btree.c

sqlite最新源码

/*
** 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 1.565 2009/02/04 01:49:30 shane 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 list of BtShared objects that are eligible for participation
** in shared cache.  This variable has file scope during normal builds,
** but the test harness needs to access it so we make it global for 
** test builds.
*/
#ifdef SQLITE_TEST
BtShared *SQLITE_WSD sqlite3SharedCacheList = 0;
#else
static BtShared *SQLITE_WSD sqlite3SharedCacheList = 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){
  sqlite3GlobalConfig.sharedCacheEnabled = 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

/*
** Set bit pgno of the BtShared.pHasContent bitvec. This is called 
** when a page that previously contained data becomes a free-list leaf 
** page.
**
** The BtShared.pHasContent bitvec exists to work around an obscure
** bug caused by the interaction of two useful IO optimizations surrounding
** free-list leaf pages:
**
**   1) When all data is deleted from a page and the page becomes
**      a free-list leaf page, the page is not written to the database
**      (as free-list leaf pages contain no meaningful data). Sometimes
**      such a page is not even journalled (as it will not be modified,
**      why bother journalling it?).
**
**   2) When a free-list leaf page is reused, its content is not read
**      from the database or written to the journal file (why should it
**      be, if it is not at all meaningful?).
**
** By themselves, these optimizations work fine and provide a handy
** performance boost to bulk delete or insert operations. However, if
** a page is moved to the free-list and then reused within the same
** transaction, a problem comes up. If the page is not journalled when
** it is moved to the free-list and it is also not journalled when it
** is extracted from the free-list and reused, then the original data
** may be lost. In the event of a rollback, it may not be possible
** to restore the database to its original configuration.
**
** The solution is the BtShared.pHasContent bitvec. Whenever a page is 
** moved to become a free-list leaf page, the corresponding bit is
** set in the bitvec. Whenever a leaf page is extracted from the free-list,
** optimization 2 above is ommitted if the corresponding bit is already
** set in BtShared.pHasContent. The contents of the bitvec are cleared
** at the end of every transaction.
*/
static int btreeSetHasContent(BtShared *pBt, Pgno pgno){
  int rc = SQLITE_OK;
  if( !pBt->pHasContent ){
    int nPage;
    rc = sqlite3PagerPagecount(pBt->pPager, &nPage);
    if( rc==SQLITE_OK ){
      pBt->pHasContent = sqlite3BitvecCreate((u32)nPage);
      if( !pBt->pHasContent ){
        rc = SQLITE_NOMEM;
      }
    }
  }
  if( rc==SQLITE_OK && pgno<=sqlite3BitvecSize(pBt->pHasContent) ){
    rc = sqlite3BitvecSet(pBt->pHasContent, pgno);
  }
  return rc;
}

/*
** Query the BtShared.pHasContent vector.
**
** This function is called when a free-list leaf page is removed from the
** free-list for reuse. It returns false if it is safe to retrieve the
** page from the pager layer with the 'no-content' flag set. True otherwise.
*/
static int btreeGetHasContent(BtShared *pBt, Pgno pgno){
  Bitvec *p = pBt->pHasContent;
  return (p && (pgno>sqlite3BitvecSize(p) || sqlite3BitvecTest(p, pgno)));
}

/*
** Clear (destroy) the BtShared.pHasContent bitvec. This should be
** invoked at the conclusion of each write-transaction.
*/
static void btreeClearHasContent(BtShared *pBt){
  sqlite3BitvecDestroy(pBt->pHasContent);
  pBt->pHasContent = 0;
}

/*
** 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->apPage[0]->intKey){
    void *pKey = sqlite3Malloc( (int)pCur->nKey );
    if( pKey ){
      rc = sqlite3BtreeKey(pCur, 0, (int)pCur->nKey, pKey);
      if( rc==SQLITE_OK ){
        pCur->pKey = pKey;
      }else{
        sqlite3_free(pKey);
      }
    }else{
      rc = SQLITE_NOMEM;
    }
  }
  assert( !pCur->apPage[0]->intKey || !pCur->pKey );

  if( rc==SQLITE_OK ){
    int i;
    for(i=0; i<=pCur->iPage; i++){
      releasePage(pCur->apPage[i]);
      pCur->apPage[i] = 0;
    }
    pCur->iPage = -1;
    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 ){