bug#69785: 29.2; Image mode can loop due to invalid images

[Rahguzar]

Dear Emacs developers,

Starting with emacs -Q, evaluate

(with-current-buffer (get-buffer-create "*image-mode-test*")
  (insert (propertize "abc" 'display (list 'image "Definitely not an image.")))
  (image-mode))

twice.

The second time results in an error and pop ups the debugger, with the
following text,

Debugger entered--Lisp error: (excessive-lisp-nesting 1601)
  display-images-p()
  image-mode()
  major-mode-restore((image-mode image-mode-as-text))
  image-mode-to-text()
  image-mode-as-text()
  image-mode--display()
  image-mode()
  major-mode-restore((image-mode image-mode-as-text))
  image-mode-to-text()
  image-mode-as-text()
  image-mode--display()
  image-mode()
  major-mode-restore((image-mode image-mode-as-text))
  image-mode-to-text()
  image-mode-as-text()
  image-mode--display()
  image-mode()
  ...

It is unclear to me why this doesn't happen the first time but I think
sequence causing it the second time is as follows:

1. 'image-mode' calls 'major-mode-suspend' when 'major' mode is
'fundamental-mode'. So 'major-mode--suspended' is not set.

2. 'image-mode--display' catches error and call 'image-mode-as-text'
which calls 'image-mode-to-text'.

3. 'image-mode-as-text' calls 'major-mode-restore' with '(image-mode
image-mode-as-text) as mode to avoid.

4. Since major-mode--suspended was not set in step 1
'major-mode-restore' falls back to 'normal-mode' which calls
'image-mode' causing the loop.

I think the simplest way of fixing this is to make sure that
'image-mode' doesn't call 'major-mode-suspend' with 'fundamental-mode'
instead calling 'text-mode' or 'special-mode' before calling
'major-mode-suspend' if it starts in 'fundamental-mode'.

Thanks,
Rahguzar


In GNU Emacs 29.2 (build 1, aarch64-redhat-linux-gnu, GTK+ Version
 3.24.41, cairo version 1.18.0) of 2024-02-02 built on
 4f70ed73c5a14276aae52125608014aa
System Description: Fedora Linux Asahi Remix 39 (Thirty Nine)

Configured using:
 'configure --build=aarch64-redhat-linux --host=aarch64-redhat-linux
 --program-prefix= --disable-dependency-tracking --prefix=/usr
 --exec-prefix=/usr --bindir=/usr/bin --sbindir=/usr/sbin
 --sysconfdir=/etc --datadir=/usr/share --includedir=/usr/include
 --libdir=/usr/lib64 --libexecdir=/usr/libexec --localstatedir=/var
 --runstatedir=/run --sharedstatedir=/var/lib --mandir=/usr/share/man
 --infodir=/usr/share/info --with-cairo --with-dbus --with-gif
 --with-gpm=no --with-harfbuzz --with-jpeg --with-json --with-modules
 --with-native-compilation=aot --with-pgtk --with-png --with-rsvg
 --with-sqlite3 --with-tiff --with-tree-sitter --with-webp --with-xpm
 --with-xwidgets build_alias=aarch64-redhat-linux
 host_alias=aarch64-redhat-linux CC=gcc 'CFLAGS=-DMAIL_USE_LOCKF -O2
 -flto=auto -ffat-lto-objects -fexceptions -g -grecord-gcc-switches
 -pipe -Wall -Werror=format-security
 -Wp,-U_FORTIFY_SOURCE,-D_FORTIFY_SOURCE=3 -Wp,-D_GLIBCXX_ASSERTIONS
 -specs=/usr/lib/rpm/redhat/redhat-hardened-cc1 -fstack-protector-strong
 -specs=/usr/lib/rpm/redhat/redhat-annobin-cc1
 -mbranch-protection=standard -fasynchronous-unwind-tables
 -fstack-clash-protection -fno-omit-frame-pointer
 -mno-omit-leaf-frame-pointer ' LDFLAGS=-Wl,-z,relro
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 'CXXFLAGS=-O2 -flto=auto -ffat-lto-objects -fexceptions -g
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 -mbranch-protection=standard -fasynchronous-unwind-tables
 -fstack-clash-protection -fno-omit-frame-pointer
 -mno-omit-leaf-frame-pointer ''

Configured features:
ACL CAIRO DBUS FREETYPE GIF GLIB GMP GNUTLS GSETTINGS HARFBUZZ JPEG JSON
LIBOTF LIBSELINUX LIBSYSTEMD LIBXML2 MODULES NATIVE_COMP NOTIFY INOTIFY
PDUMPER PGTK PNG RSVG SECCOMP SOUND SQLITE3 THREADS TIFF
TOOLKIT_SCROLL_BARS TREE_SITTER WEBP XIM XWIDGETS GTK3 ZLIB

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

Major mode: mu4e:main

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