Subject: Re: bug#14640: SA_RESTART prevents execution of signal handlers
From: Andy Wingo <[email-masked]>
To: [email-masked] (Ludovic Courtès)
Cc: [email-masked]
Date: Tue, 21 Jun 2016 08:37:37 +0200
List-Id: <debbugs-submit.debbugs.gnu.org>

On Mon 17 Jun 2013 15:54, [email-masked] (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:

  Guile handles signals asynchronously.  When it receives a signal, the
  synchronous signal handler just records the fact that a signal was
  received and sets a flag to tell the relevant Guile thread that it has a
  pending signal.  When the Guile thread checks the pending-interrupt
  flag, it will arrange to run the asynchronous part of the signal
  handler, which is the handler attached by @code{sigaction}.

  This strategy has some perhaps-unexpected interactions with the
  @code{SA_RESTART} flag, though: because the synchronous handler doesn't
  do very much, and notably it doesn't run the Guile handler, it's
  impossible to interrupt a thread stuck in a long-running system call via
  a signal handler that is installed with @code{SA_RESTART}: the
  synchronous handler just records the pending interrupt, but then the
  system call resumes and Guile doesn't have a chance to actually check
  the flag and run the asynchronous handler.  That's just how it is.

Andy