[Python-Dev] working with Python threads from C extension module?

Bill Janssen janssen at parc.com
Sat Sep 8 17:18:35 CEST 2007

> Be convoluted yourself and do this:
> #define PySSL_BEGIN_ALLOW_THREADS { if (_ssl_locks) { Py_BEGIN_ALLOW_THREADS
> (Untested, but I think it should work.)

Yes, that had occurred to me.  We want the code inside the braces
still to run if the locks aren't held, so something more like

			PyThreadState *_save;  \
			if (_ssl_locks_count>0) {_save = PyEval_SaveThread();}
  #define PySSL_BLOCK_THREADS	if (_ssl_locks_count>0){PyEval_RestoreThread(_save)};
  #define PySSL_UNBLOCK_THREADS	if (_ssl_locks_count>0){_save = PyEval_SaveThread()};
  #define PySSL_END_ALLOW_THREADS	if (_ssl_locks_count>0){PyEval_RestoreThread(_save);} \

would do the trick.  Unfortunately, this doesn't deal with the macro
behaviour.  The user has "turned on" threading; they expect reads and
writes to yield the GIL so that other threads can make progress.  But
the fact that threading has been "turned on" after the SSL module has
been initialized, means that threads don't work inside the SSL code.
So the user's understanding of the system will be broken.

No, I don't see any good way to fix this except to add a callback
chain inside PyThread_init_thread, which is run down when threads are
initialized.  Any module which needs to set up threads registers itself
on that chain, and gets called as part of PyThread_init_thread.  But
I'm far from the smartest person on this list :-), so perhaps someone
else will see a good solution.

This has got to be a problem with other extension modules linked to
libraries which have their own threading abstractions.


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