The upstream read fed the client through a pipe, so it ran at the client's
speed. The client is now fed from a bounded buffer and, once it falls
behind, continues from the stored copy where it left off.
A caller handed the stream takes no hold on an unrecorded object.
On a cache miss the proxy stored the whole artifact before sending any of
it, so clients with an idle timeout gave up on large artifacts: git-lfs
after 30 seconds, pacman after 10. The caller that starts a shared fetch
now gets the bytes as they arrive while they are stored; requests joining
the fetch, HEAD and range requests still wait for the stored copy.
The client's copy is checked against the upstream's digest as it is read,
and a mismatch aborts the response before it completes. A client going
away leaves the copy to storage running. A storage error leaves the
client's copy running: the client gets the artifact, and the failure is
logged at error level and counted in proxy_artifacts_served_uncached_total.
A block-mode scanner has to see the whole artifact first, so with one
configured for the ecosystem a miss is handled as before.
