An unpooled Hackney 4 connection that ends without a reusable socket -- a
body cut short by the peer, a bodyless 204/304, or a failed read -- parks
in its `closed` state forever, holding every refc binary it read.
read_full_body/2 hands back `socket = undefined`, so it never reaches the
reuse check; `closed(enter)` arms a stop timer only for pooled
connections; and an unpooled connection is owned by hackney_conn_sup, so
the owner-DOWN cleanup never fires.
Nothing here can work around it. Since 4.0 Hackney ignores `with_body`
and always returns a fully read binary, so a synchronous request never
yields a handle to close, and the short read still reports {:ok, body}.
Media proxy warming hits this whenever an attachment fetch is cut short
and the outbound proxy is used as a CONNECT tunnel, since tunnels are
never pooled.
Pin the fix, a single commit on top of the 4.7.2 tag, until it ships in a
release. While it is a git dep Hackney drops out of `mix deps.audit` and
`mix hex.outdated`, so this wants moving back to hex promptly.
benoitc/hackney#918, benoitc/hackney#919
