Libvirt's image caching fetches images multiple times

[John Warren]

When launching several VMs in rapid succession, it is possible that libvirt's image caching will fetch the same image several times. This can occur when all of the VMs in question are using the same base image and this base image has not been previously fetched. The inline fetch_func_sync method prevents multiple threads from fetching the same image at the same time, but it does not prevent a thread that is waiting to acquire the lock from fetching the image that was being fetched while the lock was still in use. This is because the presence of the image is checked only before the lock has been acquired, not after.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/87066

[Joe Gordon (jogo)]

Patch in progress: https://review.openstack.org/83313/

[Sean Dague (sdague)]

There are competing upstream patches, and it's not really clear either are making forward progress.

[OpenStack Infra (hudson-openstack)]

Change abandoned by Wangpan (<email address hidden>) on branch: master
Review: https://review.openstack.org/83313

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.openstack.org/83400
Committed: https://git.openstack.org/cgit/openstack/nova/commit/?id=04fa99095cd82f7b6030694996c75d2fd95e7c61
Submitter: Jenkins
Branch: master

commit 04fa99095cd82f7b6030694996c75d2fd95e7c61
Author: John Warren <email address hidden>
Date: Thu Mar 27 14:26:48 2014 +0000

    Add exists check to fetch_func_sync in libvirt imagebackend

    This change adds a path exists check to fetch_func_sync to deal with
    cases where a thread is waiting to acquire the lock while another
    thread has the lock and is downloading the same image. Without this
    check the same image template will be downloaded multiple times.

    Change-Id: Ib410903c7fd9d6954a8082391f89b42bac6a20d3
    Closes-Bug: #1298420