If upload-pack fails while generating a pack, the receiving side of a
fetch will print out a message like this:

  fatal: git remote host hung up unexpectedly
  fatal: recursion detected in die handler

You can see it by instrumenting upload-pack to fail like this:

diff --git a/upload-pack.c b/upload-pack.c
index bfa6279..721b416 100644
--- a/upload-pack.c
+++ b/upload-pack.c

@@ -777,6 +777,7 @@ static void upload_pack(void)
 	receive_needs();
 	if (want_obj.nr) {
 		get_common_commits();
+		exit(0);
 		create_pack_file();
 	}
 }

and doing a clone or fetch. What's actually happening is that we have
two threads running, each of which dies, fooling the recursion check.
This series attempts to fix it. The errors afterwards look like:

  fatal: git remote host hung up unexpectedly
  fatal: index-pack failed

which is what was printed pre-cd163d4 (usage.c: detect recursion in die
routines and bail out immediately).

-Peff