SQLite

** 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);
  btreeIntegrity(p);

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

  /* Write transactions are not possible on a read-only database */
  if( (pBt->btsFlags & BTS_READ_ONLY)!=0 && wrflag ){
    rc = SQLITE_READONLY;
    goto trans_begun;
  }

#ifndef SQLITE_OMIT_SHARED_CACHE
  {
    sqlite3 *pBlock = 0;
    /* If another database handle has already opened a write transaction 
    ** on this shared-btree structure and a second write transaction is
    ** requested, return SQLITE_LOCKED.
    */
    if( (wrflag && pBt->inTransaction==TRANS_WRITE)
     || (pBt->btsFlags & BTS_PENDING)!=0
    ){
      pBlock = pBt->pWriter->db;
    }else if( wrflag>1 ){
      BtLock *pIter;
      for(pIter=pBt->pLock; pIter; pIter=pIter->pNext){
        if( pIter->pBtree!=p ){
          pBlock = pIter->pBtree->db;
          break;
        }
      }
    }
    if( pBlock ){
      sqlite3ConnectionBlocked(p->db, pBlock);
      rc = SQLITE_LOCKED_SHAREDCACHE;
      goto trans_begun;
    }
  }
#endif

  /* Any read-only or read-write transaction implies a read-lock on 
  ** page 1. So if some other shared-cache client already has a write-lock 
  ** on page 1, the transaction cannot be opened. */
  rc = querySharedCacheTableLock(p, MASTER_ROOT, READ_LOCK);

}

/*
** Return the size of the header added to each page by this module.
*/
int sqlite3HeaderSizeBtree(void){ return ROUND8(sizeof(MemPage)); }

#if !defined(SQLITE_OMIT_SHARED_CACHE)
/*
** Return true if the Btree passed as the only argument is sharable.
*/
int sqlite3BtreeSharable(Btree *p){
  return p->sharable;
}
#endif