bug#30387: 26.0.91; lisp_align_free is hanging Emacs on macOS 10.12

bug#30387: 26.0.91; lisp_align_free is hanging Emacs on macOS 10.12

[John Wiegley]

I've been unable to find reproducible conditions for this bug, since it only
happens after Emacs (built from emacs-26 release branch) has been running for
many hours. Basically, Emacs deadlocks -- showing the spinning beach ball of
death -- and when I sample the process using Instruments.app, I find that it's
gotten stuck in lisp_align_free.

Looking at that function, I see this loop:

      while (*tem)
	{
	  if (*tem >= (struct ablock *) abase && *tem < atop)
	    {
	      i++;
	      *tem = (*tem)->x.next_free;
	    }
	  else
	    tem = &(*tem)->x.next_free;
	}

There is no loop guard here to detect cycles, if that's what is happening.

I'll start running with full debugging on over the next few days to see if
that reveals how I'm getting stuck in this situation, but as for now, it has
required kill'ing Emacs twice in the past week.

-- 
John Wiegley                  GPG fingerprint = 4710 CF98 AF9B 327B B80F
http://newartisans.com                          60E1 46C4 BD1A 7AC1 4BA2

bug#30387: Emacs 27.1 on NixOS; emacs freezes as previously described

[Moritz Schäfer]

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I face the same issue as described previously by John Wiegley. After
several hours (sometimes even less than an hour), emacs freezes completely
without the option to recover it.
A backtrace also leads to lisp_align_free:

#0  0x000000000055aa0e in lisp_align_free ()
#1  0x000000000055acd9 in sweep_floats ()
#2  0x00000000005603ac in garbage_collect ()
#3  0x00000000005607e7 in Fgarbage_collect ()
...

This is especially cumbersome since I am running EXWM and have to therefore
recover my session in a tty by restarting my desktop manager.

I will add a timer to run the garbage collector every few (5) seconds so I
can hopefully track which event changes the state in a way that leads it to
the observed freezing behavior.

I report back back if I find out anything new. In the meanwhile, I am happy
for any suggestions on how to debug this issue.

-- 

Moritz Schäfer* <http://www.bsse.ethz.ch>*

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bug#30387: Emacs 27.1 on NixOS; emacs freezes as previously described

[Moritz Schäfer]

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So far, I was not able to attribute the freezes to any provoking event.
However, I noticed that the freezes resolve after a certain amount of time
(I don't how much exactly. I just left my PC on over the night and it was
unfrozen the next morning).

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