(ITS#7813) lmdb multi threaded env creation on Win32 can fail

[clayton.stangeland@gmail.com]

Full_Name: Clayton Stangeland
Version: gitorious mdb last commit Feb 19, 2014 e2bdd44624a525d4847c22d7ebf1ea4d154ed734
OS: Windows 7
URL: ftp://ftp.openldap.org/incoming/
Submission from: (NULL) (192.94.73.31)


There is a bug in lightning mdb in mdb.c when running Windows. When creating an
environment to the same file in multiple threads, an error can occur on the
OpenMutex call and some of the creations will fail.

In mdb_env_setup_locks, mdb_env_excl_lock is called.
If it acquires an exclusive lock CreateMutex is called.
  env->me_rmutex = CreateMutex(&mdb_all_sa, FALSE, env->me_txns->mti_rmname);

If not OpenMutex is called.
  env->me_rmutex = OpenMutex(SYNCHRONIZE, FALSE, env->me_txns->mti_rmname);

The OpenMutex call can fail if there is a thread context switch between the time
the exclusive lock is acquired and the time CreateMutex is called, because the
mutex to open has not actually been created yet. To be clear OpenMutex in
another thread gets run before CreateMutex in the first thread.

The solution is to replace the OpenMutex calls with CreateMutex. It will either
open the mutex if it exists or create it if it does not. Setting the
bInitialOwner flag to FALSE as is already done is the recommended way to use
CreateMutex in this situation.