bug#40357: 26.3; delete-selection-mode stops working in some C-g cancellation cases

[Raoul Gough]

While in pending-delete-mode, user cancellation via C-g in some
circumstances can cause Emacs to remove the delete-selection-pre-hook
from pre-command-hook, meaning that pending-delete-mode is still enabled
but stops working. Example situation:

The current buffer is changed on disk
The selection is active
User types or yanks something, triggering deletion of the selection
This pops up a message <file> changed on disk; really edit the buffer?
Hitting C-g at this point causes the problem

Example to reproduce from a freshly started Emacs:

M-x delete-selection-mode
Create a new file, e.g. /tmp/foo
Add some text to the file and save it
Update the file externally, e.g. via C-M-! echo bar >>/tmp/foo RET
Select text in the buffer and type a letter (triggering delete selection)
When prompted with "really edit the buffer?" hit C-g twice

After the repro above, delete-selection-mode is active but does
nothing. Examination of pre-command-hook shows that the
delete-selection-pre-hook hook was removed.

There are some comments in delete-selection-helper which suggests this
is kind-of a known problem but the workaround there only works for
read-only text, not for modified-on-disk files.



In GNU Emacs 26.3 (build 1, x86_64-apple-darwin14.5.0, NS appkit-1348.17 
Version 10.10.5 (Build 14F2511))
 of 2019-09-02 built on builder10-10.porkrind.org
Windowing system distributor 'Apple', version 10.3.1504
Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.
Delete-Selection mode enabled
Saving file /tmp/foo...
Wrote /tmp/foo
(Shell command succeeded with no output)
Mark set
Error in pre-command-hook (delete-selection-pre-hook): (quit)
Quit

Configured using:
 'configure --with-ns '--enable-locallisppath=/Library/Application
 Support/Emacs/${version}/site-lisp:/Library/Application
 Support/Emacs/site-lisp' --with-modules'

Configured features:
NOTIFY ACL GNUTLS LIBXML2 ZLIB TOOLKIT_SCROLL_BARS NS MODULES THREADS

Important settings:
  value of $LANG: en_GB.UTF-8
  locale-coding-system: utf-8-unix

Major mode: Lisp Interaction

Minor modes in effect:
  shell-dirtrack-mode: t
  delete-selection-mode: t
  tooltip-mode: t
  global-eldoc-mode: t
  eldoc-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
  transient-mark-mode: t

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug message rmc puny seq byte-opt gv
bytecomp byte-compile cconv cl-loaddefs cl-lib dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs mm-decode mm-bodies mm-encode
mail-parse rfc2231 mailabbrev gmm-utils mailheader sendmail rfc2047
rfc2045 ietf-drums mm-util mail-prsvr mail-utils shell pcomplete comint
ansi-color ring cus-start cus-load delsel elec-pair time-date tooltip
eldoc electric uniquify ediff-hook vc-hooks lisp-float-type mwheel
term/ns-win ns-win ucs-normalize mule-util term/common-win tool-bar dnd
fontset image regexp-opt fringe tabulated-list replace newcomment
text-mode elisp-mode lisp-mode prog-mode register page menu-bar
rfn-eshadow isearch timer select scroll-bar mouse jit-lock font-lock
syntax facemenu font-core term/tty-colors frame 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 charscript charprop
case-table epa-hook jka-cmpr-hook help simple abbrev obarray minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote threads kqueue cocoa ns
multi-tty make-network-process emacs)

Memory information:
((conses 16 215383 9376)
 (symbols 48 21447 2)
 (miscs 40 58 217)
 (strings 32 31757 2312)
 (string-bytes 1 839659)
 (vectors 16 35726)
 (vector-slots 8 729632 10140)
 (floats 8 49 86)
 (intervals 56 220 0)
 (buffers 992 13))