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Subject: bug#65193: 29.1.50; SET_FRAME_ICONIFIED can cause Lisp evaluation inside read_socket_hook.
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SET_FRAME_ICONIFIED has a call to gui_consider_frame_title and it can
cause Lisp evaluation if icon-title-format or frame-title-format
contains (:eval FORM).  This is problematic because
SET_FRAME_ICONIFIED can be called inside read_socket_hook.

The call to gui_consider_frame_title was introduced by the following
change:

commit e99f41f03a97641ee05ba4a27f8b91c190f55df1
Author: Po Lu <luangruo@HIDDEN>
Date:   Thu Jun 9 21:48:19 2022 +0800

   Fix recalculation of `icon-title-format' after a frame is iconified

   Previously it would only happen upon the next mode line
   redisplay, meaning that just pressing C-z would not update the
   implicit title, since C-z doesn't cause a redisplay.

   * src/dispextern.h: Update prototypes.
   * src/frame.h (SET_FRAME_ICONIFIED): De-slugify.  Call
   `gui_consider_frame_title', since `icon-title-format' might be
   different from the current frame title.
   * src/xdisp.c (gui_consider_frame_title): Export (also in
   dispextern.h).  (bug#55850)

				     YAMAMOTO Mitsuharu
				mituharu@HIDDEN

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To: YAMAMOTO Mitsuharu <mituharu@HIDDEN>
Subject: bug#65193: Acknowledgement (29.1.50; SET_FRAME_ICONIFIED can
 cause Lisp evaluation inside read_socket_hook.)
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Subject: bug#65193: 29.1.50; SET_FRAME_ICONIFIED can cause Lisp evaluation inside read_socket_hook.
Resent-From: Po Lu <luangruo@HIDDEN>
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To: YAMAMOTO Mitsuharu <mituharu@HIDDEN>
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From: Po Lu <luangruo@HIDDEN>
In-Reply-To: <wl7cq3mjj7.wl-mituharu@HIDDEN> (YAMAMOTO
 Mitsuharu's message of "Thu, 10 Aug 2023 16:41:16 +0900")
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Date: Thu, 10 Aug 2023 16:26:00 +0800
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YAMAMOTO Mitsuharu <mituharu@HIDDEN> writes:

> SET_FRAME_ICONIFIED has a call to gui_consider_frame_title and it can
> cause Lisp evaluation if icon-title-format or frame-title-format
> contains (:eval FORM).  This is problematic because
> SET_FRAME_ICONIFIED can be called inside read_socket_hook.
>
> The call to gui_consider_frame_title was introduced by the following
> change:

What practical problems have resulted from this change?

There are already several situations under which Lisp can be called
inside read_socket_hook.  And in principle, anything that performs GC
can in turn call finalizers that subsequently run Lisp.

Message sent to bug-gnu-emacs@HIDDEN:

Subject: bug#65193: 29.1.50; SET_FRAME_ICONIFIED can cause Lisp evaluation inside read_socket_hook.
Resent-From: YAMAMOTO Mitsuharu <mituharu@HIDDEN>
Original-Sender: "Debbugs-submit" <debbugs-submit-bounces <at> debbugs.gnu.org>
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To: Po Lu <luangruo@HIDDEN>
Cc: 65193 <at> debbugs.gnu.org
Date: Thu, 10 Aug 2023 18:21:26 +0900
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From: YAMAMOTO Mitsuharu <mituharu@HIDDEN>
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 <87pm3vs3qf.fsf@HIDDEN>
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On Thu, 10 Aug 2023 17:26:00 +0900,
Po Lu wrote:
> 
> YAMAMOTO Mitsuharu <mituharu@HIDDEN> writes:
> 
> > SET_FRAME_ICONIFIED has a call to gui_consider_frame_title and it can
> > cause Lisp evaluation if icon-title-format or frame-title-format
> > contains (:eval FORM).  This is problematic because
> > SET_FRAME_ICONIFIED can be called inside read_socket_hook.
> >
> > The call to gui_consider_frame_title was introduced by the following
> > change:
> 
> What practical problems have resulted from this change?

To my understanding, the Lisp evaluator is not reentrant, and thus
Lisp evaluation inside read_socket_hook, which can be called from
fairly arbitrary places via unblock_input/maybe_quit, causes
hard-to-reproduce, spontaneous crashes.

> There are already several situations under which Lisp can be called
> inside read_socket_hook.

That's really surprising.  I've once heard we shouldn't do that.  Is
that changed?

> And in principle, anything that performs GC can in turn call
> finalizers that subsequently run Lisp.

My understanding is that GC can only be triggered by eval_sub or
Ffuncall calls (except explicit garbage_collect calls), but not by
Lisp object allocations, for example.  Avoiding Lisp evaluation inside
read_socket_hook also means avoiding GC inside read_socket_hook.

				     YAMAMOTO Mitsuharu
				mituharu@HIDDEN

Last modified: Thu, 10 Aug 2023 09:30:02 UTC

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