bug#63865: 29.0.90; call-process while owning the X selection hangs other processes

[Eli Zaretskii <eliz@gnu.org>]

> From: Spencer Baugh <sbaugh@janestreet.com>
> Cc: Eli Zaretskii <eliz@gnu.org>,  63865@debbugs.gnu.org
> Date: Sat, 03 Jun 2023 08:30:57 -0400
> 
> Po Lu <luangruo@yahoo.com> writes:
> 
> > Spencer Baugh <sbaugh@janestreet.com> writes:
> >
> >> Po Lu <luangruo@yahoo.com> writes:
> >>
> >>> Eli Zaretskii <eliz@gnu.org> writes:
> >>>
> >>>>> From: Spencer Baugh <sbaugh@janestreet.com>
> >>>>> Date: Fri, 02 Jun 2023 21:55:09 -0400
> >>>>> 
> >>>>> 
> >>>>> 1. Under X11, with the GTK or Lucid toolkits:
> >>>>>    emacs -Q
> >>>>> 2. Become owner of the clipboard selection by killing some text; the
> >>>>>    starting comments in the scratch buffer are a good candidate.
> >>>>> 3. Immediately afterwards (i.e. without copy and pasting text in another
> >>>>>    window), run:
> >>>>>    (call-process "sleep" nil nil nil "inf")
> >>>>> 4. Now other applications will hang when they attempt to paste text.
> >>>>>    Google Chrome and Slack are two examples.  (GTK-based applications
> >>>>>    seem to be fine.  So much for proprietary software...)
> >>>>
> >>>> Thanks.
> >>>>
> >>>> Does this happen also with the latest pretest, v29.0.91?
> >>>
> >>> I can't reproduce this, but the closest thing to Google Chrome on the
> >>> computer I am currently using is Firefox 10.0.7.
> >>
> >> BTW, this can also be reproduced using just Emacs.  If I try to paste in
> >> another Emacs instead of in Google Chrome, I get a hang, followed by:
> >>
> >> gui-get-selection: (error "Timed out waiting for reply from selection owner")
> >
> > Emacs doesn't hang...
> 
> Well, it hangs for 10 seconds, then times out, because Emacs is
> correctly implemented.

The value of the timeout can be customized, if you don't like the
default.

> > OK, but then this is not a bug: Emacs can not respond to selection
> > requests when it is not reading keyboard input.
> 
> "Emacs can not respond to selection requests when it is not reading
> keyboard input." sounds like a bug to me!  Even if it's hard to fix,
> it's still a bug.

We run Lisp to generate selection response, so there's no way we can
do that when the main Lisp thread is busy.  It isn't a bug, it's a
restriction of how Emacs is designed.

> If I'm implementing some package and I decide to use call-process for
> some long operation, then some user uses my package and it runs
> call-process, and they get bored while waiting and switch away from
> Emacs, they'll experience a hang in some other application.  That hang
> seems clearly undesirable!

Then don't design the package such that call-process blocks Emacs for
prolonged periods of time.  Because this will annoy the users of Emacs
even before it will be seen by other applications that request X
selections.

> I'm personally working around this by replacing call-process with
> start-process and accept-process-output.  Because otherwise my packages
> (and any other package using call-process ever) will cause random hangs
> in other applications, which is obviously bad and not something anyone
> would want.
> 
> So perhaps call-process on Unix should be reimplemented in terms of
> those functions?  Or if that would change behavior too much, perhaps
> call-process should be deprecated in favor of some new helper built on
> those?

call-process has its use cases, which are important, and we will not
deprecate it.

You can easily emulate call-process with start-process if you need to
do so, so Emacs gives you both possibilities (and expects you to use
whatever is right in each case).