upload-pack: kill pack-objects helper on signal or exit

We spawn an external pack-objects process to actually send objects to
the remote side. If we are killed by a signal during this process, then
pack-objects may continue to run. As soon as it starts producing output
for the pack, it will see a failure writing to upload-pack and exit
itself. But before then, it may do significant work traversing the
object graph, compressing deltas, etc, which will all be pointless. So
let's make sure to kill as soon as we know that the caller will not read
the result.

There's no test here, since it's inherently racy, but here's an easy
reproduction is on a large-ish repo like linux.git:

  - make sure you don't have pack bitmaps (since they make the enumerating
    phase go quickly). For linux.git it takes ~30s or so to walk the
    whole graph on my machine.

  - run "git clone --no-local -q . dst"; the "-q" is important because
    if pack-objects is writing progress to upload-pack (to get
    multiplexed over the sideband to the client), then it will notice
    pretty quickly the failure to write to stderr

  - kill the client-side clone process in another terminal (don't use
    ^C, as that will send SIGINT to all of the processes)

  - run "ps au | grep git" or similar to observe upload-pack dying
    within 5 seconds (it will send a keepalive that will notice the
    client has gone away)

  - but you'll still see pack-objects consuming 100% CPU (and 1GB+ of
    RAM) during the traversal and delta compression phases. It will exit
    as soon as it starts to write the pack (when it will notice that
    upload-pack went away).

With this patch, pack-objects exits as soon as upload-pack does.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

upload-pack.c

@@ -321,6 +321,7 @@ static void create_pack_file(struct upload_pack_data *pack_data,
 	pack_objects.in = -1;
 	pack_objects.out = -1;
 	pack_objects.err = -1;
+	pack_objects.clean_on_exit = 1;
 
 	if (start_command(&pack_objects))
 		die("git upload-pack: unable to fork git-pack-objects");