Re: waiting for process substitutions

[Chet Ramey]

On 8/5/24 2:21 PM, Zachary Santer wrote:
> On Mon, Aug 5, 2024 at 9:54 AM Chet Ramey <chet.ramey@case.edu> wrote:
> > On 7/31/24 11:48 AM, Zachary Santer wrote:
> > > $ wait -n > >( cat )
> > > would hang, in the event that there are no other un-waited-for child
> > > processes, right?
> >
> > Yes, it will wait for the next job or procsub to terminate.
>
> Basically the same situation as Oğuz's bug here:

You mean the one I referenced below, right?

https://lists.gnu.org/archive/html/bug-bash/2022-10/msg00107.html

Sort of, but they're not the same. The issue with Oguz's bug is that
$! had changed before `wait', and was no longer the same as the last
process substitution -- the `wait' command should not have waited
for it. The other issue was a bookkeeping problem -- even though the
process substitution was performed by the same shell that executed
the subshell command (because that's how bash optimizes forks), that
command shouldn't inherit the process substitutions, since they're
logically performed in a different execution context.

But in the end, if you're waiting for a process that isn't going to
terminate, you're going to be waiting for a long time.


> So $! is only set to the pid for a procsub that's being redirected
> to/from after the command (or compound command) whose output/input is
> being redirected to/from it has completed? 

Wow, that's a really complex sentence. And the answer is not really, and
it's hard to tell. Since bash performs word expansions before redirections
when executing simple commands, all the words in a simple command have been
expanded before the process substitution changes $!. So it's not after it's
completed, but it's after word expansions have been performed, so it's hard
to tell.

But if you ran something like this:

echo $! <(echo $BASHPID > /dev/tty ) $!

the second $! would be expanded to the pid of the procsub because the word
expansion changed it.

So in your example, the redirection sets $! (or its internal equivalent)
to the pid of the procsub, and wait -n waits for it. These operations are
performed by the same shell process.

> If that's the case, maybe
> the ideal solution for both 'wait' and 'wait -n' would be to wait for
> all background jobs and all procsubs that have ever been assigned to
> $! at that point.

That's all procsubs.

--
``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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