bug#51101: 29.0.50; read-char-from-minibuffer accepts Enter even when not a choice.

[David M. Koppelman]

Execute:

 (read-char-from-minibuffer "Answer y or n" '(?y ?n))

and press Enter. The form returns Enter (13) rather than re-prompting
for a y or n.

This causes a dataloss threat due to read-char-from-minibuffer being
called through ask-user-about-supersession-threat.

Even if the read-char-from-minibuffer bug is quickly fixed, I'd
sleep better if the following patch were applied to userlock.el:

@@ -194,7 +194,9 @@ ask-user-about-supersession-threat
                       (list "File reverted" filename)))
              ((eq answer ?n)
               (signal 'file-supersession
-                      (list "File changed on disk" filename)))))
+                      (list "File changed on disk" filename)))
+              ((eq answer ?y))
+              (t (setq answer nil))))
       (message
        "File on disk now will become a backup file if you save these changes.")
       (setq buffer-backed-up nil))))


In GNU Emacs 29.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.30, cairo 
version 1.17.4)
 of 2021-10-08 built on dmk-laptop-21
Repository revision: e73c9ac4ed4ce0a4b423dae6acbfb384c1afbce0
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101002
System Description: Fedora Linux 35 (KDE Plasma Prerelease)

Configured using:
 'configure 'CFLAGS=-O3 -march=native' --without-pop
 --with-native-compilation'

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GPM GSETTINGS HARFBUZZ JPEG
JSON LCMS2 LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 M17N_FLT MODULES
NATIVE_COMP NOTIFY INOTIFY PDUMPER PNG RSVG SECCOMP SOUND THREADS TIFF
TOOLKIT_SCROLL_BARS X11 XDBE XIM XPM GTK3 ZLIB

Important settings:
  value of $LC_COLLATE: C
  value of $LC_TIME: C
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-mode: t
  show-paren-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  indent-tabs-mode: t
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny dired dired-loaddefs
rfc822 mml mml-sec epa derived epg rfc6068 epg-config gnus-util rmail
rmail-loaddefs auth-source cl-seq eieio eieio-core cl-macs
eieio-loaddefs password-cache json map text-property-search seq gv
byte-opt bytecomp byte-compile cconv mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils time-date subr-x
cl-loaddefs cl-lib iso-transl tooltip eldoc paren electric uniquify
ediff-hook vc-hooks lisp-float-type elisp-mode mwheel term/x-win x-win
term/common-win x-dnd tool-bar dnd fontset image regexp-opt fringe
tabulated-list replace newcomment text-mode lisp-mode prog-mode register
page tab-bar menu-bar rfn-eshadow isearch easymenu timer select
scroll-bar mouse jit-lock font-lock syntax font-core term/tty-colors
frame minibuffer cl-generic cham georgian utf-8-lang misc-lang
vietnamese tibetan thai tai-viet lao korean japanese eucjp-ms cp51932
hebrew greek romanian slovak czech european ethiopic indian cyrillic
chinese composite emoji-zwj charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray cl-preloaded nadvice button
loaddefs faces cus-face macroexp files window text-properties overlay
sha1 md5 base64 format env code-pages mule custom widget
hashtable-print-readable backquote threads dbusbind inotify lcms2
dynamic-setting system-font-setting font-render-setting cairo
move-toolbar gtk x-toolkit x multi-tty make-network-process
native-compile emacs)

Memory information:
((conses 16 68625 10824)
 (symbols 48 6630 0)
 (strings 32 19929 1554)
 (string-bytes 1 675055)
 (vectors 16 13948)
 (vector-slots 8 286810 18217)
 (floats 8 24 38)
 (intervals 56 232 0)
 (buffers 992 11))