GNU bug report logs -
#45117
28.0.50; process-send-string mysteriously exiting non-locally when called from timer
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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.
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> From: João Távora <joaotavora <at> gmail.com>
> Cc: 45117 <at> debbugs.gnu.org
> Date: Wed, 09 Dec 2020 11:24:47 +0000
>
> [ We've been CC-ing bug-gnu-emacs <at> gnu.org for a while. My fault, the
> typical CC blunder. Wonder how debbugs was dealing with that so
> gracefully tho. ]
It should deal with this just fine, as long as you keep the same
Subject line.
> If I set breakpoints at _all_ places where we call sys_longjmp(), I risk
> tearing down my X, which I did a couple of times.
>
> So I skip those "dangerous" breakpoints. I'm guessing one of the
> interesting loci to break is unwind_to_catch in eval.c. Of course that
> gets called every dang time a signal is thrown, so it's hard for me to
> catch the precise situation, even if I set up nicely and then call M-x
> redraw-display, and only then enable the breakpoint.
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.
> It breaks near immediately, and the `bt` output I get is always from
> some other function that expectedly signalled an error as part of its
> normal control flow.
One simple method of dealing with that is to make GDB continue
immediately after hitting the breakpoint:
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.
> 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, which is normally nil. On the C level, this is a
bool variable bidi_inhibit_bpa, which is normally zero. So, you could
wrap the problematic Lisp fragment with
(let ((bidi-inhibit-bpa t))
....
)
and then make the breakpoint conditional on that:
break eval.c:NNNN if bidi_inhibit_bpa != 0
The advantage of this is that when the let-form unwinds, the variable
will be automatically reset (again, if we believe the theory of
signal/throw that cause the non-local exit).
HTH
This bug report was last modified 4 years and 211 days ago.
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