SQLite

        int exFlag = (p->db->bUnlocked && !ISAUTOVACUUM) ? -1 : (wrflag>1);
        int bSubjInMem = sqlite3TempInMemory(p->db);
        assert( p->db->bUnlocked==0 || wrflag==1 );
        rc = sqlite3PagerBegin(pBt->pPager, exFlag, bSubjInMem);
        if( rc==SQLITE_OK ){
          rc = newDatabase(pBt);
        }
#ifdef SQLITE_ENABLE_UNLOCKED
        if( rc==SQLITE_OK && sqlite3PagerIsUnlocked(pBt->pPager) ){
          rc = btreePtrmapAllocate(pBt);
        }
#endif
      }
    }
  
    if( rc!=SQLITE_OK ){
      unlockBtreeIfUnused(pBt);
    }
  }while( (rc&0xFF)==SQLITE_BUSY && pBt->inTransaction==TRANS_NONE &&

}

/*
** Free the Pager.pInJournal and Pager.pAllRead bitvec objects.
*/
static void pagerFreeBitvecs(Pager *pPager){
  sqlite3BitvecDestroy(pPager->pInJournal);
  sqlite3BitvecDestroy(pPager->pAllRead);
  pPager->pInJournal = 0;
#ifdef SQLITE_ENABLE_UNLOCKED
  sqlite3BitvecDestroy(pPager->pAllRead);
  pPager->pAllRead = 0;
#endif
}

/*
** This function is a no-op if the pager is in exclusive mode and not
** in the ERROR state. Otherwise, it switches the pager to PAGER_OPEN
** state.
**

/*
** This function is called to rollback a transaction on a WAL database.
*/
static int pagerRollbackWal(Pager *pPager){
  int rc;                         /* Return Code */
  PgHdr *pList;                   /* List of dirty pages to revert */
  int bPage1 = 0;                 /* True if page 1 has been undone */

  /* For all pages in the cache that are currently dirty or have already
  ** been written (but not committed) to the log file, do one of the 
  ** following:
  **
  **   + Discard the cached page (if refcount==0), or
  **   + Reload page content from the database (if refcount>0).
  */
  pPager->dbSize = pPager->dbOrigSize;
  rc = sqlite3WalUndo(pPager->pWal, pagerUndoCallback, (void *)pPager);
  pList = sqlite3PcacheDirtyList(pPager->pPCache);

  /* If this is an UNLOCKED transaction, then page 1 must be reread from 
  ** the db file, even if it is not dirty. This is because the b-tree layer 
  ** may have already zeroed the nFree and iTrunk header fields.  */
#ifdef SQLITE_ENABLE_UNLOCKED
  if( rc==SQLITE_OK && (pList==0 || pList->pgno!=1) && pPager->pAllRead ){
    rc = pagerUndoCallback((void*)pPager, 1);
  }
#endif

  while( pList && rc==SQLITE_OK ){
    PgHdr *pNext = pList->pDirty;
    if( pList->pgno==1 ) bPage1 = 1;
    rc = pagerUndoCallback((void *)pPager, pList->pgno);
    pList = pNext;
  }

  /* If this is an UNLOCKED transaction, then page 1 must be reread from the
  ** db file, even if it is not dirty. This is because the b-tree layer may
  ** have already zeroed the nFree and iTrunk header fields.  */
  if( rc==SQLITE_OK && bPage1==0 && pPager->pAllRead ){
    rc = pagerUndoCallback((void*)pPager, 1);
  }

  return rc;
}

/*
** This function is a wrapper around sqlite3WalFrames(). As well as logging
** the contents of the list of pages headed by pList (connected by pDirty),
** this function notifies any active backup processes that the pages have

  if( isCommit ){
    /* If a WAL transaction is being committed, there is no point in writing
    ** any pages with page numbers greater than nTruncate into the WAL file.
    ** They will never be read by any client. So remove them from the pDirty
    ** list here. */
    PgHdr **ppNext = &pList;
    nList = 0;

    for(p=pList; (*ppNext = p)!=0; p=p->pDirty){
      if( p->pgno<=nTruncate ){
        ppNext = &p->pDirty;
        nList++;
      }
    }
    assert( pList );

    return SQLITE_OK;
  }

  pPg->pDirty = 0;
  if( pagerUseWal(pPager) ){
    /* If the transaction is a "BEGIN UNLOCKED" transaction, the page 
    ** cannot be flushed to disk. Return early in this case. */
#ifdef SQLITE_ENABLE_UNLOCKED
    if( pPager->pAllRead ) return SQLITE_OK;
#endif

    /* Write a single frame for this page to the log. */
    rc = subjournalPageIfRequired(pPg); 
    if( rc==SQLITE_OK ){
      rc = pagerWalFrames(pPager, pPg, 0, 0);
    }
  }else{

    return SQLITE_CORRUPT_BKPT;
  }
  pPager->hasBeenUsed = 1;

  /* If this is an UNLOCKED transaction and the page being read was
  ** present in the database file when the transaction was opened,
  ** mark it as read in the pAllRead vector.  */
#ifdef SQLITE_ENABLE_UNLOCKED
  if( pPager->pAllRead && pgno<=pPager->dbOrigSize ){
    rc = sqlite3BitvecSet(pPager->pAllRead, pgno);
    if( rc!=SQLITE_OK ) goto pager_acquire_err;
  }
#endif

  /* If the pager is in the error state, return an error immediately. 
  ** Otherwise, request the page from the PCache layer. */
  if( pPager->errCode!=SQLITE_OK ){
    rc = pPager->errCode;
  }else{
    if( bMmapOk && pagerUseWal(pPager) ){

  if( pPager->errCode ) return pPager->errCode;
  assert( pPager->eState>=PAGER_READER && pPager->eState<PAGER_ERROR );
  pPager->subjInMemory = (u8)subjInMemory;

  if( ALWAYS(pPager->eState==PAGER_READER) ){
    assert( pPager->pInJournal==0 );
#ifdef SQLITE_ENABLE_UNLOCKED
    assert( pPager->pAllRead==0 );
#endif

    if( pagerUseWal(pPager) ){
      /* If the pager is configured to use locking_mode=exclusive, and an
      ** exclusive lock on the database is not already held, obtain it now.
      */
      if( pPager->exclusiveMode && sqlite3WalExclusiveMode(pPager->pWal, -1) ){
        rc = pagerLockDb(pPager, EXCLUSIVE_LOCK);
        if( rc!=SQLITE_OK ){
          return rc;
        }
        sqlite3WalExclusiveMode(pPager->pWal, 1);
      }

      /* Grab the write lock on the log file. If successful, upgrade to
      ** PAGER_RESERVED state. Otherwise, return an error code to the caller.
      ** The busy-handler is not invoked if another connection already
      ** holds the write-lock. If possible, the upper layer will call it.
      */
#ifdef SQLITE_ENABLE_UNLOCKED
      if( exFlag>=0 ){
      if( exFlag<0 ){
        rc = sqlite3WalBeginWriteTransaction(pPager->pWal);
      }else{
        pPager->pAllRead = sqlite3BitvecCreate(pPager->dbSize);
        if( pPager->pAllRead==0 ){
          rc = SQLITE_NOMEM;
        }
      }else
#endif
      {
        rc = sqlite3WalBeginWriteTransaction(pPager->pWal);
      }
    }else{
      /* Obtain a RESERVED lock on the database file. If the exFlag parameter
      ** is true, then immediately upgrade this to an EXCLUSIVE lock. The
      ** busy-handler callback can be used when upgrading to the EXCLUSIVE
      ** lock, but not when obtaining the RESERVED lock.
      */

}

/*
** Return TRUE if the page given in the argument was previously passed
** to sqlite3PagerWrite().  In other words, return TRUE if it is ok
** to change the content of the page.
*/
#if defined(SQLITE_ENABLE_UNLOCKED) || !defined(NDEBUG)
int sqlite3PagerIswriteable(DbPage *pPg){
  return pPg->flags & PGHDR_WRITEABLE;
}
#endif

/*
** A call to this routine tells the pager that it is not necessary to
** write the information on page pPg back to the disk, even though
** that page might be marked as dirty.  This happens, for example, when
** the page has been added as a leaf of the freelist and so its
** content no longer matters.

    assert( !MEMDB );
    rc = sqlite3OsSync(pPager->fd, pPager->syncFlags);
  }
  return rc;
}

/*
** This function may only be called while a write-transaction is active in
** rollback. If the connection is in WAL mode, this call is a no-op. 
** Otherwise, if the connection does not already have an EXCLUSIVE lock on 
** the database file, an attempt is made to obtain one.
** This function is called to ensure that all locks required to commit the
** current write-transaction to the database file are held. If the db is
** in rollback mode, this means the EXCLUSIVE lock on the database file.
**
** Or, if this is a non-UNLOCKED transaction on a wal-mode database, this
** function is a no-op.
**
** If this is an UNLOCKED transaction on a wal-mode database, this function
** attempts to obtain the WRITER lock on the wal file and also checks to
** see that the transaction can be safely committed (does not commit with 
** any other transaction committed since it was opened).
**
** If the EXCLUSIVE lock is already held or the attempt to obtain it is
** successful, or the connection is in WAL mode, SQLITE_OK is returned.
** Otherwise, either SQLITE_BUSY or an SQLITE_IOERR_XXX error code is 
** returned.
** If the required locks are already held or successfully obtained and
** the transaction can be committed, SQLITE_OK is returned. If a required lock
** cannot be obtained, SQLITE_BUSY is returned. Or, if the current transaction
** is UNLOCKED and cannot be committed due to a conflict, SQLITE_BUSY_SNAPSHOT
** is returned. Otherwise, if some other error occurs (IO error, OOM etc.),
** and SQLite error code is returned.
*/
int sqlite3PagerExclusiveLock(Pager *pPager, PgHdr *pPage1){
  int rc = SQLITE_OK;
  assert( pPager->eState==PAGER_WRITER_CACHEMOD 
       || pPager->eState==PAGER_WRITER_DBMOD 
       || pPager->eState==PAGER_WRITER_LOCKED 
  );
  assert( assert_pager_state(pPager) );
  if( 0==pagerUseWal(pPager) ){
    rc = pager_wait_on_lock(pPager, EXCLUSIVE_LOCK);
  }
#ifdef SQLITE_ENABLE_UNLOCKED
  }else{
  else{
    if( pPager->pAllRead ){
      /* This is an UNLOCKED transaction. Attempt to lock the wal database
      ** here. If SQLITE_BUSY (but not SQLITE_BUSY_SNAPSHOT) is returned,
      ** invoke the busy-handler and try again for as long as it returns
      ** non-zero.  */
      do {
        /* rc = sqlite3WalBeginWriteTransaction(pWal); */
        rc = sqlite3WalLockForCommit(pPager->pWal, pPage1, pPager->pAllRead);
      }while( rc==SQLITE_BUSY 
           && pPager->xBusyHandler(pPager->pBusyHandlerArg) 
      );
    }
  }
#endif
  return rc;
}

#ifdef SQLITE_ENABLE_UNLOCKED
/*
** This function is called as part of committing an UNLOCKED transaction.
** At this point the wal WRITER lock is held, and all pages in the cache 
** except for page 1 are compatible with the snapshot at the head of the
** wal file. 
**
** This function updates the in-memory data structures and reloads the
** contents of page 1 so that the client is operating on the snapshot 
** at the head of the wal file.
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
*/
int sqlite3PagerUpgradeSnapshot(Pager *pPager, DbPage *pPage1){
  int rc;
  u32 iFrame = 0;

  assert( pPager->pWal && pPager->pAllRead );
  rc = sqlite3WalUpgradeSnapshot(pPager->pWal);
  if( rc==SQLITE_OK ){
    rc = sqlite3WalFindFrame(pPager->pWal, 1, &iFrame);
  }
  if( rc==SQLITE_OK ){
    rc = readDbPage(pPage1, iFrame);
  }

  return rc;
}

/*
** Set the in-memory cache of the database file size to nSz pages.
*/
void sqlite3PagerSetDbsize(Pager *pPager, Pgno nFinal){
  pPager->dbSize = nFinal;
void sqlite3PagerSetDbsize(Pager *pPager, Pgno nSz){
  pPager->dbSize = nSz;
}

/*
** Return true if this pager is currently within an UNLOCKED transaction.
*/
int sqlite3PagerIsUnlocked(Pager *pPager){
  return pPager->pAllRead!=0;
}

/*
** If this is a WAL mode connection and the WRITER lock is currently held,
** relinquish it.
*/
void sqlite3PagerDropExclusiveLock(Pager *pPager){
  if( pagerUseWal(pPager) ){
    sqlite3WalEndWriteTransaction(pPager->pWal);
  }
}

#endif   /* ifdef SQLITE_ENABLE_UNLOCKED */
/*
** Return true if this pager is currently within an UNLOCKED transaction.
*/
int sqlite3PagerIsUnlocked(Pager *pPager){
  return pPager->pAllRead!=0;
}


/*
** Sync the database file for the pager pPager. zMaster points to the name
** of a master journal file that should be written into the individual
** journal file. zMaster may be NULL, which is interpreted as no master
** journal (a single database transaction).

#endif

#ifdef SQLITE_OMIT_TRUNCATE_OPTIMIZATION
  Tcl_SetVar2(interp, "sqlite_options", "truncate_opt", "0", TCL_GLOBAL_ONLY);
#else
  Tcl_SetVar2(interp, "sqlite_options", "truncate_opt", "1", TCL_GLOBAL_ONLY);
#endif

#ifdef SQLITE_ENABLE_UNLOCKED
  Tcl_SetVar2(interp, "sqlite_options", "unlocked", "1", TCL_GLOBAL_ONLY);
#else
  Tcl_SetVar2(interp, "sqlite_options", "unlocked", "0", TCL_GLOBAL_ONLY);
#endif

#ifdef SQLITE_OMIT_UTF16
  Tcl_SetVar2(interp, "sqlite_options", "utf16", "0", TCL_GLOBAL_ONLY);
#else
  Tcl_SetVar2(interp, "sqlite_options", "utf16", "1", TCL_GLOBAL_ONLY);
#endif

    if( sqlite3BtreeIsInTrans(pBt) ){
      needXcommit = 1;
      if( i!=1 ) nTrans++;
      rc = sqlite3BtreeExclusiveLock(pBt);
    }
  }

#ifdef SQLITE_ENABLE_UNLOCKED
  if( db->bUnlocked && (rc & 0xFF)==SQLITE_BUSY ){
    /* An SQLITE_BUSY or SQLITE_BUSY_SNAPSHOT was encountered while 
    ** attempting to take the WRITER lock on a wal file. Release the
    ** WRITER locks on all wal files and return early.  */
    for(i=0; i<db->nDb; i++){
      Btree *pBt = db->aDb[i].pBt;
      if( sqlite3BtreeIsInTrans(pBt) ){
        sqlite3BtreeEnter(pBt);
        sqlite3PagerDropExclusiveLock(sqlite3BtreePager(pBt));
        sqlite3BtreeLeave(pBt);
      }
    }
  }
#endif

  if( rc!=SQLITE_OK ){
    return rc;
  }

  /* If there are any write-transactions at all, invoke the commit hook */
  if( needXcommit && db->xCommitCallback ){

      pWal->writeLock = 0;
      rc = SQLITE_BUSY_SNAPSHOT;
    }
  }
  return rc;
}

/* 
** TODO: Combine some code with BeginWriteTransaction()
/*
** This function is called by a writer that has a read-lock on aReadmark[0]
** (pWal->readLock==0). This function relinquishes that lock and takes a
** lock on a different aReadmark[] slot. 
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
*/
static int walUpgradeReadlock(Wal *pWal){
  int cnt;
  int rc;
  assert( pWal->writeLock && pWal->readLock==0 );
  walUnlockShared(pWal, WAL_READ_LOCK(0));
  pWal->readLock = -1;
  cnt = 0;
  do{
    int notUsed;
    rc = walTryBeginRead(pWal, &notUsed, 1, ++cnt);
  }while( rc==WAL_RETRY );
  assert( (rc&0xff)!=SQLITE_BUSY ); /* BUSY not possible when useWal==1 */
  testcase( (rc&0xff)==SQLITE_IOERR );
  testcase( rc==SQLITE_PROTOCOL );
  testcase( rc==SQLITE_OK );
  return rc;
}


#ifdef SQLITE_ENABLE_UNLOCKED
/* 
** This function is only ever called when committing a "BEGIN UNLOCKED"
** transaction. It may be assumed that no frames have been written to
** the wal file.
** the wal file. The second parameter is a pointer to the in-memory 
** representation of page 1 of the database (which may or may not be
** dirty). The third is a bitvec with a bit set for each page in the
** database file that was read by the current unlocked transaction.
**
** This function performs three tasks:
**
**   1) It obtains the WRITER lock on the wal file,
**
**   2) It checks that there are no conflicts between the current
**      transaction and any transactions committed to the wal file since
**      it was opened, and
**
**   3) It ejects any non-dirty pages from the page-cache that have been
**      written by another client since the UNLOCKED transaction was started
**      (so as to avoid ending up with an inconsistent cache after the
**      current transaction is committed).
**
** If no error occurs and the caller may proceed with committing the 
** transaction, SQLITE_OK is returned. SQLITE_BUSY is returned if the WRITER
** lock cannot be obtained. Or, if the WRITER lock can be obtained but there
** are conflicts with a committed transaction, SQLITE_BUSY_SNAPSHOT. Finally,
** if an error (i.e. an OOM condition or IO error), an SQLite error code
** is returned.
*/
int sqlite3WalLockForCommit(Wal *pWal, PgHdr *pPage1, Bitvec *pAllRead){
  Pager *pPager = pPage1->pPager;
  int rc = walWriteLock(pWal);

  /* If the database has been modified since this transaction was started,
  ** check if it is still possible to commit. The transaction can be 

    }
  }

  return rc;
}

/*
** This function is called by a writer that has a read-lock on aReadmark[0]
** (pWal->readLock==0). This function relinquishes that lock and takes a
** This function is called as part of committing an UNLOCKED transaction.
** It is assumed that sqlite3WalLockForCommit() has already been successfully
** called and so (a) the WRITER lock is held and (b) it is known that the
** wal-index-header stored in shared memory is not corrupt.
**
** Before returning, this function upgrades the client so that it is 
** lock on a different aReadmark[] slot. 
** operating on the database snapshot currently at the head of the wal file
** (even if the UNLOCKED transaction ran against an older snapshot).
**
** SQLITE_OK is returned if successful, or an SQLite error code otherwise.
*/
static int walUpgradeReadlock(Wal *pWal){
  int cnt;
  int rc;
  assert( pWal->writeLock && pWal->readLock==0 );
  walUnlockShared(pWal, WAL_READ_LOCK(0));
  pWal->readLock = -1;
  cnt = 0;
  do{
    int notUsed;
    rc = walTryBeginRead(pWal, &notUsed, 1, ++cnt);
  }while( rc==WAL_RETRY );
  assert( (rc&0xff)!=SQLITE_BUSY ); /* BUSY not possible when useWal==1 */
  testcase( (rc&0xff)==SQLITE_IOERR );
  testcase( rc==SQLITE_PROTOCOL );
  testcase( rc==SQLITE_OK );
  return rc;
}

int sqlite3WalUpgradeSnapshot(Wal *pWal){
  int rc = SQLITE_OK;
  assert( pWal->writeLock );
  memcpy(&pWal->hdr, (void*)walIndexHdr(pWal), sizeof(WalIndexHdr));

  /* If this client has its read-lock on slot aReadmark[0] and the entire
  ** wal has not been checkpointed, switch it to a different slot. Otherwise
  ** any reads performed between now and committing the transaction will
  ** read from the old snapshot - not the one just upgraded to.  */
  if( pWal->readLock==0 && pWal->hdr.mxFrame!=walCkptInfo(pWal)->nBackfill ){
    rc = walUpgradeReadlock(pWal);
  }
  return rc;
}
#endif   /* SQLITE_ENABLE_UNLOCKED */

/*
** End a write transaction.  The commit has already been done.  This
** routine merely releases the lock.
*/
int sqlite3WalEndWriteTransaction(Wal *pWal){
  if( pWal->writeLock ){

#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
source $testdir/lock_common.tcl
set ::testprefix unlocked

ifcapable !unlocked {
  finish_test
  return
}

do_execsql_test 1.0 {
  PRAGMA journal_mode = wal;
} {wal}

do_execsql_test 1.1 {
  CREATE TABLE t1(k INTEGER PRIMARY KEY, v);

#

set testdir [file dirname $argv0]
source $testdir/tester.tcl
source $testdir/lock_common.tcl
set ::testprefix unlocked2

ifcapable !unlocked {
  finish_test
  return
}

do_multiclient_test tn {

  do_test 1.$tn.1 {
    sql1 {
      PRAGMA journal_mode = wal;
      CREATE TABLE t1(x);

set testdir [file dirname $argv0]
source $testdir/tester.tcl
source $testdir/lock_common.tcl
set ::testprefix unlocked3

if {$AUTOVACUUM} { finish_test ; return }
ifcapable !unlocked {
  finish_test
  return
}

proc create_schema {} {
  db eval {
    PRAGMA journal_mode = wal;

    CREATE TABLE t1(x, y);
    CREATE TABLE t2(x, y);