From: "Clément Pit--Claudel" <clement.pit@gmail.com>
To: Emacs developers <emacs-devel@gnu.org>
Subject: How can I rethrow an error after recording a backtrace?
Date: Thu, 4 Aug 2016 22:12:22 -0400 [thread overview]
Message-ID: <53ca9864-3a3b-8406-65c9-8458d0512299@gmail.com> (raw)
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Hey emacs-devel,
The Emacs server doesn't print backtraces, so I'm trying to get these backtraces myself and return them to the server. After some exploration, I found about the debugger variable, and realized that I could actually force code to break into the debugger regardless of handlers using debug-on-signal. So far so good. Then I wrote the following "debugger"
(defun my-handle-error (&rest args)
(let (;; Prevent recursive error catching
(debugger #'debug)
(debug-on-quit nil)
(debug-on-error nil)
(debug-on-signal nil))
(pcase args
(`(exit ,retv) retv)
(`(error ,error-args)
(setq saved-backtrace (with-output-to-string (backtrace)))
(signal (car error-args) (cdr error-args))))))
And then I called my code as follows:
(let* (;; Make sure that we'll intercept all errors
(debug-on-quit t)
(debug-on-error t)
(debug-on-signal t)
(debug-ignored-errors nil)
;; Make sure debugger has room to execute
(max-specpdl-size (+ 50 max-specpdl-size))
(max-lisp-eval-depth (+ 50 max-lisp-eval-depth))
;; Register ourselves as the debugger
(debugger #'my-handle-error))
(my code here))
This was in part inspired by similar code in allout-widgets.el (although I think that code never worked, since it calls signal with a non-list second argument).
When running on an emacs server, though, this code doesn't work. That is, it works fine the first time, but not on further invocations; so I stepped through the code in GDB, and I understood that the issue was here:
if (
/* Don't try to run the debugger with interrupts blocked.
The editing loop would return anyway. */
! input_blocked_p ()
&& NILP (Vinhibit_debugger)
/* Does user want to enter debugger for this kind of error? */
&& (EQ (sig, Qquit)
? debug_on_quit
: wants_debugger (Vdebug_on_error, conditions))
&& ! skip_debugger (conditions, combined_data)
/* RMS: What's this for? */
&& when_entered_debugger < num_nonmacro_input_events)
{
call_debugger (list2 (Qerror, combined_data));
return 1;
}
(in maybe_call_debugger). The first time around the debugger is called, but not the second time, because the `when_entered_debugger < num_nonmacro_input_events` is false (on the first round it evaluates to -1 < 0, and on the second one to 0 < 0). This reason for this clause was explained when this code was written back in 1991:
/* The value of num_nonmacro_input_events as of the last time we
started to enter the debugger. If we decide to enter the debugger
again when this is still equal to num_nonmacro_input_events, then we
know that the debugger itself has an error, and we should just
signal the error instead of entering an infinite loop of debugger
invocations. */
static EMACS_INT when_entered_debugger;
The problem is that I'm running this code using emacsclient --eval '(error "abc")', and so the num_nonmacro_input_events value never increases. I "fixed" it this way:
(defun my-handle-error (&rest args)
(let (;; Prevent recursive error catching
(debugger #'debug)
(debug-on-quit nil)
(debug-on-error nil)
(debug-on-signal nil))
;; HACK HACK HACK HACK HACK HACK HACK HACK HACK HACK HACK HACK ;;
(setq num-nonmacro-input-events (1+ num-nonmacro-input-events))
;; HACK HACK HACK HACK HACK HACK HACK HACK HACK HACK HACK HACK ;;
(pcase args
(`(exit ,retv) retv)
(`(error ,error-args)
(setq saved-backtrace (with-output-to-string (backtrace)))
(signal (car error-args) (cdr error-args))))))
Is this going to have horrible consequences? If it is, could someone more experienced point me to how I am supposed to capture a backtrace and then rethrow an error? I can provide a full example if anyone wants to experiment and finds this description incomplete.
Cheers,
Clément.
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next reply other threads:[~2016-08-05 2:12 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-08-05 2:12 Clément Pit--Claudel [this message]
2016-08-05 2:23 ` How can I rethrow an error after recording a backtrace? Stefan Monnier
2016-08-05 9:40 ` Clément Pit--Claudel
2016-08-06 2:11 ` Stefan Monnier
2016-08-06 10:03 ` Eli Zaretskii
2016-08-06 13:29 ` Stefan Monnier
2016-08-05 2:34 ` Clément Pit--Claudel
2016-08-05 6:12 ` Eli Zaretskii
[not found] ` <73dc7988-7b41-385d-1083-6a6fa4bfbd91@gmail.com>
2016-08-05 13:40 ` Eli Zaretskii
2016-08-05 15:38 ` Clément Pit--Claudel
2016-08-06 10:02 ` Eli Zaretskii
2016-08-06 15:24 ` Clément Pit--Claudel
2016-08-06 15:31 ` Eli Zaretskii
2016-08-06 15:49 ` Clément Pit--Claudel
2016-08-06 17:38 ` Eli Zaretskii
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