Re: wait -n misses signaled subprocess

[Chet Ramey]

On 1/29/24 7:12 AM, Robert Elz wrote:
>      Date:        Sun, 28 Jan 2024 18:21:42 -0500
>      From:        Chet Ramey <chet.ramey@case.edu>
>      Message-ID:  <3347f790-529b-4bee-91fd-de39bed3f49e@case.edu>
>
>    | because `wait -n' doesn't look in the table
>    | of saved statuses -- its job is to wait for `new' jobs to terminate, not
>    | ones that have already been removed from the table.
>
> That's very interesting, and most unexpected information.
>
> I always wondered why the option was 'n' - I would have made it
> be 'a' probably, as a shorthand for "any" - but then I decided
> that perhaps 'n' was a better choice, as "a" could also be "all",
> the option name would not be providing any real clue at all, so
> I assumed you'd been ultra clever and used 'n' as the next char
> in "any" and also as it can be read like the first part of "en" "ee"
> (which you need to say out loud, or at least in your head, to get the
> effect of).

You should have. You told me about your implementation using `-n' in
10/2017, long before I implemented it (4/2020).

> It never even dawned on me that 'n' might mean "new", as in only
> processes that hadn't terminated at the time the wait -n was done,
> as that's essentially a recipe for script madness, race conditions
> galore, as the one reported here.

What does `wait -n' without job arguments mean?

> What wait(1) needed was an alternative to its normal "all" semantic,
> just "wait" waits for every background job to terminate, what's needed
> is a way to wait for any one of them (whether already terminated, but
> not previously waited for or not).   That's what I always assumed
> wait -n was doing, and how I implemented it in the NetBSD shell.

OK. Since wait without options can already wait for the same pid multiple
times, the -n option has to bring some new functionality here.


> Similarly "wait pid1 pid2 pid3" waits for all 3 of those to
> terminate, so "wait -n pid1 pid2 pid3" should wait for any one
> of them - already terminated or not. 

As long as it's still in the jobs list.


>  When there's just one pid
> in the list, the -n option always seemed useless to me, there
> ought be no difference between "wait pid" and "wait -n pid"
> (as in wait for all of one, and wait for any of one, mean the
> same thing, wait for that one), but obviously should still be
> supported for consistency. 

OK. We can agree there shouldn't be any difference between `wait pid'
and `wait -n pid'.

--
``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/

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