bug#32257: 26.1; read-multiple-choice inf loops on mouse clicks

[Noam Postavsky <npostavs@gmail.com>]

Eli Zaretskii <eliz@gnu.org> writes:

>> >> > If we want this change on emacs-26, we should carefully audit all the
>> >> > other users of rmc.el
>> 
>> What should we check for though?
>
> Anything that read-event handles differently from read-char.  Maybe a
> good starting point is to compile a list of the differences in
> behavior between the two.

Looking at the implementation, they both delegate to
read_filtered_event, but read-char passes 1 for the first three args,
where read-event passes 0.

   If NO_SWITCH_FRAME, switch-frame events are stashed
   until we get a character we like, and then stuffed into
   unread_switch_frame.

   If ASCII_REQUIRED, check function key events to see
   if the unmodified version of the symbol has a Qascii_character
   property, and use that character, if present.

   If ERROR_NONASCII, signal an error if the input we
   get isn't an ASCII character with modifiers.  If it's false but
   ASCII_REQUIRED is true, just re-read until we get an ASCII
   character.

Another possibility is to use read-char-exclusive which only changes
ERROR_NON_ASCII to 0.  The downside is that when there are mouse clicks
it prints the events into the minibuffer, covering the prompt (but
typing "?"  still works to restore the prompt).

--- i/lisp/emacs-lisp/rmc.el
+++ w/lisp/emacs-lisp/rmc.el
@@ -118,7 +118,7 @@ read-multiple-choice
                             choices)))
                   (condition-case nil
                       (let ((cursor-in-echo-area t))
-                        (read-char))
+                        (read-char-exclusive))
                     (error nil))))
           (setq answer (lookup-key query-replace-map (vector tchar) t))
           (setq tchar