bug#12447: 24.1.50; Stuck in garbage collection on OS X

[Harald Hanche-Olsen <hanche@math.ntnu.no>]

This concerns emacs built with --with-ns from trunk on OS X.

To start with the symptom: Emacs seems to freeze, and spends a lot of
CPU time. Taking a sample with Activity monitor appears to indicate a
very deeply nested mark_object calls; from this I conclude that the
problem appears to happen in GC.

How to trigger the symptom: I always got it by trying to send an email
message with a large attachment. My mail client of choice is Mew, but
I suspect that any activity triggering garbage collection will also
trigger the bug.

After bisecting, I come to the conclusion that this revision is to
blame:

revno: 109470
committer: Jan D. <jan.h.d@swipnet.se>
branch nick: trunk
timestamp: Mon 2012-08-06 18:09:02 +0200
message:
  * keyboard.c (timer_check_2): Add break so timer_check returns next timeout.

That revision only adds a single line of code. I also find that the
bug disappears if I patch the current tip of trunk (revision 110013)
as follows, which undoes revision 109470:

=== modified file 'src/keyboard.c'
--- src/keyboard.c	2012-09-13 02:21:28 +0000
+++ src/keyboard.c	2012-09-14 19:12:15 +0000
@@ -4484,7 +4484,6 @@
 	    }
 
 	  nexttime = make_emacs_time (0, 0);
-          break;
 	}
       else
 	/* When we encounter a timer that is still waiting,


I suppose the break was added for a good reason, so this is probably
not the right thing. But it does cure the present problem.

(This bug is also discussed on the emacs-devel list, in the thread
titled "Emacs seems awfully unstable on OS X lately".)

There seems to be a second bug, introduced later, that causes crashes
(as opposed to hangs). I mention this because the second bug could
interfere with attempts to work on this one. I will endeavour to track
down the other bug as well.

- Harald