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

[Dmitry Gutov]

On 26.07.2021 22:54, Rajeev N via Bug reports for GNU Emacs, the Swiss 
army knife of text editors wrote:
> For me the following also hangs emacs.  The problem seems to me that 
> (accept-process-output stream 0.05) in network-stream-get-response does not 
> return even though it has a timeout of .05.
>
> (let ()
>    (run-with-timer 0 nil #'url-retrieve"https://www.gnu.org/";  #'ignore)
>    (make-process :name "echo" :command '("echo")
>                 :sentinel (lambda (_p _e)
>                             (with-current-buffer (generate-new-buffer " *debug")
>                               (run-with-timer 0 nil #'open-network-stream
>                                               "testing" (current-buffer) 
> "smtp.gmail.com" 587
>                                               :type 'starttls
>                                               :return-list t
>                                               :warn-unless-encrypted t
>                                               :capability-command 
> "EHLOwww.gnu.org\r\n"
>                                               :end-of-command "^[0-9]+ .*\r\n"
>                                               :success "^2.*\n"
>                                               :always-query-capabilities t
>                                               :starttls-function
>                                               (lambda (capabilities)
>                                                 (and (string-match "[ 
> -]STARTTLS" capabilities)
>                                                      "STARTTLS\r\n"))
>                                               :client-certificate t
>                                               :use-starttls-if-possible t)))))

Hmm, that doesn't freeze for me.

Even though if I follow 'C-x C-e' with 'C-p' right away, it will wait 
for like 0.5s before moving the cursor to the previous line.

I have tried it in a number of versions of Emacs, so far no luck.

Any chance you're not starting with 'emacs -Q' and your personal config 
affects this?