send-pack: unbreak push over stateless rpc

send-pack: unbreak push over stateless rpc

Commit 09c9957 (send-pack: avoid deadlock when pack-object
dies early, 2011-04-25) attempted to fix a hang in the
stateless rpc case by closing a file descriptor early, but
we still need that descriptor.

Basically the deadlock can happen when pack-objects fails,
and the descriptor to upstream is left open. We never send
the pack, so the upstream is left waiting for us to say
something, and we are left waiting for upstream to close the
connection.

In the non-rpc case, our descriptor points straight to the
upstream. We hand it off to run-command, which takes
ownership and closes the descriptor after pack-objects
finishes (whether it succeeds or not).

Commit 09c9957 tried to emulate that in the rpc case. That
isn't right, though. We actually have a descriptor going
back to the remote-helper, and we need to keep using it
after pack-objects is finished. Closing it early completely
breaks pushing via smart-http.

We still need to do something on error to signal the
remote-helper that we won't be sending any pack data
(otherwise we get the deadlock).  In an ideal world, we
would send a special packet back that says "Sorry, there was
an error". But the remote-helper doesn't understand any such
packet, so the best we can do is close the descriptor and
let it report that we hung up unexpectedly.

Signed-off-by: Jeff King <peff@peff.net>
Acked-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>