[Python-Dev] Extension modules, Threading, and the GIL

["Martin v. Löwis"]

Mark Hammond wrote:
> It can be done, yes.  I am not looking for a change in semantics, just a
> simple way to do it (and maybe even a fast way to do it, but that is
> secondary).  If such a way already exists, please enlighten us.  If not, but
> it is sufficiently simple to describe, then please describe it.  Otherwise,
> I do not understand your point.

There is a very simple strategy to support multiple threads in an 
extension module.

1. In all callbacks, create a thread state and acquire the current
    thread (this requires a singleton interpreter state).

2. In all API calls that may invoke callbacks, use
    Py_BEGIN_ALLOW_THREADS and Py_END_ALLOW_THREADS around the API call.

If this strategy is followed, every code always has all Python 
resources, and no deadlocks result.

Regards,
Martin