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

[Rahguzar via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Hi Eli,
   The patch fixes the problem.

Thanks,
Rahguzar

Eli Zaretskii <eliz@gnu.org> writes:

>> Date: Wed, 13 Mar 2024 22:13:55 +0100
>> From:  Rahguzar via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>
>>
>>
>> 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'.
>
> How about the patch below instead?
>
> diff --git a/lisp/image-mode.el b/lisp/image-mode.el
> index 355685e..fa64f1a 100644
> --- a/lisp/image-mode.el
> +++ b/lisp/image-mode.el
> @@ -654,8 +654,9 @@ image-mode
>    (unless (display-images-p)
>      (error "Display does not support images"))
>
> -  (major-mode-suspend)
> -  (setq major-mode 'image-mode)
> +  (unless (eq major-mode 'image-mode)
> +    (major-mode-suspend)
> +    (setq major-mode 'image-mode))
>    (setq image-transform-resize image-auto-resize)
>
>    ;; Bail out early if we have no image data.