GNU bug report logs - #33034
`unwind-protect' cleanup form is not executed if body dies in stack overflow

Previous Next

Full log

View this message in rfc822 format

From: help-debbugs <at> gnu.org (GNU bug Tracking System)
To: Paul Eggert <eggert <at> cs.ucla.edu>
Cc: tracker <at> debbugs.gnu.org
Subject: bug#33034: closed (`unwind-protect' cleanup form is not executed
 if body dies in stack overflow)
Date: Sun, 14 Oct 2018 17:04:01 +0000

[Message part 1 (text/plain, inline)]

Your message dated Sun, 14 Oct 2018 10:02:59 -0700
with message-id <3f05af02-38e2-c03a-8449-bad3d1bb7670 <at> cs.ucla.edu>
and subject line Re: `unwind-protect' cleanup form is not executed if body dies in stack overflow
has caused the debbugs.gnu.org bug report #33034,
regarding `unwind-protect' cleanup form is not executed if body dies in stack overflow
to be marked as done.

(If you believe you have received this mail in error, please contact
help-debbugs <at> gnu.org.)


-- 
33034: http://debbugs.gnu.org/cgi/bugreport.cgi?bug=33034
GNU Bug Tracking System
Contact help-debbugs <at> gnu.org with problems

[Message part 2 (message/rfc822, inline)]

From: Paul Pogonyshev <pogonyshev <at> gmail.com>
To: bug-gnu-emacs <at> gnu.org
Subject: `unwind-protect' cleanup form is not executed if body dies in stack
 overflow
Date: Sat, 13 Oct 2018 12:07:48 +0200

To reproduce:

    (defun overflow ()
      (overflow))
    (defun test ()
      (interactive)
      (message "BEFORE")
      (unwind-protect
          (overflow)
        (message "CLEANUP")))

Invocation of `test' never issues message "CLEANUP", whether it is run
interactively or non-interactively.

By comparison, if you _catch_ the error with `condition-case':

    (defun test-2 ()
      (interactive)
      (message "BEFORE")
      (unwind-protect
          (ignore-errors (overflow))
        (message "CLEANUP")))

then cleanup form is executed properly.

But if your error catcher is "above" the `unwind-protect' form, the
cleanup is not executed again, even though the error is eaten as
expected:

    (defun test-3 ()
      (interactive)
      (message "BEFORE")
      (ignore-errors
        (unwind-protect
            (overflow)
          (message "CLEANUP"))))

This is a perfect way to screw up your Emacs permanently (until full
restart): when some `unwind-protect' cleanups are not run, you can be
left with unexpected function advices, permanently altered global
state etc., without any good way to undestand what's wrong.

Paul

[Message part 3 (message/rfc822, inline)]

From: Paul Eggert <eggert <at> cs.ucla.edu>
To: Paul Pogonyshev <pogonyshev <at> gmail.com>
Cc: 33034-done <at> debbugs.gnu.org, Eli Zaretskii <eliz <at> gnu.org>
Subject: Re: `unwind-protect' cleanup form is not executed if body dies in
 stack overflow
Date: Sun, 14 Oct 2018 10:02:59 -0700

[Message part 4 (text/plain, inline)]

Thanks for the bug report; I installed the attached to fix it. The problem with 
your test case was neither C stack overflow nor failure to unwind the Lisp 
stack: it was failure to restore the Lisp evaluation depth (which is a separate 
thing from the Lisp stack size).

By the way, why are there two different limits? That slows the interpreter down 
a bit.  Why don't we simply have a limit for the Lisp stack size? Every time 
lisp_eval_depth grows, the stack size grows, so limiting the stack limits the 
evaluation depth for free. If we had done things this way, the interpreter would 
be a bit faster and this bug would never have occurred.

[0001-Fix-lisp_eval_depth-in-unwind-protect-cleanup.patch (text/x-patch, attachment)]

Previous Next

GNU bug tracking system
Copyright (C) 1999 Darren O. Benham, 1997,2003 nCipher Corporation Ltd, 1994-97 Ian Jackson.