[Python-Dev] Preventing PyEval_AcquireLock deadlock

[Guido van Rossum]

> > Let's take a step back.  Why do you need this?  I'm guessing that you
> > have a C++ library that calls C++ callbacks, and now you want to call
> > a Python callback from your C++ callback.  The proper solution is to
> > make sure that you *always* release the Python lock before entering
> > your event loop or anything else that could possibly call callbacks.
> > See _tkinter for how I handled it there.  It's ugly, but possible.
> 
> Yep, I've already got it working this way except there are a few
> code paths that result in a callback sometimes being called
> indirectly from a different part of the code where the Python lock
> is already acquired.  I was hoping to be able to use a general
> solution instead of having to find all those situations and
> special-case them.  Oh well.

I guess you could have your own global variable that says "I've
already got the Python lock".  The lock sequences would look like
this:

// before entering the event loop
clear the flag
unlock the GIL

// after returning from the event loop
lock the GIL
set the flag

--Guido van Rossum (home page: http://www.python.org/~guido/)