bug#49682: 27.2.50; accept-process-output within accept-process-output hangs emacs

[LdBeth <andpuke@foxmail.com>]

This bug seems happened many times to me (on Emacs 29.1 macOS, with
Mitsuharu's patches if that matters) whenever I open an multipart
email that contains HTML that has several image links through IMAP in
the Wanderlust email reader since I start to enable HTML images in the
email reader. (And for some reason C-g seems not able to interrupt the
hang)

The shr library uses url-queue to asynchronously download images
and Wanderlust also use `accept-process-output' call to handle IMAP.

For this particular case I used a dirty hack

(define-advice accept-process-output
    (:before (&rest _) sync-queue)
  (when (fboundp 'url-queue-check-progress)
    (funcall #'url-queue-check-progress)))

However, could we make `accept-process-output' atomic so
the timer won't interrupt it? Or there is no reliable
method yet to make any atomic operations in Emacs yet?

---
ldbeth