Comment 10 for bug 1543146

Reviewed: https://review.openstack.org/316032
Committed: https://git.openstack.org/cgit/openstack/openstack-ansible/commit/?id=ac7b2d68433f9560a5d401f846349c2e37751f0f
Submitter: Jenkins
Branch: kilo

commit ac7b2d68433f9560a5d401f846349c2e37751f0f
Author: Darren Birkett <email address hidden>
Date: Fri May 13 11:46:15 2016 +0100

    Disable slave repo servers while syncing

    ****
    While this looks like a big patch, its actually a relatively small
    change to the default rsync script. I understand that Kilo is pre EOL,
    but this is an isolated change and removes one more potential way
    that a juno-kilo upgrade could go wrong.
    ****

    Commit message from original master patch:

    Currently there is a race between the repo servers syncing and the first
    role that attempts to install a pip package. This change ensures that
    only the primary repo server is accessible until the slaves are synced.

    This is achieved by adding a hook into lsyncd that allows a command to
    be run before and after each sync. This command is an ssh command to
    connect to the relevant secondary container and stop/start nginx. As the
    nginx user is unprivileged, a sudoers file is added to allow nginx to be
    stopped and started.

    Notes on adding the hook into lsyncd:
     * There is an existing script in lsyncd/examples for postcmd. This
       works at a higher level by adding an event onto the stack for
       executing a command once the sync has finished. I experimented with
       that but events dont get fired for the initial recursive sync, only
       on subsequent changes. As it is the initial sync that causes the
       problem that this patch is addressing, I had to look at a lower level.

     * The lsync lua C lib has an exec function, but it is hidden from
       config scripts except through the spawn(...) function. However spawn
       requires an event so can't be used for the initial sync.

     * I ended up going outside the lsync framework and using lua's own
       os.execute() function for pre/post cmds.

    While this looks like a big patch, its actually a relatively small
    change to the default rsync script. See
    https://github.com/hughsaunders/lsyncd/compare/master...hughsaunders:rsync_prepost
    for a comparison.

    Bug: #1543146
    Change-Id: I045a4a6bf722d6f1e01d21fbbec733872acb87a5
    Co-Authored-By: Hugh Saunders <email address hidden>
    Based on commit: b457f3bda65a418884e1847759e6bfa26a7ed5ab