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Date: Mon, 18 Mar 2024 13:58:26 +0900
To: cygwin AT cygwin DOT com
Subject: Re: Bogus exit code 127 from a child process
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From: Takashi Yano via Cygwin <cygwin AT cygwin DOT com>
Reply-To: Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>
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On Mon, 18 Mar 2024 12:09:06 +0900
Takashi Yano wrote:
> On Sun, 17 Mar 2024 14:10:55 +0100
> Dimitry Andric wrote:
> > On 17 Mar 2024, at 13:50, Dimitry Andric <dimitry AT unified-streaming DOT com> wrote:
> > > 
> > > On 17 Mar 2024, at 13:35, Takashi Yano via Cygwin <cygwin AT cygwin DOT com> wrote:
> > > ...
> > >> 
> > >> I also test your test case:
> > >> while bash -c 'true & true & wait -n || { echo 1: $?; exit 1; } && wait -n || { echo 2: $?; exit 1; }'; do echo $((i++)); done
> > >> in Linux (Debian 12.5), and the issue reproduced!
> > > 
> > > Yeah, same here with bash 5.1.16(1)-release on Ubuntu 22.04. It errors out with 127 after ~50-200 loops.
> > 
> > Having built bash master (bash-5.2-27-gf3b6bd19) here, it consistently gives 127 in this area:
> > 
> > https://git.savannah.gnu.org/cgit/bash.git/tree/builtins/wait.def#n227
> > 
> >    211  #if defined (JOB_CONTROL)
> >    212    if (nflag)
> >    213      {
> >    214        if (list)
> >    215          {
> >    216            opt = set_waitlist (list);
> >    217            if (opt == 0)
> >    218              WAIT_RETURN (127);
> >    219            wflags |= JWAIT_WAITING;
> >    220          }
> >    221
> >    222        status = wait_for_any_job (wflags, &pstat);
> >    223        if (vname && status >= 0)
> >    224          builtin_bind_var_to_int (vname, pstat.pid, bindflags);
> >    225
> >    226        if (status < 0)
> > => 227          status = 127;
> >    228        if (list)
> >    229          unset_waitlist ();
> >    230        WAIT_RETURN (status);
> >    231      }
> >    232  #endif
> > 
> > So for some reason, wait_for_any_job() returns a negative value in this particular situation.
> 
> Line 218 looks also suspicious.

Probably, this is not a bug. man bash says:
              If  the  -n option is supplied, wait waits for a single job from
              the list of ids or, if no ids are supplied, any job, to complete
              and returns its exit status.  If none of the supplied  arguments
              is a child of the shell, or if no arguments are supplied and the
              shell  has no unwaited‐for children, the exit status is 127.

If the background process exited before calling 'wait -n', it returns 127.
This is very different from wait() system call, which is necessary for
any background joubs, otherwise zombie remains.

In the shell, it is not necessary to call wait command for background jobs,
therefore exit status of the background job which already exited is not held
anymore.

So, actual bug is in the test case.

-- 
Takashi Yano <takashi DOT yano AT nifty DOT ne DOT jp>

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