GNU bug report logs - #33784
27.0.50; some case c-backward-token-2 takes cpu more and emacs hang

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Message #37 received at 33784-done <at> debbugs.gnu.org (full text, mbox):

From: Alan Mackenzie <acm <at> muc.de>
To: xh yang <linux.xhyang <at> gmail.com>
Cc: 33784-done <at> debbugs.gnu.org, Eli Zaretskii <eliz <at> gnu.org>
Subject: Re: bug#33784:
Date: Thu, 20 Dec 2018 12:53:26 +0000

Hello, xh.

On Thu, Dec 20, 2018 at 15:18:49 +0800, xh yang wrote:
> I found if remove first '\n' from the *str,then run emacs will not hang.*
> *May be it has nothing to do with font-lock, just str parsing ?*

Sorry for all the distraction in the last couple of days.  As you
suggested in your opening post, the problem was a simple infinite loop
involving c-backward-token-2.  This was in CC Mode's font-locking code.

I've committed a fix to the emacs-26 branch, and it should find its way
to the master branch within a few days.  I'm closing the bug.

Here is the patch:


# HG changeset patch
# User Alan Mackenzie <acm <at> muc.de>
# Date 1545307557 0
#      Thu Dec 20 12:05:57 2018 +0000
# Node ID 5319aa054ccb77924e19b836093c9e1b3ff91d4b
# Parent  a3f28b92890acc5289cde497ea1335f4b39239d7
Check result from c-backward-token-2 to avoid infinite loop

This fixes bug #33784.

* cc-fonts.el (c-get-fontification-context): While moving back over enclosing
parentheses, check that c-backward-token-2 actually moves.

diff -r a3f28b92890a -r 5319aa054ccb cc-fonts.el
--- a/cc-fonts.el	Thu Dec 20 12:04:53 2018 +0000
+++ b/cc-fonts.el	Thu Dec 20 12:05:57 2018 +0000
@@ -1255,8 +1255,8 @@
 		(save-excursion
 		  (goto-char match-pos)
 		  (while
-		      (progn (c-backward-token-2)
-			     (eq (char-after) ?\()))
+		      (and (zerop (c-backward-token-2))
+			   (eq (char-after) ?\()))
 		  (looking-at c-arithmetic-op-regexp)))
 	   (cons nil nil))
 	  ;; In a C++ member initialization list.


Thanks, once more, for the bug report.

-- 
Alan Mackenzie (Nuremberg, Germany).

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