bug#61940: 29.0.60; Occasional crash when moving point continously with visual line numbers

[Eli Zaretskii <eliz@gnu.org>]

> From: Simon Pugnet <simon@polaris64.net>
> Cc: 61940@debbugs.gnu.org
> Date: Mon, 06 Mar 2023 11:53:27 +0000
> 
> Sorry, yes I realise now that I said "crash" in the subject but I 
> meant that it hung or stopped responding to input. What I see when 
> this happens is a plain white frame (background colour of the theme) 
> and no keyboard or mouse input seems to be accepted.
> 
> 
> >> From there I obtained the backtrace and was able to step, which is
> >> how I noticed that it seemed to be in a loop. I might have 
> >> diagnosed
> >> this incorrectly however.
> >
> > The file etc/DEBUG in the Emacs repository has instructions for how 
> > to
> > identify the code in which Emacs is looping, if it's looping.  Look
> > for "If the symptom of the bug is that Emacs fails to respond".
> 
> I managed to trigger this situation again this morning while working. 
> Unfortunately I was unable to trigger it while it was being debugged 
> in gdb; the problem occurs rarely so it's not easy to reproduce on 
> demand.

You can simply run Emacs under GDB at all times.  As long as Emacs
runs normally, GDB will stay out of the way.

> Thread 1 (Thread 0x7f11e55fa2c0 (LWP 35073) "emacs"):
> #0  0x00007f11e8d139df in poll () at /usr/lib/libc.so.6
> #1  0x00007f11ec43726b in  () at /usr/lib/libxcb.so.1
> #2  0x00007f11ec438d1d in xcb_wait_for_event () at 
>  /usr/lib/libxcb.so.1
> #3  0x00007f11ec49ab09 in _XReadEvents () at /usr/lib/libX11.so.6
> #4  0x00007f11ec48143a in XIfEvent () at /usr/lib/libX11.so.6
> #5  0x00007f11ec4c7f10 in  () at /usr/lib/libX11.so.6
> #6  0x00007f11ec4bf523 in  () at /usr/lib/libX11.so.6
> #7  0x00007f11ec4c847c in _XimRead () at /usr/lib/libX11.so.6
> #8  0x00007f11ec4b8d6c in  () at /usr/lib/libX11.so.6
> #9  0x00007f11ec4a3c5f in XSetICValues () at /usr/lib/libX11.so.6
> #10 0x0000563eb62f9d0b in xic_set_preeditarea (w=0x563ec43bdc60, 
>  x=287, y=493) at xfns.c:3176
> #11 0x0000563eb62e5194 in handle_one_xevent 
>  (dpyinfo=dpyinfo@entry=0x563eb8c67800, 
>  event=event@entry=0x7ffce40dc7e0, finish=finish@entry=0x563eb6994270 
>  <current_finish>, hold_quit=0x7ffce40dcac0) at xterm.c:20116
> #12 0x0000563eb62ecec1 in event_handler_gdk (gxev=0x7ffce40dc7e0, 
>  ev=<optimized out>, data=<optimized out>) at xterm.c:17447
> #13 0x00007f11ed4d1b0f in  () at /usr/lib/libgdk-3.so.0
> #14 0x00007f11ed4d96d5 in  () at /usr/lib/libgdk-3.so.0
> #15 0x00007f11ed47f029 in gdk_display_get_event () at 
>  /usr/lib/libgdk-3.so.0
> #16 0x00007f11ed4d9a68 in  () at /usr/lib/libgdk-3.so.0
> #17 0x00007f11ec60b82b in g_main_context_dispatch () at 
>  /usr/lib/libglib-2.0.so.0
> #18 0x00007f11ec662cc9 in  () at /usr/lib/libglib-2.0.so.0
> #19 0x00007f11ec60a0e2 in g_main_context_iteration () at 
>  /usr/lib/libglib-2.0.so.0
> #20 0x00007f11ecdd8eeb in gtk_main_iteration () at 
>  /usr/lib/libgtk-3.so.0
> #21 0x0000563eb62d783a in XTread_socket (terminal=<optimized out>, 
>  hold_quit=0x7ffce40dcac0) at xterm.c:24819
> #22 0x0000563eb6322f61 in gobble_input () at keyboard.c:7417
> #23 0x0000563eb6323335 in handle_async_input () at keyboard.c:7648
> #24 process_pending_signals () at keyboard.c:7662
> #25 0x0000563eb63ac12d in probably_quit () at eval.c:1661
> #26 0x0000563eb63c0a50 in maybe_quit () at 
>  /storage/Work/personal/emacs/src/lisp.h:3689

This again says that Emacs is trying to read input via XIM.  Maybe Po
Lu could help us understand what could be going on.

> I tried using the "finish" gdb command from this point and here is 
> what happened: -
> 
> (gdb) finish
> Run till exit from #0  0x00007f11e8d139df in poll () from 
> /usr/lib/libc.so.6
> 0x00007f11ec43726b in ?? () from /usr/lib/libxcb.so.1
> (gdb) finish
> Run till exit from #0  0x00007f11ec43726b in ?? () from 
> /usr/lib/libxcb.so.1
> 0x00007f11ec438d1d in xcb_wait_for_event () from /usr/lib/libxcb.so.1
> (gdb) finish
> Run till exit from #0  0x00007f11ec438d1d in xcb_wait_for_event () 
> from /usr/lib/libxcb.so.1
> 0x00007f11ec49ab09 in _XReadEvents () from /usr/lib/libX11.so.6
> (gdb) finish
> Run till exit from #0  0x00007f11ec49ab09 in _XReadEvents () from 
> /usr/lib/libX11.so.6
> 0x00007f11ec48143a in XIfEvent () from /usr/lib/libX11.so.6
> (gdb) finish
> Run till exit from #0  0x00007f11ec48143a in XIfEvent () from 
> /usr/lib/libX11.so.6
> 
> (at this point the "finish" command does not return).
> 
> I hope that helps. I will try to reproduce the problem while running 
> in gdb and I will send an update if I'm successful.