Re: `wait -n` returns 127 when it shouldn't

[Aleksey Covacevice]

On Thu, May 18, 2023 at 3:07 PM Chet Ramey <chet.ramey@case.edu> wrote:
>
> On 5/18/23 7:51 AM, Robert Elz wrote:
>
> > Apparently, in bash, if the code is running in a (shell) loop (like inside
> > a while, or similar, loop) then each iteration around the loop, any jobs 
> > that
> > have exited, but not been cleaned already, are removed from the queue (the
> > jobs table in practice, though bash may also have something else).
> >
> > That's really broken, and should be fixed (but has apparently been that
> > way for decades, and no-one noticed).
>
> This isn't a problem, and is a red herring. The code that manages that list
> makes sure to keep as many jobs in the list as POSIX requires, subject to
> the maxchild resource limit.
>
>
> > In the script in question, the offending loop isn't the one in the main
> > program - in that for each iteration the background processes are started,
> > and waited for, in each iteration, but the one in the waitjobs function.
> > which (appears at first glance, which is all the analysis shells ever do)
> > to be an infinite loop, so each time around, if there are any completed
> > jobs in the table, they're removed.
>
> No, this isn't what happens. The problem is that the shell reaps both jobs,
> but the `wait -n' code had a race condition that prevented it from finding
> a job in the list.
>

I fail to see where the race condition in `true & wait -n` is. Whether the
'underlying function' has a race condition is the true red herring here.

Also, the manual states:

'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.'

`true & wait -n` returning 127 means `wait -n` did not wait for 'any job';
in fact, it waited for no job. The subsequent part about
'unwaited-for' children is
either irrelevant or contradictory to the above given the current scenario.

> --
> ``The lyf so short, the craft so long to lerne.'' - Chaucer
>                  ``Ars longa, vita brevis'' - Hippocrates
> Chet Ramey, UTech, CWRU    chet@case.edu    http://tiswww.cwru.edu/~chet/
>