Re: bug in zsh wait builtin - rhbz#1150541

[Peter Stephenson <p.stephenson@xxxxxxxxxxx>]

On Tue, 21 Oct 2014 23:55:42 -0700
Bart Schaefer <schaefer@xxxxxxxxxxxxxxxx> wrote:
> On Oct 21,  9:02pm, Peter Stephenson wrote:
> }
> } There's an explanatory note in the latest POSIX standard about this,
> } quoted below.  It seems that the shell is basically required to remember
> } all background processes indefinitely (up to not very helpful get out
> } clauses).  As a baseline, CHILD_MAX here is 1024.  This probably needs
> } to be a special hash.
> 
> Since you've bothered to look this up ... does it go on to say what the
> shell is supposed to do if PIDs roll over so that a new background job
> gets the same $! as some previous one?

No, and as Chet says this appears to be basically "caveat usator".  The
user needs to be careful / lucky enough to perform their "wait" before
the numbers come round again.

>  Is "kill" supposed to work the same way?

There's no indication kill needs to have this.  Presumably this is
because for kill you don't need to have a sensible exit status, just a
reasonable likelihood the job is dead (or wedged in some state where
that signal doesn't work, but that's an entirely different problem).

> Note also that this is partly handled by the POSIX_JOBS option:
> 
>      When the option is set, it becomes possible to use the wait
>      builtin to wait for the last job started in the background (as
>      given by $!) even if that job has already exited.  This works even
>      if the option is turned on temporarily around the use of the wait
>      builtin.
> 
> I would say that any further change made for this should also be under
> the auspices (so to speak) of POSIX_JOBS.

That would already cover the cases in the "bug" report, in fact.

I'm not really sure why we wouldn't just implement this particular
feature generally, despite the current status.  Is there any reason why
you'd *want* "wait" to give you an error (which isn't a particularly
useful message) owing to a race condition you can't control?  POSIX_JOBS
was originally designed for things where the behaviour was clearly
incompatible.

pws