stateless-connect: send response end packet

Currently, remote-curl acts as a proxy and blindly forwards packets
between an HTTP server and fetch-pack. In the case of a stateless RPC
connection where the connection is terminated before the transaction is
complete, remote-curl will blindly forward the packets before waiting on
more input from fetch-pack. Meanwhile, fetch-pack will read the
transaction and continue reading, expecting more input to continue the
transaction. This results in a deadlock between the two processes.

This can be seen in the following command which does not terminate:

	$ git -c protocol.version=2 clone https://github.com/git/git.git --shallow-since=20151012
	Cloning into 'git'...

whereas the v1 version does terminate as expected:

	$ git -c protocol.version=1 clone https://github.com/git/git.git --shallow-since=20151012
	Cloning into 'git'...
	fatal: the remote end hung up unexpectedly

Instead of blindly forwarding packets, make remote-curl insert a
response end packet after proxying the responses from the remote server
when using stateless_connect(). On the RPC client side, ensure that each
response ends as described.

A separate control packet is chosen because we need to be able to
differentiate between what the remote server sends and remote-curl's
control packets. By ensuring in the remote-curl code that a server
cannot send response end packets, we prevent a malicious server from
being able to perform a denial of service attack in which they spoof a
response end packet and cause the described deadlock to happen.

Reported-by: Force Charlie <charlieio@outlook.com>
Helped-by: Jeff King <peff@peff.net>
Signed-off-by: Denton Liu <liu.denton@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 1 deletions

diff --git a/remote.h b/remote.h
index 11d8719b58..5cc26c1b3b 100644
--- a/remote.h
+++ b/remote.h
@@ -179,7 +179,8 @@ struct ref **get_remote_heads(struct packet_reader *reader,
 struct ref **get_remote_refs(int fd_out, struct packet_reader *reader,
 			     struct ref **list, int for_push,
 			     const struct argv_array *ref_prefixes,
-			     const struct string_list *server_options);
+			     const struct string_list *server_options,
+			     int stateless_rpc);
 
 int resolve_remote_symref(struct ref *ref, struct ref *list);