The blob route and blobPathPattern only matched sha256 digests, so a
sha512 blob got 404 although the fetch path can verify sha512. The image
mirror rejected sha512 digests in index entries, configs and layers for
the same reason. Accept sha512 in both, with exact digest lengths.
A HEAD for an uncached manifest by digest passed on whatever digest the
upstream named. GET checks the body against the requested digest; HEAD
now answers DIGEST_INVALID when the upstream names a different digest
of the same algorithm.
A stale tag was revalidated by repeating the client's GET with
If-None-Match. Docker Hub counts manifest GETs against its pull limit,
so every TTL expiry of a tag could cost a pull. The tag is now resolved
with a HEAD and the Docker-Content-Digest compared with the cached one;
the manifest is fetched by digest only when that digest isn't cached.
A HEAD without a digest falls back to the conditional GET.
A client HEAD whose digest is cached now points the tag at it, so the
GET that follows is a cache hit. Concurrent requests for the same
manifest share one upstream exchange, and instances sharing Postgres
take turns through the fetch lease.
