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Re: wait unblocks before signals processed


From: Elliott Forney
Subject: Re: wait unblocks before signals processed
Date: Mon, 5 Nov 2012 17:52:41 -0700

OK, I see in POSIX mode that a trap on SIGCHLD will cause wait to
unblock.  We are still maintaining a counter of running jobs though so
it seems to me that there could race condition in the following line

trap '((j--))' CHLD

if two processes quit in rapid succession and one trap gets preempted
in the middle of ((j--)) then the count may be off.  Is this possible?

I tried to test whether or not traps are mutually exclusive with the
following code and got more interesting warnings.  The count appears
to suggest that there is indeed a race condition going on here but I
am unsure what "bad value in trap_list" means?

#!/bin/bash

count=0

function dummy
{
  usleep $RANDOM
}

set -m
trap ': $(( ++count ))' CHLD

for i in {1..1000}
do
  dummy $i &
done

wait

echo $count


$ ./trap_race
./trap_race: warning: run_pending_traps: bad value in trap_list[17]: 0x4536e0
./trap_race: warning: run_pending_traps: bad value in trap_list[17]: 0x4536e0
./trap_race: warning: run_pending_traps: bad value in trap_list[17]: 0x4536e0
./trap_race: warning: run_pending_traps: bad value in trap_list[17]: 0x4536e0
./trap_race: warning: run_pending_traps: bad value in trap_list[17]: 0x4536e0
./trap_race: warning: run_pending_traps: bad value in trap_list[17]: 0x4536e0
./trap_race: warning: run_pending_traps: bad value in trap_list[17]: 0x4536e0
./trap_race: warning: run_pending_traps: bad value in trap_list[17]: 0x4536e0
./trap_race: warning: run_pending_traps: bad value in trap_list[17]: 0x4536e0
./trap_race: warning: run_pending_traps: bad value in trap_list[17]: 0x4536e0
983

Thanks,
-------
Elliott Forney


On Mon, Nov 5, 2012 at 5:11 PM, Dan Douglas <address@hidden> wrote:
> Hi Elliott. The behavior of wait differs depending upon whether you are in
> POSIX mode. Try this script, which I think does essentially what you're after
> (also here: https://gist.github.com/3911059 ):
>
> #!/usr/bin/env bash
>
> ${BASH_VERSION+shopt -s lastpipe extglob}
>
> if [[ -v .sh.version ]]; then
>     builtin getconf
>     function BASHPID.get {
>         read -r .sh.value _ </proc/self/stat
>     }
> fi
>
> function f {
>     printf '%d: sleeping %d sec\n' "${@:1:2}" >&2
>     sleep "$2"
>
>     printf '%d: returning %d\n' "$1" "$3" >&2
>     return "$3"
> }
>
> function main {
>     typeset -i n= j= maxj=$(getconf _NPROCESSORS_ONLN)
>
>     set -m
>     trap '((j--))' CHLD
>
>     while ((n++<30)); do
>         f "$BASHPID" $(((RANDOM%5)+1)) $((RANDOM%2)) &
>         ((++j >= maxj)) && POSIXLY_CORRECT= wait
>     done
>
>     echo 'finished, waiting for remaining jobs...' >&2
>     wait
> }
>
> main "$@"
> echo
>
> # vim: set fenc=utf-8 ff=unix ts=4 sts=4 sw=4 ft=sh nowrap et:
>
>
> The remaining issues are making it work in other shells (Bash in non-POSIX
> mode agrees with ksh, but ksh doesn't agree with POSIX), and also I can't
> think of a reasonable way to retrieve the exit statuses. The status of "wait"
> is rather useless here. Otherwise I think this is the best approach, using
> SIGCHLD and relying upon the POSIX wait behavior. See here:
> http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html#tag_18_11
>
> An issue to be aware of is that the trap will fire when any child exits
> including command/process substitutions or pipelines etc. If any are located
> within the main loop then monitor mode needs to be toggled off around them.
> --
> Dan Douglas



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