bug#44448: Fwd: bug#44448:

[Amai Kinono]

Thanks for your help. I've got a backtrace when the bug happens ;)

(The story is, there must be some problems of the developing toolchain on my
distro (tested on 2 PCs running it), so I burn a new ubuntu live CD and built &
debugged Emacs on it, works like a charm.)

The backtrace:

---

Thread 1 "emacs" hit Breakpoint 3, signal_or_quit (error_symbol=XIL(0x555555e9b520), data="" keyboard_quit=false) at eval.c:1793
1793 {
#0  signal_or_quit (error_symbol=XIL(0x555555e9b520), data="" keyboard_quit=false) at eval.c:1793
#1  0x000055555582d7ad in quit () at eval.c:1783
#2  0x000055555582d68b in process_quit_flag () at eval.c:1730
#3  0x000055555582d6d7 in maybe_quit () at eval.c:1750
#4  0x0000555555839e4a in list_length (list=XIL(0)) at fns.c:150
#5  0x000055555590036e in get_logical_fringe_bitmap (w=0x5555567d5870, bitmap=XIL(0xe430), right_p=1, partial_p=0) at fringe.c:746
#6  0x0000555555903a0b in update_window_fringes (w=0x5555567d5870, keep_current_p=true) at fringe.c:1248
#7  0x000055555560a033 in redisplay_window (window=XIL(0x5555567d5875), just_this_one_p=false) at xdisp.c:19401
#8  0x00005555555feaea in redisplay_window_0 (window=XIL(0x5555567d5875)) at xdisp.c:16617
#9  0x000055555582ce52 in internal_condition_case_1 (bfun=0x5555555feaa4 <redisplay_window_0>, arg=XIL(0x5555567d5875), handlers=XIL(0x7ffff4f39aeb), hfun=0x5555555fea68 <redisplay_window_error>) at eval.c:1502
#10 0x00005555555fea39 in redisplay_windows (window=XIL(0x5555567d5875)) at xdisp.c:16597
#11 0x00005555555fe9eb in redisplay_windows (window=XIL(0x55555626ae75)) at xdisp.c:16591
#12 0x00005555555fd36d in redisplay_internal () at xdisp.c:16065
#13 0x00005555555fe1af in redisplay_preserve_echo_area (from_where=8) at xdisp.c:16414
#14 0x000055555576221f in detect_input_pending_run_timers (do_display=true) at keyboard.c:10392
#15 0x00005555558a5422 in wait_reading_process_output (time_limit=30, nsecs=0, read_kbd=-1, do_display=true, wait_for_cell=XIL(0), wait_proc=0x0, just_wait_proc=0) at process.c:5664
#16 0x00005555555ab5b3 in sit_for (timeout=make_fixnum(30), reading=true, display_option=1) at dispnew.c:6159
#17 0x000055555574d2e1 in read_char (commandflag=1, map=XIL(0x5555564f7463), prev_event=XIL(0), used_mouse_menu=0x7fffffffd9fd, end_time=0x0) at keyboard.c:2784
#18 0x0000555555760119 in read_key_sequence (keybuf=0x7fffffffdbe0, prompt=XIL(0), dont_downcase_last=false, can_return_switch_frame=true, fix_current_buffer=true, prevent_redisplay=false) at keyboard.c:9569
#19 0x0000555555748958 in command_loop_1 () at keyboard.c:1376
#20 0x000055555582cd6f in internal_condition_case (bfun=0x5555557484ba <command_loop_1>, handlers=XIL(0x90), hfun=0x555555747906 <cmd_error>) at eval.c:1478
#21 0x000055555574807b in command_loop_2 (handlers=XIL(0x90)) at keyboard.c:1117
#22 0x000055555582be66 in internal_catch (tag=XIL(0xe280), func=0x555555748050 <command_loop_2>, arg=XIL(0x90)) at eval.c:1198
#23 0x000055555574801b in command_loop () at keyboard.c:1095
#24 0x00005555557473b1 in recursive_edit_1 () at keyboard.c:720
#25 0x00005555557475cd in Frecursive_edit () at keyboard.c:792
#26 0x0000555555743179 in main (argc=1, argv=0x7fffffffe068) at emacs.c:2310

Lisp Backtrace:
"redisplay_internal (C function)" (0x0)
$59 = 1

---

1. I've recorded several backtraces when the bug happens. They look basically
   the same so I just post one here. To me the update_window_fringes looks
   suspicious.

2. I tested with mode-line-format being nil or the default value, the backtrace
   when bug happens look the same, so I believe you've fixed the modeline part.

Eli Zaretskii <eliz@gnu.org> 于2021年8月16日周一 上午2:29写道：

> From: Amai Kinono <amaikinono@gmail.com>
> Date: Mon, 16 Aug 2021 02:10:33 +0800
> Cc: "44448@debbugs.gnu.org" <44448@debbugs.gnu.org>
>
> I tried and the bug can happen with breakpoint set. Haven't got a backtrace though.
>
> I have several questions.
>
> 1. I am not familiar with GDB. I set the breakpoint by "b signal_or_quit", is that right?

Yes.

> 2. When I reached the breakpoint, "p current_buffer" gives me "No symbol "current_buffer" in current
> context." How should I get the point position in current window?

I don't understand how this could happen.  current_buffer is a global
variable, it should be visible everywhere.  Is your Emacs built with
the -g3 switch (i.e. CFLAGS includes -g3)?  If not, please rebuild
with CFLAGS=-g3.

> 3. When the bug happens, the cursor doesn't jump before hitting the breakpoint, but immediately after I
> continue using "c". Does this seem right? (I guess so)

Yes.  But you should define commands for the breakpoint, so that Emacs
keeps running.  Like this:

  (gdb) break signal_or_quit
  (gdb) commands
    > backtrace
    > print current_buffer->pt
    > continue
    > end

> Printing the backtrace is so slow that it freezes my desktop for a long time (and I don't know if that's
> forever). I need a way to tell if the bug happens before I print the backtrace (I think that's the 2nd question
> above), so I can make sure I am getting the right backtrace and leave my PC there for a while.

I don't know how to tell that.  In my testing, I kept typing C-g until
I saw the buffer displayed at BOB, then stopped typing C-g and looked
at the last few (2 to 4) backtraces.  I think the backtraces that are
interesting are inside redisplay (under the function
redisplay_internal).

Printing backtrace should be fast enough, it definitely shouldn't
freeze anything.  It didn't freeze for me here.