from the file. If this happens, return non-zero.
  **
  ** There are two copies of the header at the beginning of the wal-index.
  ** When reading, read [0] first then [1].  Writes are in the reverse order.
  ** Memory barriers are used to prevent the compiler or the hardware from
  ** reordering the reads and writes.
  */
  aHdr = walIndexHdr(pWal);
  memcpy(pHdr, (void *)&aHdr[0], sizeof(h2));
  walShmBarrier(pWal);
  memcpy(&h2, (void *)&aHdr[1], sizeof(h2));

  if( memcmp(&h2, pHdr, sizeof(h2))!=0 ){
    return 1;   /* Dirty read */
  }  
  if( h2.isInit==0 ){
    return 1;   /* Malformed header - probably all zeros */
  }
  walChecksumBytes(1, (u8*)&h2, sizeof(h2)-sizeof(h2.aCksum), 0, aCksum);
  if( aCksum[0]!=h2.aCksum[0] || aCksum[1]!=h2.aCksum[1] ){
    return 1;   /* Checksum does not match */
  }

  return 0;
}

/*
** Try to read the wal-index header.  Return 0 on success and 1 if
** there is a problem.
**
** The wal-index is in shared memory.  Another thread or process might
** be writing the header at the same time this procedure is trying to
** read it, which might result in inconsistency.  A dirty read is detected
** by verifying that both copies of the header are the same and also by
** a checksum on the header.
**
** If and only if the read is consistent and the header is different from
** pWal->hdr, then pWal->hdr is updated to the content of the new header
** and *pChanged is set to 1.
**
** If the checksum cannot be verified return non-zero. If the header
** is read successfully and the checksum verified, return zero.
*/
static int walIndexTryHdr(Wal *pWal, int *pChanged){

  WalIndexHdr h1;                 /* Copy of the header content */




  if( walIndexLoadHdr(pWal, &h1) ){
















    return 1;







  }

  if( memcmp(&pWal->hdr, &h1, sizeof(WalIndexHdr)) ){
    *pChanged = 1;
    memcpy(&pWal->hdr, &h1, sizeof(WalIndexHdr));
    pWal->szPage = (pWal->hdr.szPage&0xfe00) + ((pWal->hdr.szPage&0x0001)<<16);
    testcase( pWal->szPage<=32768 );

  **   a) None of the pages in pList have been modified since the 
  **      transaction opened, and
  **
  **   b) The database schema cookie has not been modified since the
  **      transaction was started.
  */
  if( rc==SQLITE_OK ){

    WalIndexHdr head;

    if( walIndexLoadHdr(pWal, &head) ){
      /* This branch is taken if the wal-index header is corrupted. This 
      ** occurs if some other writer has crashed while committing a 
      ** transaction to this database since the current unlocked transaction
      ** was opened.  */
      rc = SQLITE_BUSY_SNAPSHOT;
    }else if( memcmp(&pWal->hdr, (void*)&head, sizeof(WalIndexHdr))!=0 ){
      int iHash;
      int iLastHash = walFramePage(head.mxFrame);
      u32 iFirst = pWal->hdr.mxFrame+1;     /* First wal frame to check */
      if( memcmp(pWal->hdr.aSalt, (u32*)head.aSalt, sizeof(u32)*2) ){
        assert( pWal->readLock==0 );
        iFirst = 1;
      }
      for(iHash=walFramePage(iFirst); iHash<=iLastHash; iHash++){
        volatile ht_slot *aHash;
        volatile u32 *aPgno;
        u32 iZero;

        rc = walHashGet(pWal, iHash, &aHash, &aPgno, &iZero);
        if( rc==SQLITE_OK ){
          int i;
          int iMin = (iFirst - iZero);
          int iMax = (iHash==0) ? HASHTABLE_NPAGE_ONE : HASHTABLE_NPAGE;
          if( iMin<1 ) iMin = 1;
          if( iMax>head.mxFrame ) iMax = head.mxFrame;
          for(i=iMin; i<=iMax; i++){
            PgHdr *pPg;
            if( aPgno[i]==1 ){
              /* Check that the schema cookie has not been modified. If
              ** it has not, the commit can proceed. */
              u8 aNew[4];
              u8 *aOld = &((u8*)pPage1->pData)[40];

#-------------------------------------------------------------------------
# UNLOCKED transactions may not modify the db schema.
#
foreach {tn sql} {
  1 { CREATE TABLE xx(a, b) }
  2 { DROP TABLE t1 }
  3 { CREATE INDEX i1 ON t1(a) }
  4 { CREATE VIEW v1 AS SELECT * FROM t1 }
  5 { CREATE TEMP TABLE xx(a, b) }
} {
  do_catchsql_test 1.7.$tn.1 "
    BEGIN UNLOCKED;
    $sql
  " {1 {cannot modify database schema within UNLOCKED transaction}}

  do_execsql_test 1.7.$tn.2 {
    SELECT sql FROM sqlite_master;
    SELECT sql FROM sqlite_temp_master;
  } {{CREATE TABLE t1(a, b)}}

  do_execsql_test 1.7.$tn.3 COMMIT
}

#-------------------------------------------------------------------------
# If an auto-vacuum database is written within an UNLOCKED transaction, it
# is handled in the same way as for a non-UNLOCKED transaction.
#
reset_db

  do_test 2.$tn.7.3 {
    list [catch { sql1 { COMMIT } } msg] $msg [sqlite3_errcode db]
  } {0 {} SQLITE_OK}

  do_test 2.$tn.7.4 { sql3 { PRAGMA integrity_check } } ok
}

#-------------------------------------------------------------------------
# Unlocked transactions may not modify the user_version or application_id.
#
reset_db
do_execsql_test 3.0 {
  PRAGMA journal_mode = wal;
  CREATE TABLE t1(x, y);
  INSERT INTO t1 VALUES('a', 'b');
  PRAGMA user_version = 10;
} {wal}
do_execsql_test 3.1 {
  BEGIN UNLOCKED;
    INSERT INTO t1 VALUES('c', 'd');
    SELECT * FROM t1;
} {a b c d}
do_catchsql_test 3.2 {
  PRAGMA user_version = 11;
} {1 {cannot modify user_version within UNLOCKED transaction}}
do_execsql_test 3.3 {
  PRAGMA user_version;
  SELECT * FROM t1;
} {10 a b c d}
do_catchsql_test 3.4 {
  PRAGMA application_id = 11;
} {1 {cannot modify application_id within UNLOCKED transaction}}
do_execsql_test 3.5 {
  COMMIT;
  PRAGMA user_version;
  PRAGMA application_id;
  SELECT * FROM t1;
} {10 0 a b c d}

finish_test