Comment 6 for bug 1785841

Reviewed: https://review.opendev.org/c/openstack/blazar/+/589899
Committed: https://opendev.org/openstack/blazar/commit/c92edb8a177de51862ad2a4f9cbac2c50d31ef84
Submitter: "Zuul (22348)"
Branch: master

commit c92edb8a177de51862ad2a4f9cbac2c50d31ef84
Author: Pierre Riteau <email address hidden>
Date: Wed Aug 8 12:46:28 2018 +0200

    Prevent conflicting events from running concurrently

    If two leases have compute hosts in common, and the second lease starts
    exactly when the first lease ends, there is the possibility of a race.
    The Blazar manager can first run the start_lease event of the second
    lease. This event would fail since the end_lease event of the first
    lease would still be UNDONE, and the compute hosts in common would still
    be in the aggregate associated with the first lease, instead of being in
    the freepool.

    This patch changes event execution code so that events are executed
    concurrently if possible, with the following constraints:

    - events are executed strictly in order, i.e. events are started only
      after all previous events have completed
    - when events are at the same time, we first execute before_end_lease
      events (unless there is a start_lease at the same time), then
      end_lease events, followed by start_lease events, ensuring the bug
      described above does not happen. Finally, we run any before_end_lease
      which had a corresponding start_lease event at the same time.

    It also has the side effect of providing better stack traces for event
    execution failures, since we call wait() on all GreenThread objects.

    Co-Authored-By: Jason Anderson <email address hidden>
    Change-Id: Ie2339db18e8baee379fbea082f1238ec44fca6b1
    Closes-Bug: #1785841