tstate invalid crash with threads

[Tim Peters]

Followups to the Thread-SIG, please.

[Ray Loyzaga]
> I have been playing with a multithreaded tcp based server for a while,
> and during some serious stress testing continually hit a problem
> involving the interpreter crashing with "Fatal Python error:
> PyThreadState_Delete: invalid tstate".
> ...
> It appears to be a subtle race in PyThreadState_Delete ....
> interestingly, if I uncomment the small sleep in "handle" in the server,
> ie. make the server slower, it seems to work for ever ... 4m transactions
> before I gave up. I think the problem only comes if you are creating and
> destroying threads quickly in parallel.

PyThreadState_Delete is called from very few places, and one of them strikes
me as suspicious:  at the end of threadmodule.c's t_bootstrap, we have:

	PyThreadState_Clear(tstate);
	PyEval_ReleaseThread(tstate);
	PyThreadState_Delete(tstate);
	PyThread_exit_thread();

The suspicious thing here is that PyEval_ReleaseThread releases the global
interpreter lock, so nothing is serializing calls to PyThreadState_Delete
made from the following line.  PyThreadState_Delete in turn does no locking
of its own either, but mutates a shared list.

If this isn't plain wrong, it's certainly not plain right <wink>.  Matches
your symptoms, too (very rare blowups during high rates of thread death).

Guido?  I haven't been able to provoke Ray's problem under Win95, but the
above just doesn't smell right.

win95-didn't-crash-but-the-TAB-and-ESC-keys-did-swap-their-
    meanings!-ly y'rs  - tim