SQLite

/*
** 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 1.568 2009/02/24 16:18:05 drh 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"

  ** 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 on any table
  ** other than the sqlite_master table (table 1) and if the ReadUncommitted
  ** flag is set, then the lock 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
  ** on non-schema tables 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){

  /* 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 on
  ** a non-schema table, then the lock is always granted.  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

**    May you share freely, never taking more than you give.
**
*************************************************************************
** This file contains the implementation of the sqlite3_prepare()
** interface, and routines that contribute to loading the database schema
** from disk.
**
** $Id: prepare.c,v 1.107 2009/02/24 16:18:05 drh Exp $
*/
#include "sqliteInt.h"

/*
** Fill the InitData structure with an error message that indicates
** that the database is corrupt.
*/

  *ppStmt = 0;
  if( sqlite3SafetyOn(db) ){
    return SQLITE_MISUSE;
  }
  assert( !db->mallocFailed );
  assert( sqlite3_mutex_held(db->mutex) );

  /* Check to verify that it is possible to get a read lock on all
  ** database schemas.  The inability to get a read lock indicates that

  ** some other database connection is holding a write-lock, which in
  ** turn means that the other connection has made uncommitted changes
  ** to the schema.
  **
  ** Were we to proceed and prepare the statement against the uncommitted
  ** schema changes and if those schema changes are subsequently rolled
  ** back and different changes are made in their place, then when this
  ** prepared statement goes to run the schema cookie would fail to detect
  ** the schema change.  Disaster would follow.
  **
  ** This thread is currently holding mutexes on all Btrees (because
  ** of the sqlite3BtreeEnterAll() in sqlite3LockAndPrepare()) so it
  ** is not possible for another thread to start a new schema change
  ** while this routine is running.  Hence, we do not need to hold 
  ** locks on the schema, we just need to make sure nobody else is 
  ** holding them.
  **
  ** Note that setting READ_UNCOMMITTED overrides most lock detection,
  ** but it does *not* override schema lock detection, so this all still
  ** works even if READ_UNCOMMITTED is set.
  */
  for(i=0; i<db->nDb; i++) {
    Btree *pBt = db->aDb[i].pBt;
    if( pBt ){
      assert( sqlite3BtreeHoldsMutex(pBt) );
      rc = sqlite3BtreeSchemaLocked(pBt);
      if( rc ){
        const char *zDb = db->aDb[i].zName;
        sqlite3Error(db, SQLITE_LOCKED, "database schema is locked: %s", zDb);
        (void)sqlite3SafetyOff(db);
        testcase( db->flags & SQLITE_ReadUncommitted );
        return sqlite3ApiExit(db, SQLITE_LOCKED);
      }
    }
  }
  
  memset(&sParse, 0, sizeof(sParse));
  sParse.db = db;