Comment 9 for bug 1409012

Reviewed: https://review.openstack.org/158623
Committed: https://git.openstack.org/cgit/openstack/cinder/commit/?id=60c563f72d2d6d008fca03f0a058941f3807e1a8
Submitter: Jenkins
Branch: master

commit 60c563f72d2d6d008fca03f0a058941f3807e1a8
Author: Zhiteng Huang <email address hidden>
Date: Tue Feb 24 17:05:46 2015 +0800

    Allow scheduler to receive volume stats when starting service

    Filter scheduler relies on volume services to report their stats in
    order to make decisions - this nature makes scheduler service a bit
    special when it starts because it can't really do anything without
    knowning stats of avilable volume backend. So the original
    implementation of Filter Scheduler added a hook in init_host() to
    ask all volume services to send their stats immediately (volume RPC
    call 'publish_service_capabitlies()'), this reduced the chance of a
    new volume being set to 'error' state because scheduler has no
    knowledge of backends to minimum.

                 Scheduler RPC Exchanges Volume Service(s)
    service start | RPC init |
                  | init_host() |
                  | --pub_src_caps()-->| ----------> | (vol fanout)
                  | <------------- |<-vol stats--| (sch fanout)

    However, commit 65fa80c361f71158cc492dfc520dc4a63ccfa419 moved
    init_host() ahead of service RPC initialization because for volume
    services, service initialization should be done before the service
    starts accepting RPC requests. This change unfortunately invalidated
    scheduler service's init_host() because that needs to be done *AFTER*
    RPC is ready (a queue is bound to scheduler fanout exchange). The
    result is scheduler asks vol service to send update, but it misses
    the information it asked for because its queue hasn't been created
    and bound to the fanout queue yet. So scheduler always has to wait
    for the next update from volume services to be able to work properly.

                 Scheduler RPC Exchanges Volume Service(s)
    service start | init_host() |
                  | --pub_src_caps()-->| ----------> | (vol fanout)
                  | (vol stats lost) |<-vol stats--| (sch fanout)
                  | RPC init |

    This change adds a new hook to Manager class called init_hook_with_rpc()
    to allow services like scheduler to do something once RPC is ready.
    This should restore scheduler's behavior before commit
    65fa80c361f71158cc492dfc520dc4a63ccfa419.

    Change-Id: If6cf9030eb44c39a06ec501ac5c049d460782481
    Partial-bug: #1409012