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bug#14640: SA_RESTART prevents execution of signal handlers
From: |
Andy Wingo |
Subject: |
bug#14640: SA_RESTART prevents execution of signal handlers |
Date: |
Tue, 21 Jun 2016 09:59:33 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/24.5 (gnu/linux) |
On Tue 21 Jun 2016 09:48, address@hidden (Ludovic Courtès) writes:
> Andy Wingo <address@hidden> skribis:
>
>> On Mon 17 Jun 2013 15:54, address@hidden (Ludovic Courtès) writes:
>>
>>> When using SA_RESTART, signal handlers are never executed, as in this
>>> example (checked on 2.0.9+):
>>>
>>> (sigaction SIGALRM
>>> (lambda (signum)
>>> (pk 'sig signum))
>>> SA_RESTART)
>>> (alarm 3)
>>> (pk 'char (read-char))
>>>
>>> Presumably this is because the read(2) syscall is automatically
>>> restarted, leaving no chance for the handler async to run.
>>
>> Thinking about this a bit -- since we always handle signals
>> asynchronously and have no intention of handling them synchronously,
>> then we just have to document this behavior. Done in e877e1b:
>
> I think it’s problematic though. With the current design, signal
> delivery is unreliable (with or without SA_RESTART; what we observe with
> SA_RESTART occurs similarly if you make a syscall right after queuing,
> but not running, an async.)
Can you expect any kind of reasonable behavior with SA_RESTART? I think
not.
> The more I think about it, the more I think a different approach is
> needed. On GNU/Linux, signalfd(2) may be part of the solution.
We already do the equivalent of signalfd(), with our self-pipe trick.
And an fd doesn't help you if the syscall has no associated fd. Signals
are just a problem. I agree we can do better though :)
If you are just concerned about read and write, I think the right thing
is non-blocking fd's, and making the C read/write waiters also add the
signal FD to their poll set. WDYT?
Andy