Re: coproc state wrongly inherited by subshell

[Tobias Hoffmann]

On 03/12/17 21:14, Chet Ramey wrote:
> On 11/29/17 5:36 PM, Tobias Hoffmann wrote:
>
> > Description:
> > As stated in the man page, bash will only allow a single coproc at a time.
> > Thus, when a first coproc has already ended, a second one can be created.
> > This can e.g. be verified by commenting out the upper #sleep line, below.
> > Alternatively, you can remove the parens that start the subshell.
> >
> > But if we start a subshell while the coproc still runs, then wait inside
> > the subshell
> > until the first coproc has ended and then try to start another coproc, bash
> > prints the
> > infamous warning:
> > ./test_sp1.sh: line 10: warning: execute_coproc: coproc [11030:COPROC]
> > still exists
> >
> > The process id 11030 does (verifyably - e.g. add `ps a`) not exist the time
> > the warning is printed.░
> The question is how much to reset, and when to do it. A child process can
> legitimately expect to inherit the coproc file descriptors from its parent
> and read and write them under circumstances that have been valid
> historically.
Well, there seem to be two types of subshells: Those spawned with (... 
), &, or | -- and those coming from $(...) and <( ... ).

In both of them $COPROC[] and $COPROC_PID are available, but only in the 
second kind you can actually use the file descriptors. In the first 
kind, you'll get (e.g.)
  ${COPROC[1]}: Bad file descriptor
(i.e. does not work, as documented).

You can, however, make them available even in the first kind, by duping 
them first, e.g. by using
  exec {fd}>&${COPROC[1]}

I'm not sure whether using them directly in the first kind of subshell 
is actually a good idea. Some things seem to work better when duping 
them first, but I have not investigated those cases further.

  Tobias

> The pid will never be valid for waiting in the child, because
> it's not a child of the current shell, but simply testing whether or not
> the process is alive isn't sufficient.
>
> I think the best thing to do is to invalidate the pid whenever a child
> process invalidates the coproc FDs.
>
> Chet