[sqlite] A questionable bit of code

[Kyle Jessup]

I have been trying to track down a crasher in my multi-threaded sqlite 
3.2.0 application. I was seeing crashes in highly contentious 
situations where many threads are opening and closing a database 
frequently. The crash was consistently in the following bit of code:

src/main.c in sqlite_close starting on line 527
#ifndef SQLITE_OMIT_GLOBALRECOVER
  {
    sqlite3 *pPrev = pDbList;
    sqlite3OsEnterMutex();
    while( pPrev && pPrev->pNext!=db ){
      pPrev = pPrev->pNext;
    }
    if( pPrev ){
      pPrev->pNext = db->pNext;
    }else{
      assert( pDbList==db );
      pDbList = db->pNext;
    }
    sqlite3OsLeaveMutex();
  }
#endif
The problem is that pPrev can be left dangling since the pDbList 
pointer is saved off before entering the mutex. It is possible that 
another thread already in the mutex at that time ends up sqliteFree'ing 
what was the pDbList head, invalidating pPrev for the other thread, 
causing the crash.

Swapping these two lines fixes the problem:
    sqlite3 *pPrev = pDbList;
    sqlite3OsEnterMutex();
change to:
    sqlite3OsEnterMutex();
    sqlite3 *pPrev = pDbList;
Once I made this change, the crash vanished.
I just looked in the 3.2.1 source and this bit of code is the same.
-Kyle