[issue21645] asyncio: Race condition in signal handling on FreeBSD

[STINNER Victor]

STINNER Victor added the comment:

> The intention if the code was that the Py_AddPendingCall() would complete before the thread switch.

In fact I also expected Py_AddPendingCall() to be called *before* writing the signal number into the "wakeup fd". I guess that before nobody was relying on the "wakeup fd" *and* and the callback.

> Perhaps a CPython fix could be to call it before writing a byte to the pipe??? But that would require someone else to meditate more on the C code.

It doesn't work for Tulip which targets Python 3.3.

Oh... I just saw that Python 2.7 calls Py_AddPendingCall() and *then* writes "\0" into wakeup fd... So Python 2.7 does not have the issue!

But yes, we may change the C signal handler in Python 3.5 because it makes more sense to call Py_AddPendingCall() to avoid this specific race condition.

> I think your patch to Tulip is fine.

Good :) I will test it on FreeBSD. If it fixes the issue, I will apply it. We will see if it makes all buildbots happy.

> I wouldn't worry about missing a signal when things are busy -- AFAIK the child process reaper will reap multiple processes.

Well, Py_AddPendingCall() also has a limit which is smaller and it does *not* warn when signals are dropped :-(

--

I don't know if it can be useful, but there is also the signal.pthread_sigmask() which can be used to "queue" pending signals. Unblocking signals will flush the queue. It might help to mitigate burst of signals in a short window.... I don't know if signal.pthread_sigmask() has a longer queue than Python. And it may be tricky to decide when and where the signals should be blocked and unblocked.

If a signal is blocked and the process gets the signal more than once, the signal is only raised once when the signal is unblocked. I don't know if it is an issue or not if asyncio only calls the signal handler once in this case.

----------

_______________________________________
Python tracker <report at bugs.python.org>
<http://bugs.python.org/issue21645>
_______________________________________