Race condition in imagebackend.Image.cache downloads image several times

[Alvaro Lopez]

There's a race condition in imagebackend.Image.cache that makes nova download an image N times when N requests requiring the same image are scheduled in the same host during the image feching.

The imagebackend.Image.cache method only synchronizes on the image fetching function, but the whole function should be synchronized (or the create_image function). When several requests using the same image are scheduled at the same time there's no synchronization when nova checks if an image already exists or not, therefore several requests may check that the image does not exist, and start a download for all of them (the actual download will be syncronized, but the image will be download several times, one for each request).

This can be seen requesting several instances into the same host:

    nova boot --image <image> --flavor <foo> --num-instances=4 --availability-zone <az>:<host>

In the host we can see:

    -rw-r--r-- 1 nova nova 5.0G Aug 25 14:21 243eccfbc52469947665a506145d798670e3fc88
    -rw-r--r-- 1 nova nova 1.2G Aug 25 14:22 243eccfbc52469947665a506145d798670e3fc88.part

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.openstack.org/116838

[OpenStack Infra (hudson-openstack)]

Change abandoned by Alvaro Lopez Garcia (<email address hidden>) on branch: master
Review: https://review.openstack.org/116838
Reason: Abandoned in favour of https://review.openstack.org/#/c/83400/