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Calling sem_wait() in DllMain( DLL_THREAD_DETACH) crashes

I've come across a problem whilst trying to run the twisted test suite, where
some tests just cause python to segfault.  It seems to be the same issue with
libcrypto as reported in [2],[3]

A small test case is attached to [2] as, referred to in
msg76086, which shows that manipulating a semaphore in
DllMain(DLL_THREAD_DETACH) causes a crash.

There is a patch for openssl attached to [1] which makes sense to me.  Surely
the cleanup ERR_remove_state() does should be requested using
pthread_cleanup_push() rather using DllMain(DLL_THREAD_DETACH)?  But that
doesn't seem to be done for any other POSIX target.  The documentation of
ERR_remove_state() gives me the impression that it expects the application to
arrange for that to happen, so perhaps it's just trying too hard to be
helpful here?

I suppose the alternative interpretation is that you are supposed to be able
to call anything you like from DllMain(DLL_THREAD_DETACH), then this is a
cygwin bug. But I can't see how that could be made work as pthread::exit() has
to delete the cygwin thread object before ExitThread() is called, as there's
no thread to do it in afterwards :-)


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