mitogen/ansible_mitogen/process.py

# Copyright 2017, David Wilson
#
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from __future__ import absolute_import
import os
import socket
import sys
import threading

import mitogen
import mitogen.core
import mitogen.master
import mitogen.parent
import mitogen.service
import mitogen.unix
import mitogen.utils

import ansible_mitogen.logging
import ansible_mitogen.services


class MuxProcess(object):
    """
    Implement a subprocess forked from the Ansible top-level, as a safe place
    to contain the Mitogen IO multiplexer thread, keeping its use of the
    logging package (and the logging package's heavy use of locks) far away
    from the clutches of os.fork(), which is used continuously by the
    multiprocessing package in the top-level process.

    The problem with running the multiplexer in that process is that should the
    multiplexer thread be in the process of emitting a log entry (and holding
    its lock) at the point of fork, in the child, the first attempt to log any
    log entry using the same handler will deadlock the child, as in the memory
    image the child received, the lock will always be marked held.

    See https://bugs.python.org/issue6721 for a thorough description of the
    class of problems this worker is intended to avoid.
    """

    #: In the top-level process, this references one end of a socketpair(),
    #: which the MuxProcess blocks reading from in order to determine when
    #: the master process dies. Once the read returns, the MuxProcess will
    #: begin shutting itself down.
    worker_sock = None

    #: In the worker process, this references the other end of
    #: :py:attr:`worker_sock`.
    child_sock = None

    #: In the top-level process, this is the PID of the single MuxProcess
    #: that was spawned.
    worker_pid = None

    #: In both processes, this is the temporary UNIX socket used for
    #: forked WorkerProcesses to contact the MuxProcess
    unix_listener_path = None

    #: Singleton.
    _instance = None

    @classmethod
    def start(cls):
        """
        Arrange for the subprocess to be started, if it is not already running.

        The parent process picks a UNIX socket path the child will use prior to
        fork, creates a socketpair used essentially as a semaphore, then blocks
        waiting for the child to indicate the UNIX socket is ready for use.
        """
        if cls.worker_sock is not None:
            return

        cls.unix_listener_path = mitogen.unix.make_socket_path()
        cls.worker_sock, cls.child_sock = socket.socketpair()
        mitogen.core.set_cloexec(cls.worker_sock)
        mitogen.core.set_cloexec(cls.child_sock)

        cls.child_pid = os.fork()
        ansible_mitogen.logging.setup()
        if cls.child_pid:
            cls.child_sock.close()
            cls.child_sock = None
            cls.worker_sock.recv(1)
        else:
            cls.worker_sock.close()
            cls.worker_sock = None
            self = cls()
            self.worker_main()
            sys.exit()

    def worker_main(self):
        """
        The main function of for the mux process: setup the Mitogen broker
        thread and ansible_mitogen services, then sleep waiting for the socket
        connected to the parent to be closed (indicating the parent has died).
        """
        self._setup_master()
        self._setup_services()

        # Let the parent know our listening socket is ready.
        self.child_sock.send('1')
        # Block until the socket is closed, which happens on parent exit.
        self.child_sock.recv(1)

    def _setup_master(self):
        """
        Construct a Router, Broker, and mitogen.unix listener
        """
        self.router = mitogen.master.Router()
        self.router.responder.whitelist_prefix('ansible')
        self.router.responder.whitelist_prefix('ansible_mitogen')
        mitogen.core.listen(self.router.broker, 'shutdown', self.on_broker_shutdown)
        self.listener = mitogen.unix.Listener(
            router=self.router,
            path=self.unix_listener_path,
        )
        if 'MITOGEN_ROUTER_DEBUG' in os.environ:
            self.router.enable_debug()

    def _setup_services(self):
        """
        Construct a ContextService and a thread to service requests for it
        arriving from worker processes.
        """
        self.service = ansible_mitogen.services.ContextService(self.router)
        self.service_thread = threading.Thread(target=self.service.run)
        self.service_thread.start()

    def on_broker_shutdown(self):
        """
        Respond to the Router shutdown (indirectly triggered through exit of
        the main thread) by unlinking the listening socket. Ideally this would
        happen explicitly, but Ansible provides no hook to allow it.
        """
        os.unlink(self.listener.path)
        self.service_thread.join(timeout=10)