Comment 3 for bug 1221486

Reviewed: https://review.openstack.org/45362
Committed: http://github.com/openstack/neutron/commit/fc0e7cd5ab01cb54ba7f2696e15c49e1fb827bc7
Submitter: Jenkins
Branch: master

commit fc0e7cd5ab01cb54ba7f2696e15c49e1fb827bc7
Author: Kaiwei Fan <email address hidden>
Date: Thu Sep 5 13:57:13 2013 -0700

    Fix IF checks on spawned green thread instance

    Initially the symptom looks like race condition between two threads when
    stopping the task manager. After further analysis/troubleshooting, it
    turns out that two threads are spawned if a task manager is stopped and
    started again, causing unexpected errors.

    The IF check on the spawned thread sometimes return True sometime return False
    if not compared against None explicitly. This makes start() method
    think no thread has been started or stop() method think no thread is started.
    Change the check to compare against None.

    Also fixed a problem in unit-test where a thread may never terminated when
    a stop call is invoked during db access.

    Closes-Bug: 1221486
    Change-Id: I0d67bfe8fef7a390f0d6bc0f5a42835f86a9fb27