/*

 ** 2004 April 6

 **

 ** 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.

 **

 *************************************************************************

 ** $Id: btree.c,v 1.495 2008/08/02 17:36:46 danielk1977 Exp $

 **

 ** This file implements a external (disk-based) database using BTrees.

 ** See the header comment on "btreeInt.h" for additional information.

 ** Including a description of file format and an overview of operation.

 */

 #include "btreeInt.h"

 /*

 ** The header string that appears at the beginning of every

 ** SQLite database.

 */

 static const char zMagicHeader[] = SQLITE_FILE_HEADER;

 /*

 ** Set this global variable to 1 to enable tracing using the TRACE

 ** macro.

 */

 #if 0

 int sqlite3BtreeTrace=0;  /* True to enable tracing */

 # define TRACE(X)  if(sqlite3BtreeTrace){printf X;fflush(stdout);}

 #else

 # define TRACE(X)

 #endif

 #ifndef SQLITE_OMIT_SHARED_CACHE

 /*

 ** A flag to indicate whether or not shared cache is enabled.  Also,

 ** a list of BtShared objects that are eligible for participation

 ** in shared cache.  The variables have file scope during normal builds,

 ** but the test harness needs to access these variables so we make them

 ** global for test builds.

 */

 #ifdef SQLITE_TEST

 BtShared *sqlite3SharedCacheList = 0;

 int sqlite3SharedCacheEnabled = 0;

 #else

 static BtShared *sqlite3SharedCacheList = 0;

 static int sqlite3SharedCacheEnabled = 0;

 #endif

 #endif /* SQLITE_OMIT_SHARED_CACHE */

 #ifndef SQLITE_OMIT_SHARED_CACHE

 /*

 ** Enable or disable the shared pager and schema features.

 **

 ** This routine has no effect on existing database connections.

 ** The shared cache setting effects only future calls to

 ** sqlite3_open(), sqlite3_open16(), or sqlite3_open_v2().

 */

 int sqlite3_enable_shared_cache(int enable){

   sqlite3SharedCacheEnabled = enable;

   return SQLITE_OK;

 }

 #endif

 /*

 ** Forward declaration

 */

 static int checkReadLocks(Btree*, Pgno, BtCursor*, i64);

 #ifdef SQLITE_OMIT_SHARED_CACHE

   /*

   ** The functions queryTableLock(), lockTable() and unlockAllTables()

   ** manipulate entries in the BtShared.pLock linked list used to store

   ** shared-cache table level locks. If the library is compiled with the

   ** shared-cache feature disabled, then there is only ever one user

   ** of each BtShared structure and so this locking is not necessary. 

   ** So define the lock related functions as no-ops.

   */

   #define queryTableLock(a,b,c) SQLITE_OK

   #define lockTable(a,b,c) SQLITE_OK

   #define unlockAllTables(a)

 #endif

 #ifndef SQLITE_OMIT_SHARED_CACHE

 /*

 ** Query to see if btree handle p may obtain a lock of type eLock 

 ** (READ_LOCK or WRITE_LOCK) on the table with root-page iTab. Return

 ** SQLITE_OK if the lock may be obtained (by calling lockTable()), or

 ** SQLITE_LOCKED if not.

 */

 static int queryTableLock(Btree *p, Pgno iTab, u8 eLock){

   BtShared *pBt = p->pBt;

   BtLock *pIter;

   assert( sqlite3BtreeHoldsMutex(p) );

   assert( eLock==READ_LOCK || eLock==WRITE_LOCK );

   assert( p->db!=0 );

   /* This is a no-op if the shared-cache is not enabled */

   if( !p->sharable ){

     return SQLITE_OK;

   }

   /* If some other connection is holding an exclusive lock, the

   ** requested lock may not be obtained.

   */

   if( pBt->pExclusive && pBt->pExclusive!=p ){

     return SQLITE_LOCKED;

   }

   /* This (along with lockTable()) is where the ReadUncommitted flag is

   ** dealt with. If the caller is querying for a read-lock and the flag is

   ** set, it is unconditionally granted - even if there are write-locks

   ** on the table. If a write-lock is requested, the ReadUncommitted flag

   ** is not considered.

   **

   ** In function lockTable(), if a read-lock is demanded and the 

   ** ReadUncommitted flag is set, no entry is added to the locks list 

   ** (BtShared.pLock).

   **

   ** To summarize: If the ReadUncommitted flag is set, then read cursors do

   ** not create or respect table locks. The locking procedure for a 

   ** write-cursor does not change.

   */

   if( 

     0==(p->db->flags&SQLITE_ReadUncommitted) || 

     eLock==WRITE_LOCK ||

     iTab==MASTER_ROOT

   ){

     for(pIter=pBt->pLock; pIter; pIter=pIter->pNext){

       if( pIter->pBtree!=p && pIter->iTable==iTab && 

           (pIter->eLock!=eLock || eLock!=READ_LOCK) ){

         return SQLITE_LOCKED;

       }

     }

   }

   return SQLITE_OK;

 }

 #endif /* !SQLITE_OMIT_SHARED_CACHE */

 #ifndef SQLITE_OMIT_SHARED_CACHE

 /*

 ** Add a lock on the table with root-page iTable to the shared-btree used

 ** by Btree handle p. Parameter eLock must be either READ_LOCK or 

 ** WRITE_LOCK.

 **

 ** SQLITE_OK is returned if the lock is added successfully. SQLITE_BUSY and

 ** SQLITE_NOMEM may also be returned.

 */

 static int lockTable(Btree *p, Pgno iTable, u8 eLock){

   BtShared *pBt = p->pBt;

   BtLock *pLock = 0;

   BtLock *pIter;

   assert( sqlite3BtreeHoldsMutex(p) );

   assert( eLock==READ_LOCK || eLock==WRITE_LOCK );

   assert( p->db!=0 );

   /* This is a no-op if the shared-cache is not enabled */

   if( !p->sharable ){

     return SQLITE_OK;

   }

   assert( SQLITE_OK==queryTableLock(p, iTable, eLock) );

   /* If the read-uncommitted flag is set and a read-lock is requested,

   ** return early without adding an entry to the BtShared.pLock list. See

   ** comment in function queryTableLock() for more info on handling 

   ** the ReadUncommitted flag.

   */

   if( 

     (p->db->flags&SQLITE_ReadUncommitted) && 

     (eLock==READ_LOCK) &&

     iTable!=MASTER_ROOT

   ){

     return SQLITE_OK;

   }

   /* First search the list for an existing lock on this table. */

   for(pIter=pBt->pLock; pIter; pIter=pIter->pNext){

     if( pIter->iTable==iTable && pIter->pBtree==p ){

       pLock = pIter;

       break;

     }

   }

   /* If the above search did not find a BtLock struct associating Btree p

   ** with table iTable, allocate one and link it into the list.

   */

   if( !pLock ){

     pLock = (BtLock *)sqlite3MallocZero(sizeof(BtLock));

     if( !pLock ){

       return SQLITE_NOMEM;

     }

     pLock->iTable = iTable;

     pLock->pBtree = p;

     pLock->pNext = pBt->pLock;

     pBt->pLock = pLock;

   }

   /* Set the BtLock.eLock variable to the maximum of the current lock

   ** and the requested lock. This means if a write-lock was already held

   ** and a read-lock requested, we don't incorrectly downgrade the lock.

   */

   assert( WRITE_LOCK>READ_LOCK );

   if( eLock>pLock->eLock ){

     pLock->eLock = eLock;

   }

   return SQLITE_OK;

 }

 #endif /* !SQLITE_OMIT_SHARED_CACHE */

 #ifndef SQLITE_OMIT_SHARED_CACHE

 /*

 ** Release all the table locks (locks obtained via calls to the lockTable()

 ** procedure) held by Btree handle p.

 */

 static void unlockAllTables(Btree *p){

   BtShared *pBt = p->pBt;

   BtLock **ppIter = &pBt->pLock;

   assert( sqlite3BtreeHoldsMutex(p) );

   assert( p->sharable || 0==*ppIter );

   while( *ppIter ){

     BtLock *pLock = *ppIter;

     assert( pBt->pExclusive==0 || pBt->pExclusive==pLock->pBtree );

     if( pLock->pBtree==p ){

       *ppIter = pLock->pNext;

       sqlite3_free(pLock);

     }else{

       ppIter = &pLock->pNext;

     }

   }

   if( pBt->pExclusive==p ){

     pBt->pExclusive = 0;

   }

 }

 #endif /* SQLITE_OMIT_SHARED_CACHE */

 static void releasePage(MemPage *pPage);  /* Forward reference */

 /*

 ** Verify that the cursor holds a mutex on the BtShared

 */

 #ifndef NDEBUG

 static int cursorHoldsMutex(BtCursor *p){

   return sqlite3_mutex_held(p->pBt->mutex);

 }

 #endif

 #ifndef SQLITE_OMIT_INCRBLOB

 /*

 ** Invalidate the overflow page-list cache for cursor pCur, if any.

 */

 static void invalidateOverflowCache(BtCursor *pCur){

   assert( cursorHoldsMutex(pCur) );

   sqlite3_free(pCur->aOverflow);

   pCur->aOverflow = 0;

 }

 /*

 ** Invalidate the overflow page-list cache for all cursors opened

 ** on the shared btree structure pBt.

 */

 static void invalidateAllOverflowCache(BtShared *pBt){

   BtCursor *p;

   assert( sqlite3_mutex_held(pBt->mutex) );

   for(p=pBt->pCursor; p; p=p->pNext){

     invalidateOverflowCache(p);

   }

 }

 #else

   #define invalidateOverflowCache(x)

   #define invalidateAllOverflowCache(x)

 #endif

 /*

 ** Save the current cursor position in the variables BtCursor.nKey 

 ** and BtCursor.pKey. The cursor's state is set to CURSOR_REQUIRESEEK.

 */

 static int saveCursorPosition(BtCursor *pCur){

   int rc;

   assert( CURSOR_VALID==pCur->eState );

   assert( 0==pCur->pKey );

   assert( cursorHoldsMutex(pCur) );

   rc = sqlite3BtreeKeySize(pCur, &pCur->nKey);

   /* If this is an intKey table, then the above call to BtreeKeySize()

   ** stores the integer key in pCur->nKey. In this case this value is

   ** all that is required. Otherwise, if pCur is not open on an intKey

   ** table, then malloc space for and store the pCur->nKey bytes of key 

   ** data.

   */

   if( rc==SQLITE_OK && 0==pCur->pPage->intKey){

     void *pKey = sqlite3Malloc(pCur->nKey);

     if( pKey ){

       rc = sqlite3BtreeKey(pCur, 0, pCur->nKey, pKey);

       if( rc==SQLITE_OK ){

         pCur->pKey = pKey;

       }else{

         sqlite3_free(pKey);

       }

     }else{

       rc = SQLITE_NOMEM;

     }

   }

   assert( !pCur->pPage->intKey || !pCur->pKey );

   if( rc==SQLITE_OK ){

     releasePage(pCur->pPage);

     pCur->pPage = 0;

     pCur->eState = CURSOR_REQUIRESEEK;

   }

   invalidateOverflowCache(pCur);

   return rc;

 }

 /*

 ** Save the positions of all cursors except pExcept open on the table 

 ** with root-page iRoot. Usually, this is called just before cursor

 ** pExcept is used to modify the table (BtreeDelete() or BtreeInsert()).

 */

 static int saveAllCursors(BtShared *pBt, Pgno iRoot, BtCursor *pExcept){

   BtCursor *p;

   assert( sqlite3_mutex_held(pBt->mutex) );

   assert( pExcept==0 || pExcept->pBt==pBt );

   for(p=pBt->pCursor; p; p=p->pNext){

     if( p!=pExcept && (0==iRoot || p->pgnoRoot==iRoot) && 

         p->eState==CURSOR_VALID ){

       int rc = saveCursorPosition(p);

       if( SQLITE_OK!=rc ){

         return rc;

       }

     }

   }

   return SQLITE_OK;

 }

 /*

 ** Clear the current cursor position.

 */

 static void clearCursorPosition(BtCursor *pCur){

   assert( cursorHoldsMutex(pCur) );

   sqlite3_free(pCur->pKey);

   pCur->pKey = 0;

   pCur->eState = CURSOR_INVALID;

 }

 /*

 ** Restore the cursor to the position it was in (or as close to as possible)

 ** when saveCursorPosition() was called. Note that this call deletes the 

 ** saved position info stored by saveCursorPosition(), so there can be

 ** at most one effective restoreCursorPosition() call after each 

 ** saveCursorPosition().

 */

 int sqlite3BtreeRestoreCursorPosition(BtCursor *pCur){

   int rc;

   assert( cursorHoldsMutex(pCur) );

   assert( pCur->eState>=CURSOR_REQUIRESEEK );

   if( pCur->eState==CURSOR_FAULT ){

     return pCur->skip;

   }

   pCur->eState = CURSOR_INVALID;

   rc = sqlite3BtreeMoveto(pCur, pCur->pKey, 0, pCur->nKey, 0, &pCur->skip);

   if( rc==SQLITE_OK ){

     sqlite3_free(pCur->pKey);

     pCur->pKey = 0;

     assert( pCur->eState==CURSOR_VALID || pCur->eState==CURSOR_INVALID );

   }

   return rc;

 }

 #define restoreCursorPosition(p) \

   (p->eState>=CURSOR_REQUIRESEEK ? \

          sqlite3BtreeRestoreCursorPosition(p) : \

          SQLITE_OK)

 /*

 ** Determine whether or not a cursor has moved from the position it

 ** was last placed at.  Cursor can move when the row they are pointing

 ** at is deleted out from under them.

 **

 ** This routine returns an error code if something goes wrong.  The

 ** integer *pHasMoved is set to one if the cursor has moved and 0 if not.

 */

 int sqlite3BtreeCursorHasMoved(BtCursor *pCur, int *pHasMoved){

   int rc;

   rc = restoreCursorPosition(pCur);

   if( rc ){

     *pHasMoved = 1;

     return rc;

   }

   if( pCur->eState!=CURSOR_VALID || pCur->skip!=0 ){

     *pHasMoved = 1;

   }else{

     *pHasMoved = 0;

   }

   return SQLITE_OK;

 }

 #ifndef SQLITE_OMIT_AUTOVACUUM

 /*

 ** Given a page number of a regular database page, return the page

 ** number for the pointer-map page that contains the entry for the

 ** input page number.

 */

 static Pgno ptrmapPageno(BtShared *pBt, Pgno pgno){

   int nPagesPerMapPage, iPtrMap, ret;

   assert( sqlite3_mutex_held(pBt->mutex) );

   nPagesPerMapPage = (pBt->usableSize/5)+1;

   iPtrMap = (pgno-2)/nPagesPerMapPage;

   ret = (iPtrMap*nPagesPerMapPage) + 2; 

   if( ret==PENDING_BYTE_PAGE(pBt) ){

     ret++;

   }

   return ret;

 }

 /*

 ** Write an entry into the pointer map.

 **

 ** This routine updates the pointer map entry for page number 'key'

 ** so that it maps to type 'eType' and parent page number 'pgno'.

 ** An error code is returned if something goes wrong, otherwise SQLITE_OK.

 */

 static int ptrmapPut(BtShared *pBt, Pgno key, u8 eType, Pgno parent){

   DbPage *pDbPage;  /* The pointer map page */

   u8 *pPtrmap;      /* The pointer map data */

   Pgno iPtrmap;     /* The pointer map page number */

   int offset;       /* Offset in pointer map page */

   int rc;

   assert( sqlite3_mutex_held(pBt->mutex) );

   /* The master-journal page number must never be used as a pointer map page */

   assert( 0==PTRMAP_ISPAGE(pBt, PENDING_BYTE_PAGE(pBt)) );

   assert( pBt->autoVacuum );

   if( key==0 ){

     return SQLITE_CORRUPT_BKPT;

   }

   iPtrmap = PTRMAP_PAGENO(pBt, key);

   rc = sqlite3PagerGet(pBt->pPager, iPtrmap, &pDbPage);

   if( rc!=SQLITE_OK ){

     return rc;

   }

   offset = PTRMAP_PTROFFSET(iPtrmap, key);

   pPtrmap = (u8 *)sqlite3PagerGetData(pDbPage);

   if( eType!=pPtrmap[offset] || get4byte(&pPtrmap[offset+1])!=parent ){

     TRACE(("PTRMAP_UPDATE: %d->(%d,%d)\n", key, eType, parent));

     rc = sqlite3PagerWrite(pDbPage);

     if( rc==SQLITE_OK ){

       pPtrmap[offset] = eType;

       put4byte(&pPtrmap[offset+1], parent);

     }

   }

   sqlite3PagerUnref(pDbPage);

   return rc;

 }

 /*

 ** Read an entry from the pointer map.

 **

 ** This routine retrieves the pointer map entry for page 'key', writing

 ** the type and parent page number to *pEType and *pPgno respectively.

 ** An error code is returned if something goes wrong, otherwise SQLITE_OK.

 */

 static int ptrmapGet(BtShared *pBt, Pgno key, u8 *pEType, Pgno *pPgno){

   DbPage *pDbPage;   /* The pointer map page */

   int iPtrmap;       /* Pointer map page index */

   u8 *pPtrmap;       /* Pointer map page data */

   int offset;        /* Offset of entry in pointer map */

   int rc;

   assert( sqlite3_mutex_held(pBt->mutex) );

   iPtrmap = PTRMAP_PAGENO(pBt, key);

   rc = sqlite3PagerGet(pBt->pPager, iPtrmap, &pDbPage);

   if( rc!=0 ){

     return rc;

   }

   pPtrmap = (u8 *)sqlite3PagerGetData(pDbPage);

   offset = PTRMAP_PTROFFSET(iPtrmap, key);

   assert( pEType!=0 );

   *pEType = pPtrmap[offset];

   if( pPgno ) *pPgno = get4byte(&pPtrmap[offset+1]);

   sqlite3PagerUnref(pDbPage);

   if( *pEType<1 || *pEType>5 ) return SQLITE_CORRUPT_BKPT;

   return SQLITE_OK;

 }

 #else /* if defined SQLITE_OMIT_AUTOVACUUM */

   #define ptrmapPut(w,x,y,z) SQLITE_OK

   #define ptrmapGet(w,x,y,z) SQLITE_OK

   #define ptrmapPutOvfl(y,z) SQLITE_OK

 #endif

 /*

 ** Given a btree page and a cell index (0 means the first cell on

 ** the page, 1 means the second cell, and so forth) return a pointer

 ** to the cell content.

 **

 ** This routine works only for pages that do not contain overflow cells.

 */

 #define findCell(P,I) \

   ((P)->aData + ((P)->maskPage & get2byte(&(P)->aData[(P)->cellOffset+2*(I)])))

 /*

 ** This a more complex version of findCell() that works for

 ** pages that do contain overflow cells.  See insert

 */

 static u8 *findOverflowCell(MemPage *pPage, int iCell){

   int i;

   assert( sqlite3_mutex_held(pPage->pBt->mutex) );

   for(i=pPage->nOverflow-1; i>=0; i--){

     int k;

     struct _OvflCell *pOvfl;

     pOvfl = &pPage->aOvfl[i];

     k = pOvfl->idx;

     if( k<=iCell ){

       if( k==iCell ){

         return pOvfl->pCell;

       }

       iCell--;

     }

   }

   return findCell(pPage, iCell);

 }

 /*

 ** Parse a cell content block and fill in the CellInfo structure.  There

 ** are two versions of this function.  sqlite3BtreeParseCell() takes a 

 ** cell index as the second argument and sqlite3BtreeParseCellPtr() 

 ** takes a pointer to the body of the cell as its second argument.

 **

 ** Within this file, the parseCell() macro can be called instead of

 ** sqlite3BtreeParseCellPtr(). Using some compilers, this will be faster.

 */

 void sqlite3BtreeParseCellPtr(

   MemPage *pPage,         /* Page containing the cell */

   u8 *pCell,              /* Pointer to the cell text. */

   CellInfo *pInfo         /* Fill in this structure */

 ){

   int n;                  /* Number bytes in cell content header */

   u32 nPayload;           /* Number of bytes of cell payload */

   assert( sqlite3_mutex_held(pPage->pBt->mutex) );

   pInfo->pCell = pCell;

   assert( pPage->leaf==0 || pPage->leaf==1 );

   n = pPage->childPtrSize;

   assert( n==4-4*pPage->leaf );

   if( pPage->intKey ){

     if( pPage->hasData ){

       n += getVarint32(&pCell[n], nPayload);

     }else{

       nPayload = 0;

     }

     n += getVarint(&pCell[n], (u64*)&pInfo->nKey);

     pInfo->nData = nPayload;

   }else{

     pInfo->nData = 0;

     n += getVarint32(&pCell[n], nPayload);

     pInfo->nKey = nPayload;

   }

   pInfo->nPayload = nPayload;

   pInfo->nHeader = n;

   if( likely(nPayload<=pPage->maxLocal) ){

     /* This is the (easy) common case where the entire payload fits

     ** on the local page.  No overflow is required.

     */

     int nSize;          /* Total size of cell content in bytes */

     nSize = nPayload + n;

     pInfo->nLocal = nPayload;

     pInfo->iOverflow = 0;

     if( (nSize & ~3)==0 ){

       nSize = 4;        /* Minimum cell size is 4 */

     }

     pInfo->nSize = nSize;

   }else{

     /* If the payload will not fit completely on the local page, we have

     ** to decide how much to store locally and how much to spill onto

     ** overflow pages.  The strategy is to minimize the amount of unused

     ** space on overflow pages while keeping the amount of local storage

     ** in between minLocal and maxLocal.

     **

     ** Warning:  changing the way overflow payload is distributed in any

     ** way will result in an incompatible file format.

     */

     int minLocal;  /* Minimum amount of payload held locally */

     int maxLocal;  /* Maximum amount of payload held locally */

     int surplus;   /* Overflow payload available for local storage */

     minLocal = pPage->minLocal;

     maxLocal = pPage->maxLocal;

     surplus = minLocal + (nPayload - minLocal)%(pPage->pBt->usableSize - 4);

     if( surplus <= maxLocal ){

       pInfo->nLocal = surplus;

     }else{

       pInfo->nLocal = minLocal;

     }

     pInfo->iOverflow = pInfo->nLocal + n;

     pInfo->nSize = pInfo->iOverflow + 4;

   }

 }

 #define parseCell(pPage, iCell, pInfo) \

   sqlite3BtreeParseCellPtr((pPage), findCell((pPage), (iCell)), (pInfo))

 void sqlite3BtreeParseCell(

   MemPage *pPage,         /* Page containing the cell */

   int iCell,              /* The cell index.  First cell is 0 */

   CellInfo *pInfo         /* Fill in this structure */

 ){

   parseCell(pPage, iCell, pInfo);

 }

 /*

 ** Compute the total number of bytes that a Cell needs in the cell

 ** data area of the btree-page.  The return number includes the cell

 ** data header and the local payload, but not any overflow page or

 ** the space used by the cell pointer.

 */

 #ifndef NDEBUG

 static u16 cellSize(MemPage *pPage, int iCell){

   CellInfo info;

   sqlite3BtreeParseCell(pPage, iCell, &info);

   return info.nSize;

 }

 #endif

 static u16 cellSizePtr(MemPage *pPage, u8 *pCell){

   CellInfo info;

   sqlite3BtreeParseCellPtr(pPage, pCell, &info);

   return info.nSize;

 }

 #ifndef SQLITE_OMIT_AUTOVACUUM

 /*

 ** If the cell pCell, part of page pPage contains a pointer

 ** to an overflow page, insert an entry into the pointer-map

 ** for the overflow page.

 */

 static int ptrmapPutOvflPtr(MemPage *pPage, u8 *pCell){

   CellInfo info;

   assert( pCell!=0 );

   sqlite3BtreeParseCellPtr(pPage, pCell, &info);

   assert( (info.nData+(pPage->intKey?0:info.nKey))==info.nPayload );

   if( (info.nData+(pPage->intKey?0:info.nKey))>info.nLocal ){

     Pgno ovfl = get4byte(&pCell[info.iOverflow]);

     return ptrmapPut(pPage->pBt, ovfl, PTRMAP_OVERFLOW1, pPage->pgno);

   }

   return SQLITE_OK;

 }

 /*

 ** If the cell with index iCell on page pPage contains a pointer

 ** to an overflow page, insert an entry into the pointer-map

 ** for the overflow page.

 */

 static int ptrmapPutOvfl(MemPage *pPage, int iCell){

   u8 *pCell;

   assert( sqlite3_mutex_held(pPage->pBt->mutex) );

   pCell = findOverflowCell(pPage, iCell);

   return ptrmapPutOvflPtr(pPage, pCell);

 }

 #endif

 /*

 ** Defragment the page given.  All Cells are moved to the

 ** end of the page and all free space is collected into one

 ** big FreeBlk that occurs in between the header and cell

 ** pointer array and the cell content area.

 */

 static void defragmentPage(MemPage *pPage){

   int i;                     /* Loop counter */

   int pc;                    /* Address of a i-th cell */

   int addr;                  /* Offset of first byte after cell pointer array */

   int hdr;                   /* Offset to the page header */

   int size;                  /* Size of a cell */

   int usableSize;            /* Number of usable bytes on a page */

   int cellOffset;            /* Offset to the cell pointer array */

   int brk;                   /* Offset to the cell content area */

   int nCell;                 /* Number of cells on the page */

   unsigned char *data;       /* The page data */

   unsigned char *temp;       /* Temp area for cell content */

   assert( sqlite3PagerIswriteable(pPage->pDbPage) );

   assert( pPage->pBt!=0 );

   assert( pPage->pBt->usableSize <= SQLITE_MAX_PAGE_SIZE );

   assert( pPage->nOverflow==0 );

   assert( sqlite3_mutex_held(pPage->pBt->mutex) );

   temp = sqlite3PagerTempSpace(pPage->pBt->pPager);

   data = pPage->aData;

   hdr = pPage->hdrOffset;

   cellOffset = pPage->cellOffset;

   nCell = pPage->nCell;

   assert( nCell==get2byte(&data[hdr+3]) );

   usableSize = pPage->pBt->usableSize;

   brk = get2byte(&data[hdr+5]);

   memcpy(&temp[brk], &data[brk], usableSize - brk);

   brk = usableSize;

   for(i=0; i<nCell; i++){

     u8 *pAddr;     /* The i-th cell pointer */

     pAddr = &data[cellOffset + i*2];

     pc = get2byte(pAddr);

     assert( pc<pPage->pBt->usableSize );

     size = cellSizePtr(pPage, &temp[pc]);

     brk -= size;

     memcpy(&data[brk], &temp[pc], size);

     put2byte(pAddr, brk);

   }

   assert( brk>=cellOffset+2*nCell );

   put2byte(&data[hdr+5], brk);

   data[hdr+1] = 0;

   data[hdr+2] = 0;

   data[hdr+7] = 0;

   addr = cellOffset+2*nCell;

   memset(&data[addr], 0, brk-addr);

 }

 /*

 ** Allocate nByte bytes of space on a page.

 **

 ** Return the index into pPage->aData[] of the first byte of

 ** the new allocation.  The caller guarantees that there is enough

 ** space.  This routine will never fail.

 **

 ** If the page contains nBytes of free space but does not contain

 ** nBytes of contiguous free space, then this routine automatically

 ** calls defragementPage() to consolidate all free space before 

 ** allocating the new chunk.

 */

 static int allocateSpace(MemPage *pPage, int nByte){

   int addr, pc, hdr;

   int size;

   int nFrag;

   int top;

   int nCell;

   int cellOffset;

   unsigned char *data;

   data = pPage->aData;

   assert( sqlite3PagerIswriteable(pPage->pDbPage) );

   assert( pPage->pBt );

   assert( sqlite3_mutex_held(pPage->pBt->mutex) );

   assert( nByte>=0 );  /* Minimum cell size is 4 */

   assert( pPage->nFree>=nByte );

   assert( pPage->nOverflow==0 );

   pPage->nFree -= nByte;

   hdr = pPage->hdrOffset;

   nFrag = data[hdr+7];

   if( nFrag<60 ){

     /* Search the freelist looking for a slot big enough to satisfy the

     ** space request. */

     addr = hdr+1;

     while( (pc = get2byte(&data[addr]))>0 ){

       size = get2byte(&data[pc+2]);

       if( size>=nByte ){

         if( size<nByte+4 ){

           memcpy(&data[addr], &data[pc], 2);

           data[hdr+7] = nFrag + size - nByte;

           return pc;

         }else{

           put2byte(&data[pc+2], size-nByte);

           return pc + size - nByte;

         }

       }

       addr = pc;

     }

   }

   /* Allocate memory from the gap in between the cell pointer array

   ** and the cell content area.

   */

   top = get2byte(&data[hdr+5]);

   nCell = get2byte(&data[hdr+3]);

   cellOffset = pPage->cellOffset;

   if( nFrag>=60 || cellOffset + 2*nCell > top - nByte ){

     defragmentPage(pPage);

     top = get2byte(&data[hdr+5]);

   }

   top -= nByte;

   assert( cellOffset + 2*nCell <= top );

   put2byte(&data[hdr+5], top);

   return top;

 }

 /*

 ** Return a section of the pPage->aData to the freelist.

 ** The first byte of the new free block is pPage->aDisk[start]

 ** and the size of the block is "size" bytes.

 **

 ** Most of the effort here is involved in coalesing adjacent

 ** free blocks into a single big free block.

 */

 static void freeSpace(MemPage *pPage, int start, int size){

   int addr, pbegin, hdr;

   unsigned char *data = pPage->aData;

   assert( pPage->pBt!=0 );

   assert( sqlite3PagerIswriteable(pPage->pDbPage) );

   assert( start>=pPage->hdrOffset+6+(pPage->leaf?0:4) );

   assert( (start + size)<=pPage->pBt->usableSize );

   assert( sqlite3_mutex_held(pPage->pBt->mutex) );

   assert( size>=0 );   /* Minimum cell size is 4 */

 #ifdef SQLITE_SECURE_DELETE

   /* Overwrite deleted information with zeros when the SECURE_DELETE 

   ** option is enabled at compile-time */

   memset(&data[start], 0, size);

 #endif

   /* Add the space back into the linked list of freeblocks */

   hdr = pPage->hdrOffset;

   addr = hdr + 1;

   while( (pbegin = get2byte(&data[addr]))<start && pbegin>0 ){

     assert( pbegin<=pPage->pBt->usableSize-4 );

     assert( pbegin>addr );

     addr = pbegin;

   }

   assert( pbegin<=pPage->pBt->usableSize-4 );

   assert( pbegin>addr || pbegin==0 );

   put2byte(&data[addr], start);

   put2byte(&data[start], pbegin);

   put2byte(&data[start+2], size);

   pPage->nFree += size;

   /* Coalesce adjacent free blocks */

   addr = pPage->hdrOffset + 1;

   while( (pbegin = get2byte(&data[addr]))>0 ){

     int pnext, psize;

     assert( pbegin>addr );

     assert( pbegin<=pPage->pBt->usableSize-4 );

     pnext = get2byte(&data[pbegin]);

     psize = get2byte(&data[pbegin+2]);

     if( pbegin + psize + 3 >= pnext && pnext>0 ){

       int frag = pnext - (pbegin+psize);

       assert( frag<=data[pPage->hdrOffset+7] );

       data[pPage->hdrOffset+7] -= frag;

       put2byte(&data[pbegin], get2byte(&data[pnext]));

       put2byte(&data[pbegin+2], pnext+get2byte(&data[pnext+2])-pbegin);

     }else{

       addr = pbegin;

     }

   }

   /* If the cell content area begins with a freeblock, remove it. */

   if( data[hdr+1]==data[hdr+5] && data[hdr+2]==data[hdr+6] ){

     int top;

     pbegin = get2byte(&data[hdr+1]);

     memcpy(&data[hdr+1], &data[pbegin], 2);

     top = get2byte(&data[hdr+5]);

     put2byte(&data[hdr+5], top + get2byte(&data[pbegin+2]));

   }

 }

 /*

 ** Decode the flags byte (the first byte of the header) for a page

 ** and initialize fields of the MemPage structure accordingly.

 **

 ** Only the following combinations are supported.  Anything different

 ** indicates a corrupt database files:

 **

 **         PTF_ZERODATA

 **         PTF_ZERODATA | PTF_LEAF

 **         PTF_LEAFDATA | PTF_INTKEY

 **         PTF_LEAFDATA | PTF_INTKEY | PTF_LEAF

 */

 static int decodeFlags(MemPage *pPage, int flagByte){

   BtShared *pBt;     /* A copy of pPage->pBt */

   assert( pPage->hdrOffset==(pPage->pgno==1 ? 100 : 0) );

   assert( sqlite3_mutex_held(pPage->pBt->mutex) );

   pPage->leaf = flagByte>>3;  assert( PTF_LEAF == 1<<3 );

   flagByte &= ~PTF_LEAF;

   pPage->childPtrSize = 4-4*pPage->leaf;

   pBt = pPage->pBt;

   if( flagByte==(PTF_LEAFDATA | PTF_INTKEY) ){

     pPage->intKey = 1;

     pPage->hasData = pPage->leaf;

     pPage->maxLocal = pBt->maxLeaf;

     pPage->minLocal = pBt->minLeaf;

   }else if( flagByte==PTF_ZERODATA ){

     pPage->intKey = 0;

     pPage->hasData = 0;

     pPage->maxLocal = pBt->maxLocal;

     pPage->minLocal = pBt->minLocal;

   }else{

     return SQLITE_CORRUPT_BKPT;

   }

   return SQLITE_OK;

 }

 /*

 ** Initialize the auxiliary information for a disk block.

 **

 ** The pParent parameter must be a pointer to the MemPage which

 ** is the parent of the page being initialized.  The root of a

 ** BTree has no parent and so for that page, pParent==NULL.

 **

 ** Return SQLITE_OK on success.  If we see that the page does

 ** not contain a well-formed database page, then return 

 ** SQLITE_CORRUPT.  Note that a return of SQLITE_OK does not

 ** guarantee that the page is well-formed.  It only shows that

 ** we failed to detect any corruption.

 */

 int sqlite3BtreeInitPage(

   MemPage *pPage,        /* The page to be initialized */

   MemPage *pParent       /* The parent.  Might be NULL */

 ){

   int pc;            /* Address of a freeblock within pPage->aData[] */

   int hdr;           /* Offset to beginning of page header */

   u8 *data;          /* Equal to pPage->aData */

   BtShared *pBt;        /* The main btree structure */

   int usableSize;    /* Amount of usable space on each page */

   int cellOffset;    /* Offset from start of page to first cell pointer */

   int nFree;         /* Number of unused bytes on the page */

   int top;           /* First byte of the cell content area */

   pBt = pPage->pBt;

   assert( pBt!=0 );

   assert( pParent==0 || pParent->pBt==pBt );

   assert( sqlite3_mutex_held(pBt->mutex) );

   assert( pPage->pgno==sqlite3PagerPagenumber(pPage->pDbPage) );

   assert( pPage == sqlite3PagerGetExtra(pPage->pDbPage) );

   assert( pPage->aData == sqlite3PagerGetData(pPage->pDbPage) );

   if( pPage->pParent!=pParent && (pPage->pParent!=0 || pPage->isInit) ){

     /* The parent page should never change unless the file is corrupt */

     return SQLITE_CORRUPT_BKPT;

   }

   if( pPage->isInit ) return SQLITE_OK;

   if( pPage->pParent==0 && pParent!=0 ){

     pPage->pParent = pParent;

     sqlite3PagerRef(pParent->pDbPage);

   }

   hdr = pPage->hdrOffset;

   data = pPage->aData;

   if( decodeFlags(pPage, data[hdr]) ) return SQLITE_CORRUPT_BKPT;

   assert( pBt->pageSize>=512 && pBt->pageSize<=32768 );

   pPage->maskPage = pBt->pageSize - 1;

   pPage->nOverflow = 0;

   pPage->idxShift = 0;

   usableSize = pBt->usableSize;

   pPage->cellOffset = cellOffset = hdr + 12 - 4*pPage->leaf;

   top = get2byte(&data[hdr+5]);

   pPage->nCell = get2byte(&data[hdr+3]);

   if( pPage->nCell>MX_CELL(pBt) ){

     /* To many cells for a single page.  The page must be corrupt */

     return SQLITE_CORRUPT_BKPT;

   }

   if( pPage->nCell==0 && pParent!=0 && pParent->pgno!=1 ){

     /* All pages must have at least one cell, except for root pages */

     return SQLITE_CORRUPT_BKPT;

   }

   /* Compute the total free space on the page */

   pc = get2byte(&data[hdr+1]);

   nFree = data[hdr+7] + top - (cellOffset + 2*pPage->nCell);

   while( pc>0 ){

     int next, size;

     if( pc>usableSize-4 ){

       /* Free block is off the page */

       return SQLITE_CORRUPT_BKPT; 

     }

     next = get2byte(&data[pc]);

     size = get2byte(&data[pc+2]);

     if( next>0 && next<=pc+size+3 ){

       /* Free blocks must be in accending order */

       return SQLITE_CORRUPT_BKPT; 

     }

     nFree += size;

     pc = next;

   }

   pPage->nFree = nFree;

   if( nFree>=usableSize ){

     /* Free space cannot exceed total page size */

     return SQLITE_CORRUPT_BKPT; 

   }

 #if 0

   /* Check that all the offsets in the cell offset array are within range. 

   ** 

   ** Omitting this consistency check and using the pPage->maskPage mask

   ** to prevent overrunning the page buffer in findCell() results in a

   ** 2.5% performance gain.

   */

   {

     u8 *pOff;        /* Iterator used to check all cell offsets are in range */

     u8 *pEnd;        /* Pointer to end of cell offset array */

     u8 mask;         /* Mask of bits that must be zero in MSB of cell offsets */

     mask = ~(((u8)(pBt->pageSize>>8))-1);

     pEnd = &data[cellOffset + pPage->nCell*2];

     for(pOff=&data[cellOffset]; pOff!=pEnd && !((*pOff)&mask); pOff+=2);

     if( pOff!=pEnd ){

       return SQLITE_CORRUPT_BKPT;

     }

   }

 #endif

   pPage->isInit = 1;

   return SQLITE_OK;

 }

 /*

 ** Set up a raw page so that it looks like a database page holding

 ** no entries.

 */

 static void zeroPage(MemPage *pPage, int flags){

   unsigned char *data = pPage->aData;

   BtShared *pBt = pPage->pBt;

   int hdr = pPage->hdrOffset;

   int first;

   assert( sqlite3PagerPagenumber(pPage->pDbPage)==pPage->pgno );

   assert( sqlite3PagerGetExtra(pPage->pDbPage) == (void*)pPage );

   assert( sqlite3PagerGetData(pPage->pDbPage) == data );

   assert( sqlite3PagerIswriteable(pPage->pDbPage) );

   assert( sqlite3_mutex_held(pBt->mutex) );

   /*memset(&data[hdr], 0, pBt->usableSize - hdr);*/

   data[hdr] = flags;

   first = hdr + 8 + 4*((flags&PTF_LEAF)==0);

   memset(&data[hdr+1], 0, 4);

   data[hdr+7] = 0;

   put2byte(&data[hdr+5], pBt->usableSize);

   pPage->nFree = pBt->usableSize - first;

   decodeFlags(pPage, flags);

   pPage->hdrOffset = hdr;

   pPage->cellOffset = first;

   pPage->nOverflow = 0;

   assert( pBt->pageSize>=512 && pBt->pageSize<=32768 );

   pPage->maskPage = pBt->pageSize - 1;

   pPage->idxShift = 0;

   pPage->nCell = 0;

   pPage->isInit = 1;

 }

 /*

 ** Get a page from the pager.  Initialize the MemPage.pBt and

 ** MemPage.aData elements if needed.

 **

 ** If the noContent flag is set, it means that we do not care about

 ** the content of the page at this time.  So do not go to the disk

 ** to fetch the content.  Just fill in the content with zeros for now.

 ** If in the future we call sqlite3PagerWrite() on this page, that

 ** means we have started to be concerned about content and the disk

 ** read should occur at that point.

 */

 int sqlite3BtreeGetPage(

   BtShared *pBt,       /* The btree */

   Pgno pgno,           /* Number of the page to fetch */

   MemPage **ppPage,    /* Return the page in this parameter */

   int noContent        /* Do not load page content if true */

 ){

   int rc;

   MemPage *pPage;

   DbPage *pDbPage;

   assert( sqlite3_mutex_held(pBt->mutex) );

   rc = sqlite3PagerAcquire(pBt->pPager, pgno, (DbPage**)&pDbPage, noContent);

   if( rc ) return rc;

   pPage = (MemPage *)sqlite3PagerGetExtra(pDbPage);

   pPage->aData = sqlite3PagerGetData(pDbPage);

   pPage->pDbPage = pDbPage;

   pPage->pBt = pBt;

   pPage->pgno = pgno;

   pPage->hdrOffset = pPage->pgno==1 ? 100 : 0;

   *ppPage = pPage;

   return SQLITE_OK;

 }

 /*

 ** Get a page from the pager and initialize it.  This routine

 ** is just a convenience wrapper around separate calls to

 ** sqlite3BtreeGetPage() and sqlite3BtreeInitPage().

 */

 static int getAndInitPage(

   BtShared *pBt,          /* The database file */

   Pgno pgno,           /* Number of the page to get */

   MemPage **ppPage,    /* Write the page pointer here */

   MemPage *pParent     /* Parent of the page */

 ){

   int rc;

   assert( sqlite3_mutex_held(pBt->mutex) );

   if( pgno==0 ){

     return SQLITE_CORRUPT_BKPT; 

   }

   rc = sqlite3BtreeGetPage(pBt, pgno, ppPage, 0);

   if( rc==SQLITE_OK && (*ppPage)->isInit==0 ){

     rc = sqlite3BtreeInitPage(*ppPage, pParent);

     if( rc!=SQLITE_OK ){

       releasePage(*ppPage);

       *ppPage = 0;

     }

   }

   return rc;

 }

 /*

 ** Release a MemPage.  This should be called once for each prior

 ** call to sqlite3BtreeGetPage.

 */

 static void releasePage(MemPage *pPage){

   if( pPage ){

     assert( pPage->aData );

     assert( pPage->pBt );

     assert( sqlite3PagerGetExtra(pPage->pDbPage) == (void*)pPage );

     assert( sqlite3PagerGetData(pPage->pDbPage)==pPage->aData );

     assert( sqlite3_mutex_held(pPage->pBt->mutex) );

     sqlite3PagerUnref(pPage->pDbPage);

   }

 }

 /*

 ** This routine is called when the reference count for a page

 ** reaches zero.  We need to unref the pParent pointer when that

 ** happens.

 */

 static void pageDestructor(DbPage *pData, int pageSize){

   MemPage *pPage;

   assert( (pageSize & 7)==0 );

   pPage = (MemPage *)sqlite3PagerGetExtra(pData);

   assert( pPage->isInit==0 || sqlite3_mutex_held(pPage->pBt->mutex) );

   if( pPage->pParent ){

     MemPage *pParent = pPage->pParent;

     assert( pParent->pBt==pPage->pBt );

     pPage->pParent = 0;

     releasePage(pParent);

   }

   pPage->isInit = 0;

 }

 /*

 ** During a rollback, when the pager reloads information into the cache

 ** so that the cache is restored to its original state at the start of

 ** the transaction, for each page restored this routine is called.

 **

 ** This routine needs to reset the extra data section at the end of the

 ** page to agree with the restored data.

 */

 static void pageReinit(DbPage *pData, int pageSize){

   MemPage *pPage;

   assert( (pageSize & 7)==0 );

   pPage = (MemPage *)sqlite3PagerGetExtra(pData);

   if( pPage->isInit ){

     assert( sqlite3_mutex_held(pPage->pBt->mutex) );

     pPage->isInit = 0;

     sqlite3BtreeInitPage(pPage, pPage->pParent);

   }

 }

 /*

 ** Invoke the busy handler for a btree.

 */

 static int sqlite3BtreeInvokeBusyHandler(void *pArg, int n){

   BtShared *pBt = (BtShared*)pArg;

   assert( pBt->db );

   assert( sqlite3_mutex_held(pBt->db->mutex) );

   return sqlite3InvokeBusyHandler(&pBt->db->busyHandler);

 }

 /*

 ** Open a database file.

 ** 

 ** zFilename is the name of the database file.  If zFilename is NULL

 ** a new database with a random name is created.  This randomly named

 ** database file will be deleted when sqlite3BtreeClose() is called.

 ** If zFilename is ":memory:" then an in-memory database is created

 ** that is automatically destroyed when it is closed.

 */

 int sqlite3BtreeOpen(

   const char *zFilename,  /* Name of the file containing the BTree database */

   sqlite3 *db,            /* Associated database handle */

   Btree **ppBtree,        /* Pointer to new Btree object written here */

   int flags,              /* Options */

   int vfsFlags            /* Flags passed through to sqlite3_vfs.xOpen() */

 ){

   sqlite3_vfs *pVfs;      /* The VFS to use for this btree */

   BtShared *pBt = 0;      /* Shared part of btree structure */

   Btree *p;               /* Handle to return */

   int rc = SQLITE_OK;

   int nReserve;

   unsigned char zDbHeader[100];

   /* Set the variable isMemdb to true for an in-memory database, or 

   ** false for a file-based database. This symbol is only required if

   ** either of the shared-data or autovacuum features are compiled 

   ** into the library.

   */

 #if !defined(SQLITE_OMIT_SHARED_CACHE) || !defined(SQLITE_OMIT_AUTOVACUUM)

   #ifdef SQLITE_OMIT_MEMORYDB

     const int isMemdb = 0;

   #else

     const int isMemdb = zFilename && !strcmp(zFilename, ":memory:");

   #endif

 #endif

   assert( db!=0 );

   assert( sqlite3_mutex_held(db->mutex) );

   pVfs = db->pVfs;

   p = sqlite3MallocZero(sizeof(Btree));

   if( !p ){

     return SQLITE_NOMEM;

   }

   p->inTrans = TRANS_NONE;

   p->db = db;

 #if !defined(SQLITE_OMIT_SHARED_CACHE) && !defined(SQLITE_OMIT_DISKIO)

   /*

   ** If this Btree is a candidate for shared cache, try to find an

   ** existing BtShared object that we can share with

   */

   if( isMemdb==0

    && (db->flags & SQLITE_Vtab)==0

    && zFilename && zFilename[0]

   ){

     if( sqlite3SharedCacheEnabled ){

       int nFullPathname = pVfs->mxPathname+1;

       char *zFullPathname = sqlite3Malloc(nFullPathname);

       sqlite3_mutex *mutexShared;

       p->sharable = 1;

       db->flags |= SQLITE_SharedCache;

       if( !zFullPathname ){

         sqlite3_free(p);

         return SQLITE_NOMEM;

       }

       sqlite3OsFullPathname(pVfs, zFilename, nFullPathname, zFullPathname);

       mutexShared = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MASTER);

       sqlite3_mutex_enter(mutexShared);

       for(pBt=sqlite3SharedCacheList; pBt; pBt=pBt->pNext){

         assert( pBt->nRef>0 );

         if( 0==strcmp(zFullPathname, sqlite3PagerFilename(pBt->pPager))

                  && sqlite3PagerVfs(pBt->pPager)==pVfs ){

           p->pBt = pBt;

           pBt->nRef++;

           break;

         }

       }

       sqlite3_mutex_leave(mutexShared);

       sqlite3_free(zFullPathname);

     }

 #ifdef SQLITE_DEBUG

     else{

       /* In debug mode, we mark all persistent databases as sharable

       ** even when they are not.  This exercises the locking code and

       ** gives more opportunity for asserts(sqlite3_mutex_held())

       ** statements to find locking problems.

       */

       p->sharable = 1;

     }

 #endif

   }

 #endif

   if( pBt==0 ){

     /*

     ** The following asserts make sure that structures used by the btree are

     ** the right size.  This is to guard against size changes that result

     ** when compiling on a different architecture.

     */

     assert( sizeof(i64)==8 || sizeof(i64)==4 );

     assert( sizeof(u64)==8 || sizeof(u64)==4 );

     assert( sizeof(u32)==4 );

     assert( sizeof(u16)==2 );

     assert( sizeof(Pgno)==4 );

     pBt = sqlite3MallocZero( sizeof(*pBt) );

     if( pBt==0 ){

       rc = SQLITE_NOMEM;

       goto btree_open_out;

     }

     pBt->busyHdr.xFunc = sqlite3BtreeInvokeBusyHandler;

     pBt->busyHdr.pArg = pBt;

     rc = sqlite3PagerOpen(pVfs, &pBt->pPager, zFilename,

                           EXTRA_SIZE, flags, vfsFlags);

     if( rc==SQLITE_OK ){

       rc = sqlite3PagerReadFileheader(pBt->pPager,sizeof(zDbHeader),zDbHeader);

     }

     if( rc!=SQLITE_OK ){

       goto btree_open_out;

     }

     sqlite3PagerSetBusyhandler(pBt->pPager, &pBt->busyHdr);

     p->pBt = pBt;

     sqlite3PagerSetDestructor(pBt->pPager, pageDestructor);

     sqlite3PagerSetReiniter(pBt->pPager, pageReinit);

     pBt->pCursor = 0;

     pBt->pPage1 = 0;

     pBt->readOnly = sqlite3PagerIsreadonly(pBt->pPager);

     pBt->pageSize = get2byte(&zDbHeader[16]);

     if( pBt->pageSize<512 || pBt->pageSize>SQLITE_MAX_PAGE_SIZE

          || ((pBt->pageSize-1)&pBt->pageSize)!=0 ){

       pBt->pageSize = 0;

       sqlite3PagerSetPagesize(pBt->pPager, &pBt->pageSize);

 #ifndef SQLITE_OMIT_AUTOVACUUM

       /* If the magic name ":memory:" will create an in-memory database, then

       ** leave the autoVacuum mode at 0 (do not auto-vacuum), even if

       ** SQLITE_DEFAULT_AUTOVACUUM is true. On the other hand, if

       ** SQLITE_OMIT_MEMORYDB has been defined, then ":memory:" is just a

       ** regular file-name. In this case the auto-vacuum applies as per normal.

       */

       if( zFilename && !isMemdb ){

         pBt->autoVacuum = (SQLITE_DEFAULT_AUTOVACUUM ? 1 : 0);

         pBt->incrVacuum = (SQLITE_DEFAULT_AUTOVACUUM==2 ? 1 : 0);

       }

 #endif

       nReserve = 0;

     }else{

       nReserve = zDbHeader[20];

       pBt->pageSizeFixed = 1;

 #ifndef SQLITE_OMIT_AUTOVACUUM

       pBt->autoVacuum = (get4byte(&zDbHeader[36 + 4*4])?1:0);

       pBt->incrVacuum = (get4byte(&zDbHeader[36 + 7*4])?1:0);

 #endif

     }

     pBt->usableSize = pBt->pageSize - nReserve;

     assert( (pBt->pageSize & 7)==0 );  /* 8-byte alignment of pageSize */

     sqlite3PagerSetPagesize(pBt->pPager, &pBt->pageSize);

 #if !defined(SQLITE_OMIT_SHARED_CACHE) && !defined(SQLITE_OMIT_DISKIO)

     /* Add the new BtShared object to the linked list sharable BtShareds.

     */

     if( p->sharable ){

       sqlite3_mutex *mutexShared;

       pBt->nRef = 1;

       mutexShared = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MASTER);

       if( SQLITE_THREADSAFE && sqlite3Config.bCoreMutex ){

         pBt->mutex = sqlite3MutexAlloc(SQLITE_MUTEX_FAST);

         if( pBt->mutex==0 ){

           rc = SQLITE_NOMEM;

           db->mallocFailed = 0;

           goto btree_open_out;

         }

       }

       sqlite3_mutex_enter(mutexShared);

       pBt->pNext = sqlite3SharedCacheList;

       sqlite3SharedCacheList = pBt;

       sqlite3_mutex_leave(mutexShared);

     }

 #endif

   }

 #if !defined(SQLITE_OMIT_SHARED_CACHE) && !defined(SQLITE_OMIT_DISKIO)

   /* If the new Btree uses a sharable pBtShared, then link the new

   ** Btree into the list of all sharable Btrees for the same connection.

   ** The list is kept in ascending order by pBt address.

   */

   if( p->sharable ){

     int i;

     Btree *pSib;

     for(i=0; i<db->nDb; i++){

       if( (pSib = db->aDb[i].pBt)!=0 && pSib->sharable ){

         while( pSib->pPrev ){ pSib = pSib->pPrev; }

         if( p->pBt<pSib->pBt ){

           p->pNext = pSib;

           p->pPrev = 0;

           pSib->pPrev = p;

         }else{

           while( pSib->pNext && pSib->pNext->pBt<p->pBt ){

             pSib = pSib->pNext;

           }

           p->pNext = pSib->pNext;

           p->pPrev = pSib;

           if( p->pNext ){

             p->pNext->pPrev = p;

           }

           pSib->pNext = p;

         }

         break;

       }

     }

   }

 #endif

   *ppBtree = p;

 btree_open_out:

   if( rc!=SQLITE_OK ){

     if( pBt && pBt->pPager ){

       sqlite3PagerClose(pBt->pPager);

     }

     sqlite3_free(pBt);

     sqlite3_free(p);

     *ppBtree = 0;

   }

   return rc;

 }

 /*

 ** Decrement the BtShared.nRef counter.  When it reaches zero,

 ** remove the BtShared structure from the sharing list.  Return

 ** true if the BtShared.nRef counter reaches zero and return

 ** false if it is still positive.

 */

 static int removeFromSharingList(BtShared *pBt){

 #ifndef SQLITE_OMIT_SHARED_CACHE

   sqlite3_mutex *pMaster;

   BtShared *pList;

   int removed = 0;

   assert( sqlite3_mutex_notheld(pBt->mutex) );

   pMaster = sqlite3MutexAlloc(SQLITE_MUTEX_STATIC_MASTER);

   sqlite3_mutex_enter(pMaster);

   pBt->nRef--;

   if( pBt->nRef<=0 ){

     if( sqlite3SharedCacheList==pBt ){

       sqlite3SharedCacheList = pBt->pNext;

     }else{

       pList = sqlite3SharedCacheList;

       while( ALWAYS(pList) && pList->pNext!=pBt ){

         pList=pList->pNext;

       }

       if( ALWAYS(pList) ){

         pList->pNext = pBt->pNext;

       }

     }

     if( SQLITE_THREADSAFE ){

       sqlite3_mutex_free(pBt->mutex);

     }

     removed = 1;

   }

   sqlite3_mutex_leave(pMaster);

   return removed;

 #else

   return 1;

 #endif

 }

 /*

 ** Make sure pBt->pTmpSpace points to an allocation of 

 ** MX_CELL_SIZE(pBt) bytes.

 */

 static void allocateTempSpace(BtShared *pBt){

   if( !pBt->pTmpSpace ){

     pBt->pTmpSpace = sqlite3PageMalloc( pBt->pageSize );

   }

 }

 /*

 ** Free the pBt->pTmpSpace allocation

 */

 static void freeTempSpace(BtShared *pBt){

   sqlite3PageFree( pBt->pTmpSpace);

   pBt->pTmpSpace = 0;

 }

 /*

 ** Close an open database and invalidate all cursors.

 */

 int sqlite3BtreeClose(Btree *p){

   BtShared *pBt = p->pBt;

   BtCursor *pCur;

   /* Close all cursors opened via this handle.  */

   assert( sqlite3_mutex_held(p->db->mutex) );

   sqlite3BtreeEnter(p);

   pBt->db = p->db;

   pCur = pBt->pCursor;

   while( pCur ){

     BtCursor *pTmp = pCur;

     pCur = pCur->pNext;

     if( pTmp->pBtree==p ){

       sqlite3BtreeCloseCursor(pTmp);

     }

   }

   /* Rollback any active transaction and free the handle structure.

   ** The call to sqlite3BtreeRollback() drops any table-locks held by

   ** this handle.

   */

   sqlite3BtreeRollback(p);

   sqlite3BtreeLeave(p);

   /* If there are still other outstanding references to the shared-btree

   ** structure, return now. The remainder of this procedure cleans 

   ** up the shared-btree.

   */

   assert( p->wantToLock==0 && p->locked==0 );

   if( !p->sharable || removeFromSharingList(pBt) ){

     /* The pBt is no longer on the sharing list, so we can access

     ** it without having to hold the mutex.

     **

     ** Clean out and delete the BtShared object.

     */

     assert( !pBt->pCursor );

     sqlite3PagerClose(pBt->pPager);

     if( pBt->xFreeSchema && pBt->pSchema ){

       pBt->xFreeSchema(pBt->pSchema);

     }

     sqlite3_free(pBt->pSchema);

     freeTempSpace(pBt);

     sqlite3_free(pBt);

   }

 #ifndef SQLITE_OMIT_SHARED_CACHE

   assert( p->wantToLock==0 );

   assert( p->locked==0 );

   if( p->pPrev ) p->pPrev->pNext = p->pNext;

   if( p->pNext ) p->pNext->pPrev = p->pPrev;

 #endif

   sqlite3_free(p);

   return SQLITE_OK;

 }

 /*

 ** Change the limit on the number of pages allowed in the cache.

 **

 ** The maximum number of cache pages is set to the absolute

 ** value of mxPage.  If mxPage is negative, the pager will

 ** operate asynchronously - it will not stop to do fsync()s

 ** to insure data is written to the disk surface before

 ** continuing.  Transactions still work if synchronous is off,

 ** and the database cannot be corrupted if this program

 ** crashes.  But if the operating system crashes or there is

 ** an abrupt power failure when synchronous is off, the database

 ** could be left in an inconsistent and unrecoverable state.

 ** Synchronous is on by default so database corruption is not

 ** normally a worry.

 */

 int sqlite3BtreeSetCacheSize(Btree *p, int mxPage){

   BtShared *pBt = p->pBt;

   assert( sqlite3_mutex_held(p->db->mutex) );

   sqlite3BtreeEnter(p);

   sqlite3PagerSetCachesize(pBt->pPager, mxPage);

   sqlite3BtreeLeave(p);

   return SQLITE_OK;

 }

 /*

 ** Change the way data is synced to disk in order to increase or decrease

 ** how well the database resists damage due to OS crashes and power

 ** failures.  Level 1 is the same as asynchronous (no syncs() occur and

 ** there is a high probability of damage)  Level 2 is the default.  There

 ** is a very low but non-zero probability of damage.  Level 3 reduces the

 ** probability of damage to near zero but with a write performance reduction.

 */

 #ifndef SQLITE_OMIT_PAGER_PRAGMAS

 int sqlite3BtreeSetSafetyLevel(Btree *p, int level, int fullSync){

   BtShared *pBt = p->pBt;

   assert( sqlite3_mutex_held(p->db->mutex) );

   sqlite3BtreeEnter(p);

   sqlite3PagerSetSafetyLevel(pBt->pPager, level, fullSync);

   sqlite3BtreeLeave(p);

   return SQLITE_OK;

 }

 #endif

 /*

 ** Return TRUE if the given btree is set to safety level 1.  In other

 ** words, return TRUE if no sync() occurs on the disk files.

 */

 int sqlite3BtreeSyncDisabled(Btree *p){

   BtShared *pBt = p->pBt;

   int rc;

   assert( sqlite3_mutex_held(p->db->mutex) );  

   sqlite3BtreeEnter(p);

   assert( pBt && pBt->pPager );

   rc = sqlite3PagerNosync(pBt->pPager);

   sqlite3BtreeLeave(p);

   return rc;

 }

 #if !defined(SQLITE_OMIT_PAGER_PRAGMAS) || !defined(SQLITE_OMIT_VACUUM)

 /*

 ** Change the default pages size and the number of reserved bytes per page.

 **

 ** The page size must be a power of 2 between 512 and 65536.  If the page

 ** size supplied does not meet this constraint then the page size is not

 ** changed.

 **

 ** Page sizes are constrained to be a power of two so that the region

 ** of the database file used for locking (beginning at PENDING_BYTE,

 ** the first byte past the 1GB boundary, 0x40000000) needs to occur

 ** at the beginning of a page.

 **

 ** If parameter nReserve is less than zero, then the number of reserved

 ** bytes per page is left unchanged.

 */

 int sqlite3BtreeSetPageSize(Btree *p, int pageSize, int nReserve){

   int rc = SQLITE_OK;

   BtShared *pBt = p->pBt;

   sqlite3BtreeEnter(p);

   if( pBt->pageSizeFixed ){

     sqlite3BtreeLeave(p);

     return SQLITE_READONLY;

   }

   if( nReserve<0 ){

     nReserve = pBt->pageSize - pBt->usableSize;

   }

   if( pageSize>=512 && pageSize<=SQLITE_MAX_PAGE_SIZE &&

         ((pageSize-1)&pageSize)==0 ){

     assert( (pageSize & 7)==0 );

     assert( !pBt->pPage1 && !pBt->pCursor );

     pBt->pageSize = pageSize;

     freeTempSpace(pBt);

     rc = sqlite3PagerSetPagesize(pBt->pPager, &pBt->pageSize);

   }

   pBt->usableSize = pBt->pageSize - nReserve;

   sqlite3BtreeLeave(p);

   return rc;

 }

 /*

 ** Return the currently defined page size

 */

 int sqlite3BtreeGetPageSize(Btree *p){

   return p->pBt->pageSize;

 }

 int sqlite3BtreeGetReserve(Btree *p){

   int n;

   sqlite3BtreeEnter(p);

   n = p->pBt->pageSize - p->pBt->usableSize;

   sqlite3BtreeLeave(p);

   return n;

 }

 /*

 ** Set the maximum page count for a database if mxPage is positive.

 ** No changes are made if mxPage is 0 or negative.

 ** Regardless of the value of mxPage, return the maximum page count.

 */

 int sqlite3BtreeMaxPageCount(Btree *p, int mxPage){

   int n;

   sqlite3BtreeEnter(p);

   n = sqlite3PagerMaxPageCount(p->pBt->pPager, mxPage);

   sqlite3BtreeLeave(p);

   return n;

 }

 #endif /* !defined(SQLITE_OMIT_PAGER_PRAGMAS) || !defined(SQLITE_OMIT_VACUUM) */

 /*

 ** Change the 'auto-vacuum' property of the database. If the 'autoVacuum'

 ** parameter is non-zero, then auto-vacuum mode is enabled. If zero, it

 ** is disabled. The default value for the auto-vacuum property is 

 ** determined by the SQLITE_DEFAULT_AUTOVACUUM macro.

 */

 int sqlite3BtreeSetAutoVacuum(Btree *p, int autoVacuum){

 #ifdef SQLITE_OMIT_AUTOVACUUM

   return SQLITE_READONLY;

 #else

   BtShared *pBt = p->pBt;

   int rc = SQLITE_OK;

   int av = (autoVacuum?1:0);

   sqlite3BtreeEnter(p);

   if( pBt->pageSizeFixed && av!=pBt->autoVacuum ){

     rc = SQLITE_READONLY;

   }else{

     pBt->autoVacuum = av;

   }

   sqlite3BtreeLeave(p);

   return rc;

 #endif

 }

 /*

 ** Return the value of the 'auto-vacuum' property. If auto-vacuum is 

 ** enabled 1 is returned. Otherwise 0.

 */

 int sqlite3BtreeGetAutoVacuum(Btree *p){

 #ifdef SQLITE_OMIT_AUTOVACUUM

   return BTREE_AUTOVACUUM_NONE;

 #else

   int rc;

   sqlite3BtreeEnter(p);

   rc = (

     (!p->pBt->autoVacuum)?BTREE_AUTOVACUUM_NONE:

     (!p->pBt->incrVacuum)?BTREE_AUTOVACUUM_FULL:

     BTREE_AUTOVACUUM_INCR

   );

   sqlite3BtreeLeave(p);

   return rc;

 #endif

 }

 /*

 ** Get a reference to pPage1 of the database file.  This will

 ** also acquire a readlock on that file.

 **

 ** SQLITE_OK is returned on success.  If the file is not a

 ** well-formed database file, then SQLITE_CORRUPT is returned.

 ** SQLITE_BUSY is returned if the database is locked.  SQLITE_NOMEM

 ** is returned if we run out of memory. 

 */

 static int lockBtree(BtShared *pBt){

   int rc;

   MemPage *pPage1;

   int nPage;

   assert( sqlite3_mutex_held(pBt->mutex) );

   if( pBt->pPage1 ) return SQLITE_OK;

   rc = sqlite3BtreeGetPage(pBt, 1, &pPage1, 0);

   if( rc!=SQLITE_OK ) return rc;

   /* Do some checking to help insure the file we opened really is

   ** a valid database file. 

   */

   rc = sqlite3PagerPagecount(pBt->pPager, &nPage);

   if( rc!=SQLITE_OK ){

     goto page1_init_failed;

   }else if( nPage>0 ){

     int pageSize;

     int usableSize;

     u8 *page1 = pPage1->aData;

     rc = SQLITE_NOTADB;

     if( memcmp(page1, zMagicHeader, 16)!=0 ){

       goto page1_init_failed;

     }

     if( page1[18]>1 ){

       pBt->readOnly = 1;

     }

     if( page1[19]>1 ){

       goto page1_init_failed;

     }

     /* The maximum embedded fraction must be exactly 25%.  And the minimum

     ** embedded fraction must be 12.5% for both leaf-data and non-leaf-data.

     ** The original design allowed these amounts to vary, but as of

     ** version 3.6.0, we require them to be fixed.

     */

     if( memcmp(&page1[21], "\100\040\040",3)!=0 ){

       goto page1_init_failed;

     }

     pageSize = get2byte(&page1[16]);

     if( ((pageSize-1)&pageSize)!=0 || pageSize<512 ||

         (SQLITE_MAX_PAGE_SIZE<32768 && pageSize>SQLITE_MAX_PAGE_SIZE)

     ){

       goto page1_init_failed;

     }

     assert( (pageSize & 7)==0 );

     usableSize = pageSize - page1[20];

     if( pageSize!=pBt->pageSize ){

       /* After reading the first page of the database assuming a page size

       ** of BtShared.pageSize, we have discovered that the page-size is

       ** actually pageSize. Unlock the database, leave pBt->pPage1 at

       ** zero and return SQLITE_OK. The caller will call this function

       ** again with the correct page-size.

       */

       releasePage(pPage1);

       pBt->usableSize = usableSize;

       pBt->pageSize = pageSize;

       freeTempSpace(pBt);

       sqlite3PagerSetPagesize(pBt->pPager, &pBt->pageSize);

       return SQLITE_OK;

     }

     if( usableSize<500 ){

       goto page1_init_failed;

     }

     pBt->pageSize = pageSize;

     pBt->usableSize = usableSize;

 #ifndef SQLITE_OMIT_AUTOVACUUM

     pBt->autoVacuum = (get4byte(&page1[36 + 4*4])?1:0);

     pBt->incrVacuum = (get4byte(&page1[36 + 7*4])?1:0);

 #endif

   }

   /* maxLocal is the maximum amount of payload to store locally for

   ** a cell.  Make sure it is small enough so that at least minFanout

   ** cells can will fit on one page.  We assume a 10-byte page header.

   ** Besides the payload, the cell must store:

   **     2-byte pointer to the cell

   **     4-byte child pointer

   **     9-byte nKey value

   **     4-byte nData value

   **     4-byte overflow page pointer

   ** So a cell consists of a 2-byte poiner, a header which is as much as

   ** 17 bytes long, 0 to N bytes of payload, and an optional 4 byte overflow

   ** page pointer.

   */

   pBt->maxLocal = (pBt->usableSize-12)*64/255 - 23;

   pBt->minLocal = (pBt->usableSize-12)*32/255 - 23;

   pBt->maxLeaf = pBt->usableSize - 35;

   pBt->minLeaf = (pBt->usableSize-12)*32/255 - 23;

   assert( pBt->maxLeaf + 23 <= MX_CELL_SIZE(pBt) );

   pBt->pPage1 = pPage1;

   return SQLITE_OK;

 page1_init_failed:

   releasePage(pPage1);

   pBt->pPage1 = 0;

   return rc;

 }

 /*

 ** This routine works like lockBtree() except that it also invokes the

 ** busy callback if there is lock contention.

 */

 static int lockBtreeWithRetry(Btree *pRef){

   int rc = SQLITE_OK;

   assert( sqlite3BtreeHoldsMutex(pRef) );

   if( pRef->inTrans==TRANS_NONE ){

     u8 inTransaction = pRef->pBt->inTransaction;

     btreeIntegrity(pRef);

     rc = sqlite3BtreeBeginTrans(pRef, 0);

     pRef->pBt->inTransaction = inTransaction;

     pRef->inTrans = TRANS_NONE;

     if( rc==SQLITE_OK ){

       pRef->pBt->nTransaction--;

     }

     btreeIntegrity(pRef);

   }

   return rc;

 }

 /*

 ** If there are no outstanding cursors and we are not in the middle

 ** of a transaction but there is a read lock on the database, then

 ** this routine unrefs the first page of the database file which 

 ** has the effect of releasing the read lock.

 **

 ** If there are any outstanding cursors, this routine is a no-op.

 **

 ** If there is a transaction in progress, this routine is a no-op.

 */

 static void unlockBtreeIfUnused(BtShared *pBt){

   assert( sqlite3_mutex_held(pBt->mutex) );

   if( pBt->inTransaction==TRANS_NONE && pBt->pCursor==0 && pBt->pPage1!=0 ){

     if( sqlite3PagerRefcount(pBt->pPager)>=1 ){

       assert( pBt->pPage1->aData );

 #if 0

       if( pBt->pPage1->aData==0 ){

         MemPage *pPage = pBt->pPage1;

         pPage->aData = sqlite3PagerGetData(pPage->pDbPage);

         pPage->pBt = pBt;

         pPage->pgno = 1;

       }

 #endif

       releasePage(pBt->pPage1);

     }

     pBt->pPage1 = 0;

     pBt->inStmt = 0;

   }

 }

 /*

 ** Create a new database by initializing the first page of the

 ** file.

 */

 static int newDatabase(BtShared *pBt){

   MemPage *pP1;

   unsigned char *data;

   int rc;

   int nPage;

   assert( sqlite3_mutex_held(pBt->mutex) );

   rc = sqlite3PagerPagecount(pBt->pPager, &nPage);

   if( rc!=SQLITE_OK || nPage>0 ){

     return rc;

   }

   pP1 = pBt->pPage1;

   assert( pP1!=0 );

   data = pP1->aData;

   rc = sqlite3PagerWrite(pP1->pDbPage);

   if( rc ) return rc;

   memcpy(data, zMagicHeader, sizeof(zMagicHeader));

   assert( sizeof(zMagicHeader)==16 );

   put2byte(&data[16], pBt->pageSize);

   data[18] = 1;

   data[19] = 1;

   data[20] = pBt->pageSize - pBt->usableSize;

   data[21] = 64;

   data[22] = 32;

   data[23] = 32;

   memset(&data[24], 0, 100-24);

   zeroPage(pP1, PTF_INTKEY|PTF_LEAF|PTF_LEAFDATA );

   pBt->pageSizeFixed = 1;

 #ifndef SQLITE_OMIT_AUTOVACUUM

   assert( pBt->autoVacuum==1 || pBt->autoVacuum==0 );

   assert( pBt->incrVacuum==1 || pBt->incrVacuum==0 );

   put4byte(&data[36 + 4*4], pBt->autoVacuum);

   put4byte(&data[36 + 7*4], pBt->incrVacuum);

 #endif

   return SQLITE_OK;

 }

 /*

 ** Attempt to start a new transaction. A write-transaction

 ** is started if the second argument is nonzero, otherwise a read-

 ** transaction.  If the second argument is 2 or more and exclusive

 ** transaction is started, meaning that no other process is allowed

 ** to access the database.  A preexisting transaction may not be

 ** upgraded to exclusive by calling this routine a second time - the

 ** exclusivity flag only works for a new transaction.

 **

 ** A write-transaction must be started before attempting any 

 ** changes to the database.  None of the following routines 

 ** will work unless a transaction is started first:

 **

 **      sqlite3BtreeCreateTable()

 **      sqlite3BtreeCreateIndex()

 **      sqlite3BtreeClearTable()

 **      sqlite3BtreeDropTable()

 **      sqlite3BtreeInsert()

 **      sqlite3BtreeDelete()

 **      sqlite3BtreeUpdateMeta()

 **

 ** If an initial attempt to acquire the lock fails because of lock contention

 ** and the database was previously unlocked, then invoke the busy handler

 ** if there is one.  But if there was previously a read-lock, do not

 ** invoke the busy handler - just return SQLITE_BUSY.  SQLITE_BUSY is 

 ** returned when there is already a read-lock in order to avoid a deadlock.

 **

 ** Suppose there are two processes A and B.  A has a read lock and B has

 ** a reserved lock.  B tries to promote to exclusive but is blocked because

 ** of A's read lock.  A tries to promote to reserved but is blocked by B.

 ** One or the other of the two processes must give way or there can be

 ** no progress.  By returning SQLITE_BUSY and not invoking the busy callback

 ** when A already has a read lock, we encourage A to give up and let B

 ** proceed.

 */

 int sqlite3BtreeBeginTrans(Btree *p, int wrflag){

   BtShared *pBt = p->pBt;

   int rc = SQLITE_OK;

   sqlite3BtreeEnter(p);

   pBt->db = p->db;

   btreeIntegrity(p);

   /* If the btree is already in a write-transaction, or it

   ** is already in a read-transaction and a read-transaction

   ** is requested, this is a no-op.

   */

   if( p->inTrans==TRANS_WRITE || (p->inTrans==TRANS_READ && !wrflag) ){

     goto trans_begun;

   }

   /* Write transactions are not possible on a read-only database */

   if( pBt->readOnly && wrflag ){

     rc = SQLITE_READONLY;

     goto trans_begun;

   }

   /* If another database handle has already opened a write transaction 

   ** on this shared-btree structure and a second write transaction is

   ** requested, return SQLITE_BUSY.

   */

   if( pBt->inTransaction==TRANS_WRITE && wrflag ){

     rc = SQLITE_BUSY;

     goto trans_begun;

   }

 #ifndef SQLITE_OMIT_SHARED_CACHE

   if( wrflag>1 ){

     BtLock *pIter;

     for(pIter=pBt->pLock; pIter; pIter=pIter->pNext){

       if( pIter->pBtree!=p ){

         rc = SQLITE_BUSY;

         goto trans_begun;

       }

     }

   }

 #endif

   do {

     if( pBt->pPage1==0 ){

       do{

         rc = lockBtree(pBt);

       }while( pBt->pPage1==0 && rc==SQLITE_OK );

     }

     if( rc==SQLITE_OK && wrflag ){

       if( pBt->readOnly ){

         rc = SQLITE_READONLY;

       }else{

         rc = sqlite3PagerBegin(pBt->pPage1->pDbPage, wrflag>1);

         if( rc==SQLITE_OK ){

           rc = newDatabase(pBt);

         }

       }

     }

     if( rc==SQLITE_OK ){

       if( wrflag ) pBt->inStmt = 0;

     }else{

       unlockBtreeIfUnused(pBt);

     }

   }while( rc==SQLITE_BUSY && pBt->inTransaction==TRANS_NONE &&

           sqlite3BtreeInvokeBusyHandler(pBt, 0) );

   if( rc==SQLITE_OK ){

     if( p->inTrans==TRANS_NONE ){

       pBt->nTransaction++;

     }

     p->inTrans = (wrflag?TRANS_WRITE:TRANS_READ);

     if( p->inTrans>pBt->inTransaction ){

       pBt->inTransaction = p->inTrans;

     }

 #ifndef SQLITE_OMIT_SHARED_CACHE

     if( wrflag>1 ){

       assert( !pBt->pExclusive );

       pBt->pExclusive = p;

     }

 #endif

   }

 trans_begun:

   btreeIntegrity(p);

   sqlite3BtreeLeave(p);

   return rc;

 }

 /*

 ** Return the size of the database file in pages.  Or return -1 if

 ** there is any kind of error.

 */

 static int pagerPagecount(Pager *pPager){

   int rc;

   int nPage;

   rc = sqlite3PagerPagecount(pPager, &nPage);

   return (rc==SQLITE_OK?nPage:-1);

 }

 #ifndef SQLITE_OMIT_AUTOVACUUM

 /*

 ** Set the pointer-map entries for all children of page pPage. Also, if

 ** pPage contains cells that point to overflow pages, set the pointer

 ** map entries for the overflow pages as well.

 */

 static int setChildPtrmaps(MemPage *pPage){

   int i;                             /* Counter variable */

   int nCell;                         /* Number of cells in page pPage */

   int rc;                            /* Return code */

   BtShared *pBt = pPage->pBt;

   int isInitOrig = pPage->isInit;

   Pgno pgno = pPage->pgno;

   assert( sqlite3_mutex_held(pPage->pBt->mutex) );

   rc = sqlite3BtreeInitPage(pPage, pPage->pParent);

   if( rc!=SQLITE_OK ){

     goto set_child_ptrmaps_out;

   }

   nCell = pPage->nCell;

   for(i=0; i<nCell; i++){

     u8 *pCell = findCell(pPage, i);

     rc = ptrmapPutOvflPtr(pPage, pCell);

     if( rc!=SQLITE_OK ){

       goto set_child_ptrmaps_out;

     }

     if( !pPage->leaf ){

       Pgno childPgno = get4byte(pCell);

       rc = ptrmapPut(pBt, childPgno, PTRMAP_BTREE, pgno);

        if( rc!=SQLITE_OK ) goto set_child_ptrmaps_out;

     }

   }

   if( !pPage->leaf ){

     Pgno childPgno = get4byte(&pPage->aData[pPage->hdrOffset+8]);

     rc = ptrmapPut(pBt, childPgno, PTRMAP_BTREE, pgno);

   }

 set_child_ptrmaps_out:

   pPage->isInit = isInitOrig;

   return rc;

 }

 /*

 ** Somewhere on pPage, which is guarenteed to be a btree page, not an overflow

 ** page, is a pointer to page iFrom. Modify this pointer so that it points to

 ** iTo. Parameter eType describes the type of pointer to be modified, as 

 ** follows:

 **

 ** PTRMAP_BTREE:     pPage is a btree-page. The pointer points at a child 

 **                   page of pPage.

 **

 ** PTRMAP_OVERFLOW1: pPage is a btree-page. The pointer points at an overflow

 **                   page pointed to by one of the cells on pPage.

 **

 ** PTRMAP_OVERFLOW2: pPage is an overflow-page. The pointer points at the next

 **                   overflow page in the list.

 */

 static int modifyPagePointer(MemPage *pPage, Pgno iFrom, Pgno iTo, u8 eType){

   assert( sqlite3_mutex_held(pPage->pBt->mutex) );

   if( eType==PTRMAP_OVERFLOW2 ){

     /* The pointer is always the first 4 bytes of the page in this case.  */

     if( get4byte(pPage->aData)!=iFrom ){

       return SQLITE_CORRUPT_BKPT;

     }

     put4byte(pPage->aData, iTo);

   }else{

     int isInitOrig = pPage->isInit;

     int i;

     int nCell;

     sqlite3BtreeInitPage(pPage, 0);

     nCell = pPage->nCell;

     for(i=0; i<nCell; i++){

       u8 *pCell = findCell(pPage, i);

       if( eType==PTRMAP_OVERFLOW1 ){

         CellInfo info;

         sqlite3BtreeParseCellPtr(pPage, pCell, &info);

         if( info.iOverflow ){

           if( iFrom==get4byte(&pCell[info.iOverflow]) ){

             put4byte(&pCell[info.iOverflow], iTo);

             break;

           }

         }

       }else{

         if( get4byte(pCell)==iFrom ){

           put4byte(pCell, iTo);

           break;

         }

       }

     }

     if( i==nCell ){

       if( eType!=PTRMAP_BTREE || 

           get4byte(&pPage->aData[pPage->hdrOffset+8])!=iFrom ){

         return SQLITE_CORRUPT_BKPT;

       }

       put4byte(&pPage->aData[pPage->hdrOffset+8], iTo);

     }

     pPage->isInit = isInitOrig;

   }

   return SQLITE_OK;

 }

 /*

 ** Move the open database page pDbPage to location iFreePage in the 

 ** database. The pDbPage reference remains valid.

 */

 static int relocatePage(

   BtShared *pBt,           /* Btree */

   MemPage *pDbPage,        /* Open page to move */

   u8 eType,                /* Pointer map 'type' entry for pDbPage */

   Pgno iPtrPage,           /* Pointer map 'page-no' entry for pDbPage */

   Pgno iFreePage,          /* The location to move pDbPage to */

   int isCommit

 ){

   MemPage *pPtrPage;   /* The page that contains a pointer to pDbPage */

   Pgno iDbPage = pDbPage->pgno;

   Pager *pPager = pBt->pPager;

   int rc;

   assert( eType==PTRMAP_OVERFLOW2 || eType==PTRMAP_OVERFLOW1 || 

       eType==PTRMAP_BTREE || eType==PTRMAP_ROOTPAGE );

   assert( sqlite3_mutex_held(pBt->mutex) );

   assert( pDbPage->pBt==pBt );

   /* Move page iDbPage from its current location to page number iFreePage */

   TRACE(("AUTOVACUUM: Moving %d to free page %d (ptr page %d type %d)\n", 

       iDbPage, iFreePage, iPtrPage, eType));

   rc = sqlite3PagerMovepage(pPager, pDbPage->pDbPage, iFreePage, isCommit);

   if( rc!=SQLITE_OK ){

     return rc;

   }

   pDbPage->pgno = iFreePage;

   /* If pDbPage was a btree-page, then it may have child pages and/or cells

   ** that point to overflow pages. The pointer map entries for all these

   ** pages need to be changed.

   **

   ** If pDbPage is an overflow page, then the first 4 bytes may store a

   ** pointer to a subsequent overflow page. If this is the case, then

   ** the pointer map needs to be updated for the subsequent overflow page.

   */

   if( eType==PTRMAP_BTREE || eType==PTRMAP_ROOTPAGE ){

     rc = setChildPtrmaps(pDbPage);

     if( rc!=SQLITE_OK ){

       return rc;

     }

   }else{

     Pgno nextOvfl = get4byte(pDbPage->aData);

     if( nextOvfl!=0 ){

       rc = ptrmapPut(pBt, nextOvfl, PTRMAP_OVERFLOW2, iFreePage);

       if( rc!=SQLITE_OK ){

         return rc;

       }

     }

   }

   /* Fix the database pointer on page iPtrPage that pointed at iDbPage so

   ** that it points at iFreePage. Also fix the pointer map entry for

   ** iPtrPage.

   */

   if( eType!=PTRMAP_ROOTPAGE ){

     rc = sqlite3BtreeGetPage(pBt, iPtrPage, &pPtrPage, 0);

     if( rc!=SQLITE_OK ){

       return rc;

     }

     rc = sqlite3PagerWrite(pPtrPage->pDbPage);

     if( rc!=SQLITE_OK ){

       releasePage(pPtrPage);

       return rc;

     }

     rc = modifyPagePointer(pPtrPage, iDbPage, iFreePage, eType);

     releasePage(pPtrPage);

     if( rc==SQLITE_OK ){

       rc = ptrmapPut(pBt, iFreePage, eType, iPtrPage);

     }

   }

   return rc;

 }

 /* Forward declaration required by incrVacuumStep(). */

 static int allocateBtreePage(BtShared *, MemPage **, Pgno *, Pgno, u8);

 /*

 ** Perform a single step of an incremental-vacuum. If successful,

 ** return SQLITE_OK. If there is no work to do (and therefore no

 ** point in calling this function again), return SQLITE_DONE.

 **

 ** More specificly, this function attempts to re-organize the 

 ** database so that the last page of the file currently in use

 ** is no longer in use.

 **

 ** If the nFin parameter is non-zero, the implementation assumes

 ** that the caller will keep calling incrVacuumStep() until

 ** it returns SQLITE_DONE or an error, and that nFin is the

 ** number of pages the database file will contain after this 

 ** process is complete.

 */

 static int incrVacuumStep(BtShared *pBt, Pgno nFin){

   Pgno iLastPg;             /* Last page in the database */

   Pgno nFreeList;           /* Number of pages still on the free-list */

   assert( sqlite3_mutex_held(pBt->mutex) );

   iLastPg = pBt->nTrunc;

   if( iLastPg==0 ){

     iLastPg = pagerPagecount(pBt->pPager);

   }

   if( !PTRMAP_ISPAGE(pBt, iLastPg) && iLastPg!=PENDING_BYTE_PAGE(pBt) ){

     int rc;

     u8 eType;

     Pgno iPtrPage;

     nFreeList = get4byte(&pBt->pPage1->aData[36]);

     if( nFreeList==0 || nFin==iLastPg ){

       return SQLITE_DONE;

     }

     rc = ptrmapGet(pBt, iLastPg, &eType, &iPtrPage);

     if( rc!=SQLITE_OK ){

       return rc;

     }

     if( eType==PTRMAP_ROOTPAGE ){

       return SQLITE_CORRUPT_BKPT;

     }

     if( eType==PTRMAP_FREEPAGE ){

       if( nFin==0 ){

         /* Remove the page from the files free-list. This is not required

         ** if nFin is non-zero. In that case, the free-list will be

         ** truncated to zero after this function returns, so it doesn't 

         ** matter if it still contains some garbage entries.

         */

         Pgno iFreePg;

         MemPage *pFreePg;

         rc = allocateBtreePage(pBt, &pFreePg, &iFreePg, iLastPg, 1);

         if( rc!=SQLITE_OK ){

           return rc;

         }

         assert( iFreePg==iLastPg );

         releasePage(pFreePg);

       }

     } else {

       Pgno iFreePg;             /* Index of free page to move pLastPg to */

       MemPage *pLastPg;

       rc = sqlite3BtreeGetPage(pBt, iLastPg, &pLastPg, 0);

       if( rc!=SQLITE_OK ){

         return rc;

       }

       /* If nFin is zero, this loop runs exactly once and page pLastPg

       ** is swapped with the first free page pulled off the free list.

       **

       ** On the other hand, if nFin is greater than zero, then keep

       ** looping until a free-page located within the first nFin pages

       ** of the file is found.

       */

       do {

         MemPage *pFreePg;

         rc = allocateBtreePage(pBt, &pFreePg, &iFreePg, 0, 0);

         if( rc!=SQLITE_OK ){

           releasePage(pLastPg);

           return rc;

         }

         releasePage(pFreePg);

       }while( nFin!=0 && iFreePg>nFin );

       assert( iFreePg<iLastPg );

       rc = sqlite3PagerWrite(pLastPg->pDbPage);

       if( rc==SQLITE_OK ){

         rc = relocatePage(pBt, pLastPg, eType, iPtrPage, iFreePg, nFin!=0);

       }

       releasePage(pLastPg);

       if( rc!=SQLITE_OK ){

         return rc;

       }

     }

   }

   pBt->nTrunc = iLastPg - 1;

   while( pBt->nTrunc==PENDING_BYTE_PAGE(pBt)||PTRMAP_ISPAGE(pBt, pBt->nTrunc) ){

     pBt->nTrunc--;

   }

   return SQLITE_OK;

 }

 /*

 ** A write-transaction must be opened before calling this function.

 ** It performs a single unit of work towards an incremental vacuum.

 **

 ** If the incremental vacuum is finished after this function has run,

 ** SQLITE_DONE is returned. If it is not finished, but no error occured,

 ** SQLITE_OK is returned. Otherwise an SQLite error code. 

 */

 int sqlite3BtreeIncrVacuum(Btree *p){

   int rc;

   BtShared *pBt = p->pBt;

   sqlite3BtreeEnter(p);

   pBt->db = p->db;

   assert( pBt->inTransaction==TRANS_WRITE && p->inTrans==TRANS_WRITE );

   if( !pBt->autoVacuum ){

     rc = SQLITE_DONE;

   }else{

     invalidateAllOverflowCache(pBt);

     rc = incrVacuumStep(pBt, 0);

   }

   sqlite3BtreeLeave(p);

   return rc;

 }

 /*

 ** This routine is called prior to sqlite3PagerCommit when a transaction

 ** is commited for an auto-vacuum database.

 **

 ** If SQLITE_OK is returned, then *pnTrunc is set to the number of pages

 ** the database file should be truncated to during the commit process. 

 ** i.e. the database has been reorganized so that only the first *pnTrunc

 ** pages are in use.

 */

 static int autoVacuumCommit(BtShared *pBt, Pgno *pnTrunc){

   int rc = SQLITE_OK;

   Pager *pPager = pBt->pPager;

 #ifndef NDEBUG

   int nRef = sqlite3PagerRefcount(pPager);

 #endif

   assert( sqlite3_mutex_held(pBt->mutex) );

   invalidateAllOverflowCache(pBt);

   assert(pBt->autoVacuum);

   if( !pBt->incrVacuum ){

     Pgno nFin = 0;

     if( pBt->nTrunc==0 ){

       Pgno nFree;

       Pgno nPtrmap;

       const int pgsz = pBt->pageSize;

       int nOrig = pagerPagecount(pBt->pPager);

       if( PTRMAP_ISPAGE(pBt, nOrig) ){

         return SQLITE_CORRUPT_BKPT;

       }

       if( nOrig==PENDING_BYTE_PAGE(pBt) ){

         nOrig--;

       }

       nFree = get4byte(&pBt->pPage1->aData[36]);

       nPtrmap = (nFree-nOrig+PTRMAP_PAGENO(pBt, nOrig)+pgsz/5)/(pgsz/5);

       nFin = nOrig - nFree - nPtrmap;

       if( nOrig>PENDING_BYTE_PAGE(pBt) && nFin<=PENDING_BYTE_PAGE(pBt) ){

         nFin--;

       }

       while( PTRMAP_ISPAGE(pBt, nFin) || nFin==PENDING_BYTE_PAGE(pBt) ){

         nFin--;

       }

     }

     while( rc==SQLITE_OK ){

       rc = incrVacuumStep(pBt, nFin);

     }

     if( rc==SQLITE_DONE ){

       assert(nFin==0 || pBt->nTrunc==0 || nFin<=pBt->nTrunc);

       rc = SQLITE_OK;

       if( pBt->nTrunc && nFin ){

         rc = sqlite3PagerWrite(pBt->pPage1->pDbPage);

         put4byte(&pBt->pPage1->aData[32], 0);

         put4byte(&pBt->pPage1->aData[36], 0);

         pBt->nTrunc = nFin;

       }

     }

     if( rc!=SQLITE_OK ){

       sqlite3PagerRollback(pPager);

     }

   }

   if( rc==SQLITE_OK ){

     *pnTrunc = pBt->nTrunc;

     pBt->nTrunc = 0;

   }

   assert( nRef==sqlite3PagerRefcount(pPager) );

   return rc;

 }

 #endif

 /*

 ** This routine does the first phase of a two-phase commit.  This routine

 ** causes a rollback journal to be created (if it does not already exist)

 ** and populated with enough information so that if a power loss occurs

 ** the database can be restored to its original state by playing back

 ** the journal.  Then the contents of the journal are flushed out to

 ** the disk.  After the journal is safely on oxide, the changes to the

 ** database are written into the database file and flushed to oxide.

 ** At the end of this call, the rollback journal still exists on the

 ** disk and we are still holding all locks, so the transaction has not

 ** committed.  See sqlite3BtreeCommit() for the second phase of the

 ** commit process.

 **

 ** This call is a no-op if no write-transaction is currently active on pBt.

 **

 ** Otherwise, sync the database file for the btree pBt. zMaster points to

 ** the name of a master journal file that should be written into the

 ** individual journal file, or is NULL, indicating no master journal file 

 ** (single database transaction).

 **

 ** When this is called, the master journal should already have been

 ** created, populated with this journal pointer and synced to disk.

 **

 ** Once this is routine has returned, the only thing required to commit

 ** the write-transaction for this database file is to delete the journal.

 */

 int sqlite3BtreeCommitPhaseOne(Btree *p, const char *zMaster){

   int rc = SQLITE_OK;

   if( p->inTrans==TRANS_WRITE ){

     BtShared *pBt = p->pBt;

     Pgno nTrunc = 0;

     sqlite3BtreeEnter(p);

     pBt->db = p->db;

 #ifndef SQLITE_OMIT_AUTOVACUUM

     if( pBt->autoVacuum ){

       rc = autoVacuumCommit(pBt, &nTrunc); 

       if( rc!=SQLITE_OK ){

         sqlite3BtreeLeave(p);

         return rc;

       }

     }

 #endif

     rc = sqlite3PagerCommitPhaseOne(pBt->pPager, zMaster, nTrunc, 0);

     sqlite3BtreeLeave(p);

   }

   return rc;

 }

 /*

 ** Commit the transaction currently in progress.

 **

 ** This routine implements the second phase of a 2-phase commit.  The

 ** sqlite3BtreeSync() routine does the first phase and should be invoked

 ** prior to calling this routine.  The sqlite3BtreeSync() routine did

 ** all the work of writing information out to disk and flushing the

 ** contents so that they are written onto the disk platter.  All this

 ** routine has to do is delete or truncate the rollback journal

 ** (which causes the transaction to commit) and drop locks.

 **

 ** This will release the write lock on the database file.  If there

 ** are no active cursors, it also releases the read lock.

 */

 int sqlite3BtreeCommitPhaseTwo(Btree *p){

   BtShared *pBt = p->pBt;

   sqlite3BtreeEnter(p);

   pBt->db = p->db;

   btreeIntegrity(p);

   /* If the handle has a write-transaction open, commit the shared-btrees 

   ** transaction and set the shared state to TRANS_READ.

   */

   if( p->inTrans==TRANS_WRITE ){

     int rc;

     assert( pBt->inTransaction==TRANS_WRITE );

     assert( pBt->nTransaction>0 );

     rc = sqlite3PagerCommitPhaseTwo(pBt->pPager);

     if( rc!=SQLITE_OK ){

       sqlite3BtreeLeave(p);

       return rc;

     }

     pBt->inTransaction = TRANS_READ;

     pBt->inStmt = 0;

   }

   unlockAllTables(p);

   /* If the handle has any kind of transaction open, decrement the transaction

   ** count of the shared btree. If the transaction count reaches 0, set

   ** the shared state to TRANS_NONE. The unlockBtreeIfUnused() call below

   ** will unlock the pager.

   */

   if( p->inTrans!=TRANS_NONE ){

     pBt->nTransaction--;

     if( 0==pBt->nTransaction ){

       pBt->inTransaction = TRANS_NONE;

     }

   }

   /* Set the handles current transaction state to TRANS_NONE and unlock

   ** the pager if this call closed the only read or write transaction.

   */

   p->inTrans = TRANS_NONE;

   unlockBtreeIfUnused(pBt);

   btreeIntegrity(p);

   sqlite3BtreeLeave(p);

   return SQLITE_OK;

 }

 /*

 ** Do both phases of a commit.

 */

 int sqlite3BtreeCommit(Btree *p){

   int rc;

   sqlite3BtreeEnter(p);

   rc = sqlite3BtreeCommitPhaseOne(p, 0);

   if( rc==SQLITE_OK ){

     rc = sqlite3BtreeCommitPhaseTwo(p);

   }

   sqlite3BtreeLeave(p);

   return rc;

 }

 #ifndef NDEBUG

 /*

 ** Return the number of write-cursors open on this handle. This is for use

 ** in assert() expressions, so it is only compiled if NDEBUG is not

 ** defined.

 **

 ** For the purposes of this routine, a write-cursor is any cursor that

 ** is capable of writing to the databse.  That means the cursor was

 ** originally opened for writing and the cursor has not be disabled

 ** by having its state changed to CURSOR_FAULT.

 */

 static int countWriteCursors(BtShared *pBt){

   BtCursor *pCur;

   int r = 0;

   for(pCur=pBt->pCursor; pCur; pCur=pCur->pNext){

     if( pCur->wrFlag && pCur->eState!=CURSOR_FAULT ) r++; 

   }

   return r;

 }

 #endif

 /*

 ** This routine sets the state to CURSOR_FAULT and the error

 ** code to errCode for every cursor on BtShared that pBtree

 ** references.

 **

 ** Every cursor is tripped, including cursors that belong

 ** to other database connections that happen to be sharing

 ** the cache with pBtree.

 **

 ** This routine gets called when a rollback occurs.

 ** All cursors using the same cache must be tripped

 ** to prevent them from trying to use the btree after

 ** the rollback.  The rollback may have deleted tables

 ** or moved root pages, so it is not sufficient to

 ** save the state of the cursor.  The cursor must be

 ** invalidated.

 */

 void sqlite3BtreeTripAllCursors(Btree *pBtree, int errCode){

   BtCursor *p;

   sqlite3BtreeEnter(pBtree);

   for(p=pBtree->pBt->pCursor; p; p=p->pNext){

     clearCursorPosition(p);

     p->eState = CURSOR_FAULT;

     p->skip = errCode;

   }

   sqlite3BtreeLeave(pBtree);

 }

 /*

 ** Rollback the transaction in progress.  All cursors will be

 ** invalided by this operation.  Any attempt to use a cursor

 ** that was open at the beginning of this operation will result

 ** in an error.

 **

 ** This will release the write lock on the database file.  If there

 ** are no active cursors, it also releases the read lock.

 */

 int sqlite3BtreeRollback(Btree *p){

   int rc;

   BtShared *pBt = p->pBt;

   MemPage *pPage1;

   sqlite3BtreeEnter(p);

   pBt->db = p->db;

   rc = saveAllCursors(pBt, 0, 0);

 #ifndef SQLITE_OMIT_SHARED_CACHE

   if( rc!=SQLITE_OK ){

     /* This is a horrible situation. An IO or malloc() error occured whilst

     ** trying to save cursor positions. If this is an automatic rollback (as

     ** the result of a constraint, malloc() failure or IO error) then 

     ** the cache may be internally inconsistent (not contain valid trees) so

     ** we cannot simply return the error to the caller. Instead, abort 

     ** all queries that may be using any of the cursors that failed to save.

     */

     sqlite3BtreeTripAllCursors(p, rc);

   }

 #endif

   btreeIntegrity(p);

   unlockAllTables(p);

   if( p->inTrans==TRANS_WRITE ){

     int rc2;

 #ifndef SQLITE_OMIT_AUTOVACUUM

     pBt->nTrunc = 0;

 #endif

     assert( TRANS_WRITE==pBt->inTransaction );

     rc2 = sqlite3PagerRollback(pBt->pPager);

     if( rc2!=SQLITE_OK ){

       rc = rc2;

     }

     /* The rollback may have destroyed the pPage1->aData value.  So

     ** call sqlite3BtreeGetPage() on page 1 again to make

     ** sure pPage1->aData is set correctly. */

     if( sqlite3BtreeGetPage(pBt, 1, &pPage1, 0)==SQLITE_OK ){

       releasePage(pPage1);

     }

     assert( countWriteCursors(pBt)==0 );

     pBt->inTransaction = TRANS_READ;

   }