bug#17561: 24.4.50; Emacs can forget processes

[Jorgen Schaefer <contact@jorgenschaefer.de>]

From time to time, my Emacs tends to become very slow. I have tracked
this down to a weird interaction with certain processes.

Sometimes, when a process dies (e.g. after its buffer is killed), Emacs
apparently does not clean up correctly. The sentinel is called, the
process itself receives a termination signal, but the process object
still shows up in M-x list-processes, and there is a zombie process in
ps.

When this happens, Emacs is in a tight loop trying to read from the dead
process(es):

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>

Killing the processes in M-x list-processes using
`process-menu-delete-process' returns Emacs to a usable state, albeit
the zombie processes stay around.

I have been able to reliably reproduce the behavior using this snippet:

(with-temp-buffer
  (let ((process-connection-type t))
    (set-process-query-on-exit-flag
     (start-process "cat" (current-buffer) "cat")
     nil)))

I.e. create a buffer, associate a process with it, remove the query on
exit flag, and kill the buffer. The `process-connection-type' nil is the
relevant part. With t, it works correctly.

The snippet does *not* work on a freshly-started Emacs. A random time
after the bug shows up, Emacs also cleans up the zombie processes and
the bug can not be triggered anymore. Until a whole while later, it
shows up again.

I have seen this happen in 24.3.50, 24.3.91 (pretest 2), and current
trunk.