[Python-Dev] pystate.c changes for Python 2.4.2

[Tim Peters]

[Gabriel Becedillas]
> Can anybody tell me if the patch I suggested is ok ?
> That will be to add the following code at the end of PyThreadState_Delete:
>
> if (autoTLSkey && PyThread_get_key_value(autoTLSkey) == tstate)
>      PyThread_delete_key_value(autoTLSkey);

It needs a little work, but I think it should be fine, and people tend
to listen to me on issues like this ;-)

The reason it can't work as-is is shallow:  autoTLSkey doesn't exist
unless Python is compiled with WITH_THREAD defined.  So you need to
wrap that inside an "#ifdef WITH_THREAD" block; or add it similarly to
tstate_delete_common(), and remove it from
PyThreadState_DeleteCurrent().

The primary reason people find this so confusing is because it is :-).
 In the dim mists of history, neither multiple interpreters nor
PyThreadState_DeleteCurrent() existed.  While multiple interpreters do
exist now, they're poorly understood, and the PEP that introduced the
GIL state API explicitly disavowed any responsibility for the new API
working at all with multiple interpreters:

    http://www.python.org/peps/pep-0311.html

    This proposal identifies a solution for extension authors with
    complex multi-threaded requirements, but that only require a
    single "PyInterpreterState".  There is no attempt to cater for
    extensions that require multiple interpreter states.  At the time
    of writing, no extension has been identified that requires
    multiple PyInterpreterStates, and indeed it is not clear if that
    facility works correctly in Python itself.

In a parallel development, thread races during Python shutdown were
discovered that could lead to segfaults, and
PyThreadState_DeleteCurrent() was introduced to repair those.  As a
result, it so happens that core Python never uses the original
PyThreadState_Delete() anymore, except when Py_NewInterpreter() has to
throw away the brand new thread state it created because it turns out
it can't create a new interpreter.

Since core Python never calls Py_NewInterpreter() or
PyThreadState_Delete(), you're in an intersection of areas no current
Python developer even thinks about, let alone tests.  But by the same
token, _because_ it's unused, there's nothing you can do to
PyThreadState_Delete() that can hurt core Python either.  That's why
people should be more encouraging here ;-)  It certainly makes sense
to me that if a thread state is going away, any record of it in the
auto-GIL-state machinery must also go away.

> Thank you very much.

Thank you!  If you put a patch on SourceForge, I'll probably be happy
to check it in.