Strange Emacs "hiccup" bug

[Jorgen Schaefer <forcer@forcix.cx>]

Hello!

I am trying to debug a problem I have encountered a few times now, but
I'm at a loss as to what to do next to figure out what is happening.
Maybe someone here has an idea.

I have witnessed this bug, or at least one that looks *very much* like
this, a number of times over the last months. I have re-built Emacs a
few times in between. The current build is from trunk on 2014-05-01.

At some point, apparently after some time of continuous use, Emacs
starts to pause intermittently. It stops responding to keys or
redisplaying for half a second, then it goes on. It does not matter
which mode or buffer is in use.

In this particular case, after a few attempts to figure out what's
going on with no effect, I simply killed all buffers. The behavior
continued, even in *Messages*.

A look at the process list showed a bunch of zombie processes around:

15361 ?        Ss     0:33 SCREEN
15362 pts/1    Ss     0:00  \_ /bin/bash
18175 pts/1    R+    37:06  |   \_ emacs
18206 ?        Zs     0:00  |       \_ [openssl] <defunct>
23385 ?        Zs     0:04  |       \_ [openssl] <defunct>
24238 ?        Zs     0:00  |       \_ [aspell] <defunct>
24999 ?        Zs     0:00  |       \_ [aspell] <defunct>
28117 ?        Zs     0:00  |       \_ [bash] <defunct>
18706 ?        Zs     0:00  |       \_ [python] <defunct>
20096 ?        Zs     0:00  |       \_ [python] <defunct>

A strace -fttT showed a very fast loop doing this:

21:24:00.948071 pselect6(12, [3 4 9 11], [], NULL, {0, 1376013},
                {NULL, 8}) = 2 (in [9 11], left {0, 1373467}) <0.000011>
21:24:00.948529 read(9, "", 4096)       = 0 <0.000009>
21:24:00.948564 read(11, "", 4096)      = 0 <0.000008>

It turned out 9 and 11 were two pipes to openssl. Emacs still had
three processes active: The emacsclient server and two openssl
connections. Even though both openssl connections had died.

Deleting the processes in Emacs improved responsiveness somewhat, but
did not completely remove the "hiccups". The strace behavior returned
to normal, though.

A M-x toggle-debug-on-quit and some lucky C-g's got me the following:

Error during redisplay: (#[128 "\300\301^B\"\210\300\302^B\"\207"
[apply redisplay--update-region-highlights ignore nil] 4 nil nil] nil)
signaled (quit)

But I'm not sure if that is really related, or if it just happened to
be what Emacs was evaluating at that time.

Now I'm stuck. Any ideas what I can do to debug this further?

Regards,
Jorgen