[Python-Dev] Signals, threads, blocking C functions (original) (raw)

Chris McDonough chrism at plope.com
Mon Sep 4 04:36:23 CEST 2006

Would adding an API for sigprocmask help here?

(Although it has been tried before -- http://mail.python.org/ pipermail/python-dev/2003-February/033016.html and died in the womb
due to threading-related issues -- http://mail.mems-exchange.org/ durusmail/quixote-users/1248/)

On Sep 2, 2006, at 8:10 AM, Gustavo Carneiro wrote:

We have to resort to timeouts in pygtk in order to catch unix signals in threaded mode. The reason is this. We call gtkmain() (mainloop function) which blocks forever. Suppose there are threads in the program; then any thread can receive a signal (e.g. SIGINT). Python catches the signal, but doesn't do anything; it simply sets a flag in a global structure and calls PyAddPendingCall(), and I guess it expects someone to call PyMakePendingCalls(). However, the main thread is blocked calling a C function and has no way of being notified it needs to give control back to python to handle the signal. Hence, we use a 100ms timeout for polling. Unfortunately, timeouts needlessly consume CPU time and drain laptop batteries.

According to [1], all python needs to do to avoid this problem is block all signals in all but the main thread; then we can guarantee signal handlers are always called from the main thread, and pygtk doesn't need a timeout. Another alternative would be to add a new API like PyAddPendingCallNotification, which would let python notify extensions that new pending calls exist and need to be processed. But I would really prefer the first alternative, as it could be fixed within python 2.5; no need to wait for 2.6. Please, let's make Python ready for the enterprise! [2] [1] https://bugzilla.redhat.com/bugzilla/processbug.cgi#c3 [2] http://perkypants.org/blog/2006/09/02/rfte-python/

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