Package: emacs;
Reported by: João Távora <joaotavora <at> gmail.com>
Date: Tue, 8 Dec 2020 11:45:02 UTC
Severity: normal
Found in version 28.0.50
Done: João Távora <joaotavora <at> gmail.com>
Bug is archived. No further changes may be made.
Message #26 received at 45117 <at> debbugs.gnu.org (full text, mbox):
From: João Távora <joaotavora <at> gmail.com> To: Eli Zaretskii <eliz <at> gnu.org> Cc: 45117 <at> debbugs.gnu.org Subject: Re: 28.0.50; process-send-string mysteriously exiting non-locally when called from timer Date: Thu, 10 Dec 2020 15:00:58 +0000
Eli Zaretskii <eliz <at> gnu.org> writes:
> AFAICT, the only relevant call to sys_longjmp is in eval.c. That is,
> if we think Emacs signals an error or otherwise throws to top-level.
I thought that, but now I'm confused. I'm uncertain about possible,
different ways of "exiting non-locally" from a function, which I define
by (foo) running and (bar) never running in (progn (foo) (bar)). When
that happens, (foo) has exited non-locally.
As far as I know, Elisp has no CL-style TAGBODY or GO, right? So indeed
I would expect that throw/catch/signal things at the C-level are the
only possible responsibles for these situations.
> break eval.c:NNNN
> commands
> > bt
> > continue
> > end
>
> (the ">" prompt is printed by GDB). Then you will have a lot of
> backtraces, but only the last one will be relevant. This simple
> method has a disadvantage that it slows down Emacs, and also produces
> a lot of possibly uninteresting stuff.
Thanks. That's the "tracer" strategy I remember you telling me. It was
useful in the past, not so much here.
>> 1. I have to find a way to set the unwind_to_catch() breakpoint
>> conditional on some Elisp/near-elisp context, in this case something
>> inside the Elisp function sly-net-send() or Fprocess_send_string.
>>
>> Do you think setting a silly global in Fprocess_send_string() and
>> then checking that as the breakpoint condition would be a good idea?
>> Where would I reset the flag? Is there some C-version of
>> "unwind-protect"?
>
> The C version of unwind-protect is record_unwind_protect.
>
> But I think it will be easier to use an existing variable that is
> usually not touched. For example, you could piggy-back
> bidi-inhibit-bpa,
That's an excellent idea, and I've verified that it works. But it
didn't help here. Or rather, not in the way I had anticipated. It did
help me determine that unwind_to_catch() doesn't seem to be the only
responsible for the non-local exit.
To be clear, I now have this that I put around the "suspicious" places:
(cl-defmacro DEBUG-45117 ((message) &rest body)
(declare (indent defun))
(let ((var (cl-gensym)))
`(let ((,var nil)
(bidi-inhibit-bpa t)) ; for your conditional break trick
(unwind-protect
(prog1 (progn ,@body)
(setq ,var t))
(unless ,var
(message ,message))))))
Here's how I use it in sly.el, in the code that's called from the idle
timer.
(defun sly-net-send (sexp proc)
"Send a SEXP to Lisp over the socket PROC.
This is the lowest level of communication. The sexp will be READ and
EVAL'd by Lisp."
(DEBUG-45117 ("SOMETHING in SLY-NET-SEND bailed")
(let* ((print-circle nil)
(print-quoted nil)
(payload (DEBUG-45117 ("ENCODE-CODING-STRING????")
(encode-coding-string
(concat (sly-prin1-to-string sexp) "\n")
'utf-8-unix)))
(string (DEBUG-45117 ("LENGTH-ENCODING????")
(concat (sly-net-encode-length (length payload))
payload))))
(DEBUG-45117 ("PROCESS-SEND-STRING?????")
(process-send-string proc string)))))
I then launch Emacs as I explained earlier:
gdb -i=mi --args ~/Source/Emacs/emacs-27/src/emacs -Q \
-L ~/Source/Emacs/sly \
-l sly-autoloads \
-f sly \
--eval "(setq eldoc-idle-delay 0.01)" \
~/Source/Emacs/sly/slynk/slynk.lisp
Then ensure that breakpoints looks more or less like this (a couple more
than the one you recommended there.)
1 breakpoint keep y 0x00005555557e2580 in terminate_due_to_signal at emacs.c:378
2 breakpoint keep y 0x000055555576f4f5 in x_error_quitter at xterm.c:10131
3 breakpoint keep y 0x00005555555aa32d in Fredraw_display at dispnew.c:3123
breakpoint already hit 1 time
6 breakpoint keep y 0x0000555555966de5 in unwind_to_catch at eval.c:1178
stop only if bidi_inhibit_bpa != 0
7 breakpoint keep y 0x000055555580b985 in quit_throw_to_read_char at keyboard.c:10970
stop only if bidi_inhibit_bpa != 0
10 breakpoint keep y 0x0000555555963f1a in call_debugger at eval.c:283
stop only if bidi_inhibit_bpa != 0
Then 'r' to run, then start the debugging process I explained,
basically just scroll up and down in the slynk.lisp file. After a
while, in *Messages*, some of these start appearing.
ENCODE-CODING-STRING????
SOMETHING in SLY-NET-SEND bailed
[sly] [issue#385] likely `process-send-string' exited non-locally from timer.
... more scrolling ...
SOMETHING in SLY-NET-SEND bailed
[aly] [issue#385] likely `process-send-string' exited non-locally from timer. [2 times]
Note that ENCODE-CODING-STRING???? is missing from the second
observation! In this last session I didn't capture the
"PROCESS-SEND-STRING???", but I'm pretty sure I have in the past.
It does seem though, that contrary to my original expectation, this is
not exclusive to process-send-string, but it happens in normal elisp
execution from quickly firing idle timers.
Anyway.
1. Shouldn't all of these have triggered the breakpoint?? I'm setting
the Elisp/C variable in the macro. I tested the technique
separately.
2. Are we sure that no other mechanisms other than throw/catch/signal
can trigger a non-local exit (that unwind-protect can still somehow
catch?).
Thanks for any insight you may have,
João
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