bug#84: Acknowledgement (23.0.60; Occasional hangs in flyspell-mode and ispell-word)

[Markus Triska <markus.triska@gmx.at>]

> while flyspell-word waits for output; one can interrupt the wait with
> C-g

I've now seen that this wasn't the case in the backtrace, i.e., I
specifically could NOT interrupt accept-process-output; also, had I been
able to interrupt it, a subsequent process-send-string to the dead
process from flyspell-word would have triggered an error and thus
removed flyspell-post-command-hook from post-command-hook.

To sum up, I think it remains valid that accept-process-output should
throw an error (instead of waiting, as it currently does) if the process
it is waiting for is dead, and an independent issue is that
wait_reading_process_output can sometimes not be interrupted manually
and can make Emacs hang indefinitely. So far, the hangs seem to occur
when I have previously interrupted wait_reading_process_output when it
was waiting for output from a dead process. For example, I'm using the
following to enable flyspell mode when it was disabled:

   (defun reactivate-flyspell ()
     (unless (memq 'flyspell-post-command-hook post-command-hook)
       (flyspell-mode 1)))

   (setq my-idle (run-with-idle-timer 0.1 t 'reactivate-flyspell))

then I continuously kill aspell with:

   $ while true; do killall -s 9 aspell; sleep 0.1; done

Then I go into a buffer with a lot of text and move the cursor over a
few words (to flyspell-them) until Emacs hangs due to
accept-process-output from a dead aspell process. I can always interrupt
this with C-g. I do this a few times, then stop the continuous killing
of aspell, and then start Gnus a few times. This almost always makes
Emacs hang completely; I'm now trying to make this test case shorter.