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Ok, this issue I'm running into seems pretty weird. I'm running an async process during init and I have set a process sentinel to tell me when it exits. This works fine in terminal Emacs but in GUI Emacs the process sentinel never runs.

Here is a minimal working example:

(setq my/test-flag nil)
(defun my/test-sentinel (proc &optional _change)
  (setq my/test-flag t))
(let* ((name "test")
       (buf (generate-new-buffer (concat "*" name "*")))
       (proc (let ((process-connection-type nil))
               (apply #'start-process name buf "sleep" '("3")))))
  (set-process-sentinel proc #'my/test-sentinel)
  (while (not my/test-flag)
    (sleep-for 0.05)))

If you put this in init.el and run terminal Emacs, it will hang for 3 seconds as expected before continuing on. GUI Emacs, on the other hand, will hang forever, indicating that my/test-sentinel is never run, even though the process does finish after 3 seconds.

Background info:

  • I am using Emacs 27.1 from the Arch Linux repositories.
  • I failed to reproduce this on macOS using the emacs Cask.
  • Replacing sleep-for with accept-process-output gives the same result.
  • If the snippet is run by the user after init, the sentinel runs as expected in GUI Emacs.
  • This issue arose while debugging use of async-wait from the async package during init.
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  • 1
    I put the code in the end of my init.el then restart my Emacs (Emacs Mac Port), it does not hang forever and after Emacs finishes startup, my/test-flag is t.
    – xuchunyang
    Commented Aug 31, 2020 at 2:24
  • 1
    I see the same result as @xuchunyang with GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars) of 2020-08-12, on Ubuntu 18.04.
    – phils
    Commented Aug 31, 2020 at 2:32
  • 1
    I see the same behavior as @PythonNut on emacs 28.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.13, cairo version 1.16.0) of 2020-04-23 (a self-built version). I did emacs -q -l foo.el where foo.el contains just the code above.
    – NickD
    Commented Aug 31, 2020 at 4:12
  • 1
    I also tried toggling debug-on-quit and added a debugging print of (process-list) in the loop: the test process appears in that output for as long as I wait: after pressing C-g I call process-list and it returns nil.
    – NickD
    Commented Aug 31, 2020 at 4:15
  • I just failed to reproduce on macOS.
    – PythonNut
    Commented Aug 31, 2020 at 6:47

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