Re: No thread yield when using synchronous processes?

[Philipp Stephani]

Am Do., 13. Dez. 2018 um 14:46 Uhr schrieb Eli Zaretskii <eliz@gnu.org>:
>
> > From: Michael Albinus <michael.albinus@gmx.de>
> > Date: Thu, 13 Dec 2018 10:47:32 +0100
> > Cc: help-gnu-emacs <help-gnu-emacs@gnu.org>
> >
> > > it seems like Emacs doesn't yield when using synchronous processes:
> > >
> > > (make-thread (lambda () (call-process "sleep" nil nil nil "5s")))
> > >
> > > ⇒ Emacs freezes for 5 seconds.
> > >
> > > Is there a reason for this behavior? The manual claims that Emacs
> > > yields when waiting for process output, but that only seems to be the
> > > case for asynchronous processes.
> >
> > The manual is clear about this: "when waiting for ... process output
> > (e.g., during ‘accept-process-output’)". `accept-process-output' is used
> > only for asynchronous processes.
>
> Right.  Though perhaps we should say that more explicitly, as "e.g."
> could be interpreted like just one example, not necessarily a
> representative of a class of examples.

Yes, I think this should be much more explicit. Given that thread
yields are highly observable (buffer contents, point, etc. can
change), it would be best to explicitly document each function that
yields, and state that other functions don't yield.

>
> Technically, thread switches happen when Emacs calls 'pselect', which
> it does as part of its idle loop.  call-process doesn't enter the idle
> loop, it busy-waits until the process exits.  One could think about
> calling the idle loop while we wait, but this will have significant
> Emacs-wide effects, which need to be considered carefully before we
> decide to do it.
>
> (Btw, pedantically, the example above doesn't really _wait_ for
> process output, it explicitly discards that output.)
>