bug#21424: Bug#685919: guile-1.6: SIGALRM signal handler does not get called when guile blocks on I/O

[Rob Browning <rlb@defaultvalue.org>]

[If possible, please preserve the -forwarded address in any replies.]

Reference: https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=765497

Panu Kalliokoski <panu.kalliokoski@gmail.com> writes:

> While playing with guile on my system, I discovered a weird anomaly
> which I could not reproduce on other systems running guile.  If I
> install a signal handler for SIGALRM, it won't get called while guile is
> making an I/O system call.  To demonstrate:
>
> [atehwa@karaihin ~/proj/psyk]$ guile
> guile> (alarm 2)
> 0
> guile> Herätyskello
> [atehwa@karaihin ~/proj/psyk]$ guile
> guile> (sigaction SIGALRM (lambda (x) (display "now!") (newline)))
> (0 . 335544320)
> guile> (alarm 2)
> 0
> guile> now a lot more than two seconds has passed, while I wrote this
> now!
> <unnamed port>: In expression now:
> <unnamed port>: Unbound variable: now
> ABORT: (unbound-variable)
> [...]
>
> As you can see, the signal handler gets called as soon as guile returns
> from read(2), already before calling (eval).
>
> I can't get to understand what causes this on my system, because another
> Debian system with exact same versions of guile-1.6, libc6 and
> libguile-ltdl-1 seems to work fine, and interrupts the read(2) call with
> the signal handler.

This appears to still be the case with at least Debian's 2.0.11+1-10
package, and setting the handler to something that doesn't perform IO
has the same effect (i.e. no alarm until you hit return):

  (sigaction SIGALRM (lambda (x) (exit 1)))

Thanks
-- 
Rob Browning
rlb @defaultvalue.org and @debian.org
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