Gunicorn + Subprocesses raises exception [Errno 10]
Question: I've stumbled across a weird exception I haven't been able to resolve... can
anyone suggest what is wrong or a new design? I'm running a Gunicorn/Flask
application. In the configuration file, I specify some work to do with an
`on_starting` hook [1]. Inside that hook code, I have some code like this
(nothing fancy):

    
    
    # Called before the server is started
    my_thread = package.MyThread()
    my_thread.start()
    

The package.MyThread class looks like the following. The `ls` command is
unimportant, it can be any command.

    
    
    class MyThread(threading.Thread):
      """
        Run a command every 60 seconds.
    
      """
      def __init__(self):
        threading.Thread.__init__(self)
        self.event = threading.Event()
    
      def run(self):
        while not self.event.is_set():
          ptest = subprocess.Popen(["ls"], stdout=subprocess.PIPE)
          ptest.communicate()
          self.event.wait(60)
    
      def stop(self):
        self.event.set()
    

Upon starting up the server, I'm always presented with this exception:

    
    
    Exception in thread Thread-1:
    Traceback (most recent call last):
      File "/usr/lib64/python2.6/threading.py", line 532, in __bootstrap_inner
        self.run()
      File "__init__.py", line 195, in run
        ptest.communicate()
      File "/usr/lib64/python2.6/subprocess.py", line 721, in communicate
        self.wait()
      File "/usr/lib64/python2.6/subprocess.py", line 1288, in wait
        pid, sts = _eintr_retry_call(os.waitpid, self.pid, 0)
      File "/usr/lib64/python2.6/subprocess.py", line 462, in _eintr_retry_call
        return func(*args)
    OSError: [Errno 10] No child processes
    

Can anyone suggest what is going on here? I haven't tried implementing the
changes in [2], they seemed hacky.

[1] - <http://gunicorn.org/configure.html#server-hooks>

[2] - [Popen.communicate() throws OSError: "[Errno 10] No child
processes"](http://stackoverflow.com/questions/1008858/popen-communicate-
throws-oserror-errno-10-no-child-processes)


Answer: The error turns out to be related to signal handling of `SIGCHLD`.

The `gunicorn` arbiter intercepts `SIGCHLD`, which breaks `subprocess.Popen`.
The `subprocess.Popen` module requires that `SIGCHLD` not be intercepted (at
least, this is true for Python 2.6 and earlier).

According to [bugs.python.org](http://bugs.python.org/issue9127) this bug has
been fixed in Python 2.7.

