bug#43844: 27.1; Mouse-3 avoid "Buffer is read-only" *patch*

[Corwin Brust]

In Dired, ERC, and, I assume, any read-only buffer the default behavior of
mouse-3 causes errors (E.g. "kill-region: Buffer is read-only: #<buffer
*Messages*>").

The attached patch attempts to prevent this by detecting a
read-only-buffer.  Unfortunately, it seems that ERC has some other means
of causing its buffers to be rendered read-only that doesn't affect
this var, thus the special-case logic.  I tested with dired, erc, and
(regression tested) while composing this bug report.

Are there other edge cases like erc-mode hanging around, where
read-only-buffer might not be accurate?  Is this oddity with ERC worth a
separate bug report?  Finally, s there a more elegant way to special
case whatever erc-mode is doing?

This is my first attempt at submitting a patch.  I will dig a bit more
into my own questions, above; tia for tips, suggestions, etc.

In GNU Emacs 27.1 (build 1, x86_64-w64-mingw32)
 of 2020-08-21 built on CIRROCUMULUS
Repository revision: 86d8d76aa36037184db0b2897c434cdaab1a9ae8
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.18363
System Description: Microsoft Windows 10 Home (v10.0.1909.18363.1082)

Recent messages:
For information about GNU Emacs and the GNU system, type C-h C-a.

Configured using:
 'configure --without-dbus --host=x86_64-w64-mingw32
 --without-compress-install 'CFLAGS=-O2 -static''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY W32NOTIFY ACL GNUTLS LIBXML2
HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS MODULES THREADS JSON PDUMPER LCMS2 GMP

Important settings:
  value of $LANG: ENU
  locale-coding-system: cp1252

Major mode: Lisp Interaction

Minor modes in effect:
  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 dired dired-loaddefs
format-spec rfc822 mml easymenu mml-sec password-cache epa derived epg
epg-config gnus-util rmail rmail-loaddefs text-property-search time-date
subr-x seq byte-opt gv bytecomp byte-compile cconv mm-decode mm-bodies
mm-encode mail-parse rfc2231 mailabbrev gmm-utils mailheader cl-loaddefs
cl-lib sendmail rfc2047 rfc2045 ietf-drums mm-util mail-prsvr mail-utils
tooltip eldoc electric uniquify ediff-hook vc-hooks lisp-float-type
mwheel dos-w32 ls-lisp disp-table term/w32-win w32-win w32-vars
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 tab-bar menu-bar rfn-eshadow isearch timer
select scroll-bar mouse jit-lock font-lock syntax facemenu 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 charscript charprop case-table epa-hook
jka-cmpr-hook help simple abbrev obarray 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 w32notify w32 lcms2 multi-tty make-network-process
emacs)

Memory information:
((conses 16 46659 8583)
 (symbols 48 6075 1)
 (strings 32 16982 2021)
 (string-bytes 1 519943)
 (vectors 16 9274)
 (vector-slots 8 123598 13156)
 (floats 8 22 312)
 (intervals 56 222 0)
 (buffers 1000 12))


Corwin

mouse-3_no-kill-when-readonly-or-erc-mode.patch
Description: Binary data