Concurrent uploads which share layers worked correctly as of #18353,
but unfortunately #18785 caused a regression. This PR removed the logic
that shares digests between different push sessions. This overlooked the
case where one session was waiting for another session to upload a
layer.
This commit adds back the ability to propagate this digest information,
using the distribution.Descriptor type because this is what is received
from stats and uploads, and also what is ultimately needed for building
the manifest.
Surprisingly, there was no test covering this case. This commit adds
one. It fails without the fix.
See recent comments on #9132.
Signed-off-by: Aaron Lehmann <aaron.lehmann@docker.com>
5c99eebe81