Re: wait -n misses signaled subprocess

[Steven Pelley]

Apologies for a typo:
With the discussed change this would return 44080: 1 in an endless loop.
1, not 0

On Tue, Jan 30, 2024 at 10:14 AM Steven Pelley <stevenpelley@gmail.com> wrote:
>
> > OK. Can you think of a use case that would break if wait -n looked at
> > terminated processes?
>
> Yes.  If one were to start a number of bg jobs and repeatedly send the
> list of pids to wait -n (probably redirecting stderr to /dev/null to
> ignore messages about unknown jobs) today you'd process the jobs one
> at a time, assuming no races between job completion.  If wait -n
> looked at terminated processes you'd return jobs repeatedly and
> possibly end up in an infinite loop.
>
> Example:
> # associate array used for consistency with later example
> declare -A pids
> { sleep 1; exit 1; } &
> pids[$!]=""
> { sleep 2; exit 2; } &
> pids[$!]=""
> { sleep 3; exit 3; } &
> pids[$!]=""
>
> status=0
> while [ $status -ne 127 ]; do
>     unset finished_pid
>     wait -n -p finished_pid "${!pids[@]}" 2>/dev/null
>     status=$?
>     if [ -n "$finished_pid" ]; then
>         echo "$finished_pid: $status @${SECONDS}"
>     fi;
> done
>
> gives a simple output like:
> 44080: 1 @1
> 44081: 2 @2
> 44083: 3 @3
>
> With the discussed change this would return 44080: 0 in an endless loop.
> It would need to change to:
>
> while [ ${#pids[@]} -ne 0 ]; do
>     unset finished_pid
>     wait -n -p finished_pid "${!pids[@]}"
>     status=$?
>     if [ -n "$finished_pid" ]; then
>         echo "$finished_pid: $status @${SECONDS}"
>     fi;
>     unset pids[$finished_pid]
> done
>
> Where the returned pid is unset in the array.  I like this more but it
> will break scripts that run correctly today.