I've hit the emacs_abort at line 5510 in process.c a few times in the
last week.  I haven't found a way to make it reproduce on demand.  I
tried to narrow the code it's happening in down to a smaller test case,
without success.  I'd appreciate suggestions for how to track down what
is going wrong.

I'm working on a Lisp program which has work to do which can be done in
parallel, and I'm implementing it using threads.  My code has 4 worker
threads which pick jobs to do off of a queue (which is made thread-safe
with a mutex and condition variables).  The jobs consist of an argument
to a shell script, which the threads run asynchronously using
start-file-process and accept-process-output.  This allows the worker
threads to be responsive to a user command to cancel the work in
progress, although I haven't been using that cancel command when the bug
happens.  When it has happened, it's been after I run a command which
adds 6 jobs to the queue for the 4 threads to process.

The crash has happened with two different shell scripts, one which just
consists of "exit 1" and another which makes a directory and a symlink.
Neither script prints anything to standard output.

I've tried using the process object instead of nil as the first argument
to accept-process-output and have seen the same crash both ways.

Here are the two main functions in my worker threads,
'erb--builder-func' which is passed to 'make-thread' to create the
threads, and 'erb--build' which runs the child processes.