A client that fell behind is switched to the stored copy, which for an
unrecorded refetch is deleted once the waiters close. It now takes its
own hold, released when it closes.
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.
