Package: emacs;
Reported by: Juri Linkov <juri <at> linkov.net>
Date: Mon, 25 Aug 2025 06:34:01 UTC
Severity: normal
Done: Stefan Monnier <monnier <at> iro.umontreal.ca>
Message #8 received at 79306 <at> debbugs.gnu.org (full text, mbox):
From: Eli Zaretskii <eliz <at> gnu.org> To: Juri Linkov <juri <at> linkov.net>, Stefan Monnier <monnier <at> iro.umontreal.ca> Cc: 79306 <at> debbugs.gnu.org Subject: Re: bug#79306: Broken follow-mode Date: Mon, 25 Aug 2025 15:54:17 +0300
> From: Juri Linkov <juri <at> linkov.net>
> Date: Mon, 25 Aug 2025 09:30:10 +0300
>
> Some recent change in master broke display for follow-mode:
>
> 0. emacs -Q
> 1. visit a file in fundamental-mode, e.g.
> C-x C-f etc/COPYING RET
> 2. split horizontally
> C-x 3
> 3. M-x follow-mode RET
> 4. scroll down a few screens
> 5. type 'M-<' (beginning-of-buffer)
>
> It displays the message "Back to top level",
> but doesn't move to the beginning of the buffer.
Looks like the trap set by Stefan a month ago finally caught its first
victim: follow-mode calls redisplay recursively, from
pre-redisplay-function:
Thread 1 hit Breakpoint 4, redisplay_internal () at xdisp.c:17122
17122 struct window *w = XWINDOW (selected_window);
(gdb) bt
#0 redisplay_internal () at xdisp.c:17122
#1 0x0104197e in redisplay_preserve_echo_area (from_where=2) at xdisp.c:18040
#2 0x00fe51f4 in Fredisplay (force=XIL(0)) at dispnew.c:7004
#3 0x0123da19 in funcall_subr (subr=0x13ee240 <Sredisplay>, numargs=0,
args=0xa019120) at eval.c:3242
#4 0x012aa6a8 in exec_byte_code (fun=XIL(0xa0000000103368c0),
args_template=514, nargs=2, args=0xa019128) at bytecode.c:805
#5 0x0123e30c in funcall_lambda (fun=XIL(0xa000000010336cd0), nargs=1,
arg_vector=0xa019050) at eval.c:3333
#6 0x0123d3d2 in funcall_general (fun=XIL(0xa000000010336cd0), numargs=1,
args=0xa019050) at eval.c:3125
#7 0x0123d6fb in Ffuncall (nargs=2, args=0xa019048) at eval.c:3174
#8 0x0123c472 in Fapply (nargs=2, args=0xa019048) at eval.c:2788
#9 0x0123dea4 in funcall_subr (subr=0x13fc600 <Sapply>, numargs=2,
args=0xa019048) at eval.c:3265
#10 0x012aa6a8 in exec_byte_code (fun=XIL(0xa00000001033a090),
args_template=128, nargs=1, args=0xbfdcd0) at bytecode.c:805
#11 0x0123e30c in funcall_lambda (fun=XIL(0xa00000001033a090), nargs=1,
arg_vector=0xbfdcd0) at eval.c:3333
#12 0x0123d3d2 in funcall_general (fun=XIL(0xa00000001033a090), numargs=1,
args=0xbfdcd0) at eval.c:3125
#13 0x0123d6fb in Ffuncall (nargs=2, args=0xbfdcc8) at eval.c:3174
#14 0x012386f0 in internal_condition_case_n (bfun=0x123d5af <Ffuncall>,
nargs=2, args=0xbfdcc8, handlers=XIL(0x30),
hfun=0x10151d3 <dsafe_eval_handler>) at eval.c:1770
#15 0x01015307 in dsafe__call (inhibit_quit=true, f=0x123d5af <Ffuncall>,
nargs=2, args=0xbfdcc8) at xdisp.c:3102
#16 0x0103711d in prepare_menu_bars () at xdisp.c:14054
#17 0x0103f95e in redisplay_internal () at xdisp.c:17283
#18 0x0103e65d in redisplay () at xdisp.c:16844
#19 0x011456ef in read_char (commandflag=1, map=XIL(0xc00000001711a630),
prev_event=XIL(0), used_mouse_menu=0xbff31f, end_time=0x0)
at keyboard.c:2672
#20 0x0115ee45 in read_key_sequence (keybuf=0xbff5f8, prompt=XIL(0),
dont_downcase_last=false, can_return_switch_frame=true,
fix_current_buffer=true, prevent_redisplay=false,
disable_text_conversion_p=false) at keyboard.c:11146
#21 0x011412ca in command_loop_1 () at keyboard.c:1424
#22 0x01238416 in internal_condition_case (bfun=0x1140c68 <command_loop_1>,
handlers=XIL(0x90), hfun=0x113fcc1 <cmd_error>) at eval.c:1690
#23 0x011406cd in command_loop_2 (handlers=XIL(0x90)) at keyboard.c:1163
#24 0x012375a2 in internal_catch (tag=XIL(0x12a80),
func=0x1140696 <command_loop_2>, arg=XIL(0x90)) at eval.c:1370
#25 0x01140638 in command_loop () at keyboard.c:1141
#26 0x0113f721 in recursive_edit_1 () at keyboard.c:749
#27 0x0113f9bf in Frecursive_edit () at keyboard.c:832
#28 0x0113a9aa in main (argc=2, argv=0xf02638) at emacs.c:2629
Lisp Backtrace:
"redisplay" (0xa019120)
"follow-adjust-window" (0xa0190b8)
"follow-post-command-hook" (0xa019080)
"follow-pre-redisplay-function" (0xa019050)
"apply" (0xa019048)
0x1033a090 PVEC_CLOSURE
"redisplay_internal (C function)" (0x0)
Then we catch this and throw to top-level:
int redisplay_counter_before = redisplay_counter;
/* Use Qt to ensure debugger does not run,
to reduce the risk of wanting to redisplay. */
val = internal_condition_case_n (f, nargs, args, Qt,
dsafe_eval_handler);
if (redisplay_counter_before != redisplay_counter)
/* A nested redisplay happened, abort this one! */
/* FIXME: Rather than jump all the way to `top-level`
we should exit only the current redisplay. */
Ftop_level (); <<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<<
Is it allowed to perform redisplay from pre-redisplay-function? If
so, we need to refine the above test, or maybe disable it when we call
pre-redisplay-function.
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