SQLite

static void unlockBtreeMutex(Btree *p){
  BtShared *pBt = p->pBt;
  assert( p->locked==1 );
  assert( sqlite3_mutex_held(pBt->mutex) );
  assert( sqlite3_mutex_held(p->db->mutex) );
  assert( p->db==pBt->db );

  pBt->iMutexCounter++;
  sqlite3_mutex_leave(pBt->mutex);
  p->locked = 0;
}

#ifdef SQLITE_DEBUG
/*
** Return the number of times that the mutex has been exited for
** the given btree.
**
** This is a small circular counter that wraps around to zero on
** overflow.  It is used only for sanity checking - to verify that
** mutexes are held continously by asserting that the value of
** this counter at the beginning of a region is the same as at
** the end.
*/
u32 sqlite3BtreeMutexCounter(Btree *p){
  assert( p->locked==1 || p->sharable==0 );
  return p->pBt->iMutexCounter;
}
#endif

/*
** Enter a mutex on the given BTree object.
**
** If the object is not sharable, then no mutex is ever required
** and this routine is a no-op.  The underlying mutex is non-recursive.
** But we keep a reference count in Btree.wantToLock so the behavior
** of this interface is recursive.

  ** should already be set correctly. */
  assert( (p->locked==0 && p->sharable) || p->pBt->db==p->db );

  if( !p->sharable ) return;
  p->wantToLock++;
  if( p->locked ) return;

  /* Increment the mutex counter on all locked btrees in the same
  ** database connection.  This simulates the unlocking that would
  ** occur on a worst-case mutex dead-lock avoidance scenario.
  */
#ifdef SQLITE_DEBUG
  {
    int ii;
    sqlite3 *db = p->db;
    Btree *pOther;
    for(ii=0; ii<db->nDb; ii++){
      if( ii==1 ) continue;
      pOther = db->aDb[ii].pBt;
      if( pOther==0 || pOther->sharable==0 || pOther->locked==0 ) continue;
      pOther->pBt->iMutexCounter++;
    }
  }
#endif

  /* In most cases, we should be able to acquire the lock we
  ** want without having to go throught the ascending lock
  ** procedure that follows.  Just be sure not to block.
  */
  if( sqlite3_mutex_try(p->pBt->mutex)==SQLITE_OK ){
    p->pBt->db = p->db;
    p->locked = 1;

  void sqlite3BtreeLeaveCursor(BtCursor*);
  void sqlite3BtreeLeaveAll(sqlite3*);
#ifndef NDEBUG
  /* These routines are used inside assert() statements only. */
  int sqlite3BtreeHoldsMutex(Btree*);
  int sqlite3BtreeHoldsAllMutexes(sqlite3*);
  int sqlite3SchemaMutexHeld(sqlite3*,int,Schema*);
  u32 sqlite3BtreeMutexCounter(Btree*);
#endif
#else

# define sqlite3BtreeLeave(X)
# define sqlite3BtreeMutexCounter(X) 0
# define sqlite3BtreeEnterCursor(X)
# define sqlite3BtreeLeaveCursor(X)
# define sqlite3BtreeLeaveAll(X)

# define sqlite3BtreeHoldsMutex(X) 1
# define sqlite3BtreeHoldsAllMutexes(X) 1
# define sqlite3BtreeSchemaMutexHeld(X,Y) 1
#endif


#endif /* _BTREE_H_ */

#ifndef SQLITE_OMIT_SHARED_CACHE
  int nRef;             /* Number of references to this structure */
  BtShared *pNext;      /* Next on a list of sharable BtShared structs */
  BtLock *pLock;        /* List of locks held on this shared-btree struct */
  Btree *pWriter;       /* Btree with currently open write transaction */
  u8 isExclusive;       /* True if pWriter has an EXCLUSIVE lock on the db */
  u8 isPending;         /* If waiting for read-locks to clear */
  u16 iMutexCounter;    /* The number of mutex_leave(mutex) calls */
#endif
  u8 *pTmpSpace;        /* BtShared.pageSize bytes of space for tmp use */
};

/*
** An instance of the following structure is used to hold information
** about a cell.  The parseCellPtr() function fills in this structure

  if( ctx.pFunc->flags & SQLITE_FUNC_NEEDCOLL ){
    assert( pOp>aOp );
    assert( pOp[-1].p4type==P4_COLLSEQ );
    assert( pOp[-1].opcode==OP_CollSeq );
    ctx.pColl = pOp[-1].p4.pColl;
  }
  (*ctx.pFunc->xFunc)(&ctx, n, apVal); /* IMP: R-24505-23230 */
  sqlite3VdbeMutexResync(p);
  if( db->mallocFailed ){
    /* Even though a malloc() has failed, the implementation of the
    ** user function may have called an sqlite3_result_XXX() function
    ** to return a value. The following call releases any resources
    ** associated with such a value.
    */
    sqlite3VdbeMemRelease(&ctx.s);

          if( rc!=SQLITE_OK ){
            goto abort_due_to_error;
          }
        }
        if( p1==SAVEPOINT_ROLLBACK && (db->flags&SQLITE_InternChanges)!=0 ){
          sqlite3ExpirePreparedStatements(db);
          sqlite3ResetInternalSchema(db, -1);
          sqlite3VdbeMutexResync(p);
          db->flags = (db->flags | SQLITE_InternChanges);
        }
      }
  
      /* Regardless of whether this is a RELEASE or ROLLBACK, destroy all 
      ** savepoints nested inside of the savepoint being operated on. */
      while( db->pSavepoint!=pSavepoint ){

  if( ctx.pFunc->flags & SQLITE_FUNC_NEEDCOLL ){
    assert( pOp>p->aOp );
    assert( pOp[-1].p4type==P4_COLLSEQ );
    assert( pOp[-1].opcode==OP_CollSeq );
    ctx.pColl = pOp[-1].p4.pColl;
  }
  (ctx.pFunc->xStep)(&ctx, n, apVal); /* IMP: R-24505-23230 */
  sqlite3VdbeMutexResync(p);
  if( ctx.isError ){
    sqlite3SetString(&p->zErrMsg, db, "%s", sqlite3_value_text(&ctx.s));
    rc = ctx.isError;
  }

  /* The app-defined function has done something that as caused this
  ** statement to expire.  (Perhaps the function called sqlite3_exec()
  ** with a CREATE TABLE statement.)
  */
#if 0
  if( p->expired ){
    rc = SQLITE_ABORT;
    break;
  }
#endif

  sqlite3VdbeMemRelease(&ctx.s);

  break;
}

/* Opcode: AggFinal P1 P2 * P4 *
**

*/
case OP_AggFinal: {
  Mem *pMem;
  assert( pOp->p1>0 && pOp->p1<=p->nMem );
  pMem = &aMem[pOp->p1];
  assert( (pMem->flags & ~(MEM_Null|MEM_Agg))==0 );
  rc = sqlite3VdbeMemFinalize(pMem, pOp->p4.pFunc);
  sqlite3VdbeMutexResync(p);
  if( rc ){
    sqlite3SetString(&p->zErrMsg, db, "%s", sqlite3_value_text(pMem));
  }else if( p->expired ){
    rc = SQLITE_ABORT;
  }
  sqlite3VdbeChangeEncoding(pMem, encoding);
  UPDATE_MAX_BLOBSIZE(pMem);
  if( sqlite3VdbeMemTooBig(pMem) ){
    goto too_big;
  }
  break;

  u8 minWriteFileFormat;  /* Minimum file format for writable database files */
  u8 inVtabMethod;        /* See comments above */
  u8 usesStmtJournal;     /* True if uses a statement journal */
  u8 readOnly;            /* True for read-only statements */
  u8 isPrepareV2;         /* True if prepared with prepare_v2() */
  int nChange;            /* Number of db changes made since last reset */
  yDbMask btreeMask;      /* Bitmask of db->aDb[] entries referenced */
  u32 iMutexCounter;      /* Mutex counter upon sqlite3VdbeEnter() */
  int iStatement;         /* Statement number (or 0 if has not opened stmt) */
  int aCounter[3];        /* Counters used by sqlite3_stmt_status() */
#ifndef SQLITE_OMIT_TRACE
  i64 startTime;          /* Time when query started - used for profiling */
#endif
  i64 nFkConstraint;      /* Number of imm. FK constraints this VM */
  i64 nStmtDefCons;       /* Number of def. constraints when stmt started */

int sqlite3VdbeMemFinalize(Mem*, FuncDef*);
const char *sqlite3OpcodeName(int);
int sqlite3VdbeMemGrow(Mem *pMem, int n, int preserve);
int sqlite3VdbeCloseStatement(Vdbe *, int);
void sqlite3VdbeFrameDelete(VdbeFrame*);
int sqlite3VdbeFrameRestore(VdbeFrame *);
void sqlite3VdbeMemStoreType(Mem *pMem);


void sqlite3VdbeEnter(Vdbe*);
void sqlite3VdbeLeave(Vdbe*);

void sqlite3VdbeMutexResync(Vdbe*);



#ifdef SQLITE_DEBUG
void sqlite3VdbeMemPrepareToChange(Vdbe*,Mem*);
#endif

#ifndef SQLITE_OMIT_FOREIGN_KEY
int sqlite3VdbeCheckFk(Vdbe *, int);

*/
void sqlite3VdbeUsesBtree(Vdbe *p, int i){
  assert( i>=0 && i<p->db->nDb && i<sizeof(yDbMask)*8 );
  assert( i<(int)sizeof(p->btreeMask)*8 );
  p->btreeMask |= ((yDbMask)1)<<i;
}

/*
** Compute the sum of all mutex counters for all btrees in the
** given prepared statement.
*/
#ifndef SQLITE_OMIT_SHARED_CACHE
static u32 mutexCounterSum(Vdbe *p){
  u32 cntSum = 0;
#ifdef SQLITE_DEBUG
  int i;
  yDbMask mask;
  sqlite3 *db = p->db;
  Db *aDb = db->aDb;
  int nDb = db->nDb;
  for(i=0, mask=1; i<nDb; i++, mask += mask){
    if( i!=1 && (mask & p->btreeMask)!=0 && ALWAYS(aDb[i].pBt!=0) ){
      cntSum += sqlite3BtreeMutexCounter(aDb[i].pBt);
    }
  }
#else
  UNUSED_PARAMETER(p);
#endif
  return cntSum;
}
#endif

/*
** If SQLite is compiled to support shared-cache mode and to be threadsafe,
** this routine obtains the mutex associated with each BtShared structure
** that may be accessed by the VM passed as an argument. In doing so it also
** sets the BtShared.db member of each of the BtShared structures, ensuring
** that the correct busy-handler callback is invoked if required.
**

** The p->btreeMask field is a bitmask of all btrees that the prepared 
** statement p will ever use.  Let N be the number of bits in p->btreeMask
** corresponding to btrees that use shared cache.  Then the runtime of
** this routine is N*N.  But as N is rarely more than 1, this should not
** be a problem.
*/
void sqlite3VdbeEnter(Vdbe *p){
#ifndef SQLITE_OMIT_SHARED_CACHE
  int i;
  yDbMask mask;
  sqlite3 *db = p->db;
  Db *aDb = db->aDb;
  int nDb = db->nDb;
  for(i=0, mask=1; i<nDb; i++, mask += mask){
    if( i!=1 && (mask & p->btreeMask)!=0 && ALWAYS(aDb[i].pBt!=0) ){
      sqlite3BtreeEnter(aDb[i].pBt);
    }
  }
  p->iMutexCounter = mutexCounterSum(p);
#else
  UNUSED_PARAMETER(p);

#endif
}

/*
** Unlock all of the btrees previously locked by a call to sqlite3VdbeEnter().
*/
void sqlite3VdbeLeave(Vdbe *p){
#ifndef SQLITE_OMIT_SHARED_CACHE
  int i;
  yDbMask mask;
  sqlite3 *db = p->db;
  Db *aDb = db->aDb;
  int nDb = db->nDb;

  /* Assert that the all mutexes have been held continously since
  ** the most recent sqlite3VdbeEnter() or sqlite3VdbeMutexResync().
  */
  assert( mutexCounterSum(p) == p->iMutexCounter );

  for(i=0, mask=1; i<nDb; i++, mask += mask){
    if( i!=1 && (mask & p->btreeMask)!=0 && ALWAYS(aDb[i].pBt!=0) ){
      sqlite3BtreeLeave(aDb[i].pBt);
    }
  }
#else
  UNUSED_PARAMETER(p);
#endif
}

/*
** Recompute the sum of the mutex counters on all btrees used by the
** prepared statement p.
**
** Call this routine while holding a sqlite3VdbeEnter() after doing something
** that might cause one or more of the individual mutexes held by the
** prepared statement to be released.  Calling sqlite3BtreeEnter() on 
** any BtShared mutex which is not used by the prepared statement is one
** way to cause one or more of the mutexes in the prepared statement
** to be temporarily released.  The anti-deadlocking logic in
** sqlite3BtreeEnter() can cause mutexes to be released temporarily then
** reacquired.
**
** Calling this routine is an acknowledgement that some of the individual
** mutexes in the prepared statement might have been released and reacquired.
** So checks to verify that mutex-protected content did not change
** unexpectedly should accompany any call to this routine.
*/
void sqlite3VdbeMutexResync(Vdbe *p){
#if !defined(SQLITE_OMIT_SHARED_CACHE) && defined(SQLITE_DEBUG)
  p->iMutexCounter = mutexCounterSum(p);
#else
  UNUSED_PARAMETER(p);
#endif
}

#if defined(VDBE_PROFILE) || defined(SQLITE_DEBUG)
/*
** Print a single opcode.  This routine is used for debugging only.
*/
void sqlite3VdbePrintOp(FILE *pOut, int pc, Op *pOp){
  char *zP4;

    /* Rollback or commit any schema changes that occurred. */
    if( p->rc!=SQLITE_OK && db->flags&SQLITE_InternChanges ){
      sqlite3ResetInternalSchema(db, -1);
      db->flags = (db->flags | SQLITE_InternChanges);
    }

    /* Release the locks */
    sqlite3VdbeMutexResync(p);
    sqlite3VdbeLeave(p);
  }

  /* We have successfully halted and closed the VM.  Record this fact. */
  if( p->pc>=0 ){
    db->activeVdbeCnt--;
    if( !p->readOnly ){