GNU bug report logs - #69785
29.2; Image mode can loop due to invalid images

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Message #5 received at submit <at> debbugs.gnu.org (full text, mbox):

From: Rahguzar <rahguzar <at> zohomail.eu>
To: bug-gnu-emacs <at> gnu.org
Subject: 29.2; Image mode can loop due to invalid images
Date: Wed, 13 Mar 2024 22:13:55 +0100

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
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Message #8 received at 69785 <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Rahguzar <rahguzar <at> zohomail.eu>
Cc: 69785 <at> debbugs.gnu.org
Subject: Re: bug#69785: 29.2; Image mode can loop due to invalid images
Date: Thu, 21 Mar 2024 12:18:44 +0200

> 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 <at> 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.

Message #11 received at 69785 <at> debbugs.gnu.org (full text, mbox):

From: Rahguzar <rahguzar <at> zohomail.eu>
To: Eli Zaretskii <eliz <at> gnu.org>
Cc: 69785 <at> debbugs.gnu.org
Subject: Re: bug#69785: 29.2; Image mode can loop due to invalid images
Date: Fri, 22 Mar 2024 18:24:09 +0100

Hi Eli,
   The patch fixes the problem.

Thanks,
Rahguzar

Eli Zaretskii <eliz <at> 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 <at> 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.

Message #16 received at 69785-done <at> debbugs.gnu.org (full text, mbox):

From: Eli Zaretskii <eliz <at> gnu.org>
To: Rahguzar <rahguzar <at> zohomail.eu>
Cc: 69785-done <at> debbugs.gnu.org
Subject: Re: bug#69785: 29.2; Image mode can loop due to invalid images
Date: Sat, 23 Mar 2024 09:53:16 +0200

> From: Rahguzar <rahguzar <at> zohomail.eu>
> Cc: 69785 <at> debbugs.gnu.org
> Date: Fri, 22 Mar 2024 18:24:09 +0100
> 
> Hi Eli,
>    The patch fixes the problem.

Thanks, installed on master, and closing the bug.

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