/*

** 2007 August 27

**

** 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: btmutex.cpp 1282 2008-11-13 09:31:33Z LarsPson $

**

** This file contains code used to implement mutexes on Btree objects.

** This code really belongs in btree.c.  But btree.c is getting too

** big and we want to break it down some.  This packaged seemed like

** a good breakout.

*/

#include "btreeInt.h"

#if SQLITE_THREADSAFE && !defined(SQLITE_OMIT_SHARED_CACHE)

/*

** 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.

**

** To avoid deadlocks, multiple Btrees are locked in the same order

** by all database connections.  The p->pNext is a list of other

** Btrees belonging to the same database connection as the p Btree

** which need to be locked after p.  If we cannot get a lock on

** p, then first unlock all of the others on p->pNext, then wait

** for the lock to become available on p, then relock all of the

** subsequent Btrees that desire a lock.

*/

void sqlite3BtreeEnter(Btree *p){

  Btree *pLater;

  /* Some basic sanity checking on the Btree.  The list of Btrees

  ** connected by pNext and pPrev should be in sorted order by

  ** Btree.pBt value. All elements of the list should belong to

  ** the same connection. Only shared Btrees are on the list. */

  assert( p->pNext==0 || p->pNext->pBt>p->pBt );

  assert( p->pPrev==0 || p->pPrev->pBt<p->pBt );

  assert( p->pNext==0 || p->pNext->db==p->db );

  assert( p->pPrev==0 || p->pPrev->db==p->db );

  assert( p->sharable || (p->pNext==0 && p->pPrev==0) );

  /* Check for locking consistency */

  assert( !p->locked || p->wantToLock>0 );

  assert( p->sharable || p->wantToLock==0 );

  /* We should already hold a lock on the database connection */

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

  if( !p->sharable ) return;

  p->wantToLock++;

  if( p->locked ) return;

  /* 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->locked = 1;

    return;

  }

  /* To avoid deadlock, first release all locks with a larger

  ** BtShared address.  Then acquire our lock.  Then reacquire

  ** the other BtShared locks that we used to hold in ascending

  ** order.

  */

  for(pLater=p->pNext; pLater; pLater=pLater->pNext){

    assert( pLater->sharable );

    assert( pLater->pNext==0 || pLater->pNext->pBt>pLater->pBt );

    assert( !pLater->locked || pLater->wantToLock>0 );

    if( pLater->locked ){

      sqlite3_mutex_leave(pLater->pBt->mutex);

      pLater->locked = 0;

    }

  }

  sqlite3_mutex_enter(p->pBt->mutex);

  p->locked = 1;

  for(pLater=p->pNext; pLater; pLater=pLater->pNext){

    if( pLater->wantToLock ){

      sqlite3_mutex_enter(pLater->pBt->mutex);

      pLater->locked = 1;

    }

  }

}

/*

** Exit the recursive mutex on a Btree.

*/

void sqlite3BtreeLeave(Btree *p){

  if( p->sharable ){

    assert( p->wantToLock>0 );

    p->wantToLock--;

    if( p->wantToLock==0 ){

      assert( p->locked );

      sqlite3_mutex_leave(p->pBt->mutex);

      p->locked = 0;

    }

  }

}

#ifndef NDEBUG

/*

** Return true if the BtShared mutex is held on the btree.  

**

** This routine makes no determination one why or another if the

** database connection mutex is held.

**

** This routine is used only from within assert() statements.

*/

int sqlite3BtreeHoldsMutex(Btree *p){

  return (p->sharable==0 ||

             (p->locked && p->wantToLock && sqlite3_mutex_held(p->pBt->mutex)));

}

#endif

#ifndef SQLITE_OMIT_INCRBLOB

/*

** Enter and leave a mutex on a Btree given a cursor owned by that

** Btree.  These entry points are used by incremental I/O and can be

** omitted if that module is not used.

*/

void sqlite3BtreeEnterCursor(BtCursor *pCur){

  sqlite3BtreeEnter(pCur->pBtree);

}

void sqlite3BtreeLeaveCursor(BtCursor *pCur){

  sqlite3BtreeLeave(pCur->pBtree);

}

#endif /* SQLITE_OMIT_INCRBLOB */

/*

** Enter the mutex on every Btree associated with a database

** connection.  This is needed (for example) prior to parsing

** a statement since we will be comparing table and column names

** against all schemas and we do not want those schemas being

** reset out from under us.

**

** There is a corresponding leave-all procedures.

**

** Enter the mutexes in accending order by BtShared pointer address

** to avoid the possibility of deadlock when two threads with

** two or more btrees in common both try to lock all their btrees

** at the same instant.

*/

void sqlite3BtreeEnterAll(sqlite3 *db){

  int i;

  Btree *p, *pLater;

  assert( sqlite3_mutex_held(db->mutex) );

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

    p = db->aDb[i].pBt;

    if( p && p->sharable ){

      p->wantToLock++;

      if( !p->locked ){

        assert( p->wantToLock==1 );

        while( p->pPrev ) p = p->pPrev;

        while( p->locked && p->pNext ) p = p->pNext;

        for(pLater = p->pNext; pLater; pLater=pLater->pNext){

          if( pLater->locked ){

            sqlite3_mutex_leave(pLater->pBt->mutex);

            pLater->locked = 0;

          }

        }

        while( p ){

          sqlite3_mutex_enter(p->pBt->mutex);

          p->locked++;

          p = p->pNext;

        }

      }

    }

  }

}

void sqlite3BtreeLeaveAll(sqlite3 *db){

  int i;

  Btree *p;

  assert( sqlite3_mutex_held(db->mutex) );

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

    p = db->aDb[i].pBt;

    if( p && p->sharable ){

      assert( p->wantToLock>0 );

      p->wantToLock--;

      if( p->wantToLock==0 ){

        assert( p->locked );

        sqlite3_mutex_leave(p->pBt->mutex);

        p->locked = 0;

      }

    }

  }

}

#ifndef NDEBUG

/*

** Return true if the current thread holds the database connection

** mutex and all required BtShared mutexes.

**

** This routine is used inside assert() statements only.

*/

int sqlite3BtreeHoldsAllMutexes(sqlite3 *db){

  int i;

  if( !sqlite3_mutex_held(db->mutex) ){

    return 0;

  }

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

    Btree *p;

    p = db->aDb[i].pBt;

    if( p && p->sharable &&

         (p->wantToLock==0 || !sqlite3_mutex_held(p->pBt->mutex)) ){

      return 0;

    }

  }

  return 1;

}

#endif /* NDEBUG */

/*

** Potentially dd a new Btree pointer to a BtreeMutexArray.

** Really only add the Btree if it can possibly be shared with

** another database connection.

**

** The Btrees are kept in sorted order by pBtree->pBt.  That

** way when we go to enter all the mutexes, we can enter them

** in order without every having to backup and retry and without

** worrying about deadlock.

**

** The number of shared btrees will always be small (usually 0 or 1)

** so an insertion sort is an adequate algorithm here.

*/

void sqlite3BtreeMutexArrayInsert(BtreeMutexArray *pArray, Btree *pBtree){

  int i, j;

  BtShared *pBt;

  if( pBtree==0 || pBtree->sharable==0 ) return;

#ifndef NDEBUG

  {

    for(i=0; i<pArray->nMutex; i++){

      assert( pArray->aBtree[i]!=pBtree );

    }

  }

#endif

  assert( pArray->nMutex>=0 );

  assert( pArray->nMutex<sizeof(pArray->aBtree)/sizeof(pArray->aBtree[0])-1 );

  pBt = pBtree->pBt;

  for(i=0; i<pArray->nMutex; i++){

    assert( pArray->aBtree[i]!=pBtree );

    if( pArray->aBtree[i]->pBt>pBt ){

      for(j=pArray->nMutex; j>i; j--){

        pArray->aBtree[j] = pArray->aBtree[j-1];

      }

      pArray->aBtree[i] = pBtree;

      pArray->nMutex++;

      return;

    }

  }

  pArray->aBtree[pArray->nMutex++] = pBtree;

}

/*

** Enter the mutex of every btree in the array.  This routine is

** called at the beginning of sqlite3VdbeExec().  The mutexes are

** exited at the end of the same function.

*/

void sqlite3BtreeMutexArrayEnter(BtreeMutexArray *pArray){

  int i;

  for(i=0; i<pArray->nMutex; i++){

    Btree *p = pArray->aBtree[i];

    /* Some basic sanity checking */

    assert( i==0 || pArray->aBtree[i-1]->pBt<p->pBt );

    assert( !p->locked || p->wantToLock>0 );

    /* We should already hold a lock on the database connection */

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

    p->wantToLock++;

    if( !p->locked && p->sharable ){

      sqlite3_mutex_enter(p->pBt->mutex);

      p->locked = 1;

    }

  }

}

/*

** Leave the mutex of every btree in the group.

*/

void sqlite3BtreeMutexArrayLeave(BtreeMutexArray *pArray){

  int i;

  for(i=0; i<pArray->nMutex; i++){

    Btree *p = pArray->aBtree[i];

    /* Some basic sanity checking */

    assert( i==0 || pArray->aBtree[i-1]->pBt<p->pBt );

    assert( p->locked || !p->sharable );

    assert( p->wantToLock>0 );

    /* We should already hold a lock on the database connection */

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

    p->wantToLock--;

    if( p->wantToLock==0 && p->locked ){

      sqlite3_mutex_leave(p->pBt->mutex);

      p->locked = 0;

    }

  }

}

#endif  /* SQLITE_THREADSAFE && !SQLITE_OMIT_SHARED_CACHE */