transport-helper: introduce stateless-connect

Introduce the transport-helper capability 'stateless-connect'.  This
capability indicates that the transport-helper can be requested to run
the 'stateless-connect' command which should attempt to make a
stateless connection with a remote end.  Once established, the
connection can be used by the git client to communicate with
the remote end natively in a stateless-rpc manner as supported by
protocol v2.  This means that the client must send everything the server
needs in a single request as the client must not assume any
state-storing on the part of the server or transport.

If a stateless connection cannot be established then the remote-helper
will respond in the same manner as the 'connect' command indicating that
the client should fallback to using the dumb remote-helper commands.

A future patch will implement the 'stateless-connect' capability in our
http remote-helper (remote-curl) so that protocol v2 can be used using
the http transport.

Signed-off-by: Brandon Williams <bmwill@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/gitremote-helpers.txt

@@ -102,6 +102,14 @@ Capabilities for Pushing
 +
 Supported commands: 'connect'.
 
+'stateless-connect'::
+	Experimental; for internal use only.
+	Can attempt to connect to a remote server for communication
+	using git's wire-protocol version 2.  See the documentation
+	for the stateless-connect command for more information.
++
+Supported commands: 'stateless-connect'.
+
 'push'::
 	Can discover remote refs and push local commits and the
 	history leading up to them to new or existing remote refs.
@@ -136,6 +144,14 @@ Capabilities for Fetching
 +
 Supported commands: 'connect'.
 
+'stateless-connect'::
+	Experimental; for internal use only.
+	Can attempt to connect to a remote server for communication
+	using git's wire-protocol version 2.  See the documentation
+	for the stateless-connect command for more information.
++
+Supported commands: 'stateless-connect'.
+
 'fetch'::
 	Can discover remote refs and transfer objects reachable from
 	them to the local object store.
@@ -375,6 +391,22 @@ Supported if the helper has the "export" capability.
 +
 Supported if the helper has the "connect" capability.
 
+'stateless-connect' <service>::
+	Experimental; for internal use only.
+	Connects to the given remote service for communication using
+	git's wire-protocol version 2.  Valid replies to this command
+	are empty line (connection established), 'fallback' (no smart
+	transport support, fall back to dumb transports) and just
+	exiting with error message printed (can't connect, don't bother
+	trying to fall back).  After line feed terminating the positive
+	(empty) response, the output of the service starts.  Messages
+	(both request and response) must consist of zero or more
+	PKT-LINEs, terminating in a flush packet. The client must not
+	expect the server to store any state in between request-response
+	pairs.  After the connection ends, the remote helper exits.
++
+Supported if the helper has the "stateless-connect" capability.
+
 If a fatal error occurs, the program writes the error message to
 stderr and exits. The caller should expect that a suitable error
 message has been printed if the child closes the connection without

transport-helper.c

@@ -12,6 +12,7 @@
 #include "argv-array.h"
 #include "refs.h"
 #include "transport-internal.h"
+#include "protocol.h"
 
 static int debug;
 
@@ -26,6 +27,7 @@ struct helper_data {
 		option : 1,
 		push : 1,
 		connect : 1,
+		stateless_connect : 1,
 		signed_tags : 1,
 		check_connectivity : 1,
 		no_disconnect_req : 1,
@@ -188,6 +190,8 @@ static struct child_process *get_helper(struct transport *transport)
 			refspecs[refspec_nr++] = xstrdup(arg);
 		} else if (!strcmp(capname, "connect")) {
 			data->connect = 1;
+		} else if (!strcmp(capname, "stateless-connect")) {
+			data->stateless_connect = 1;
 		} else if (!strcmp(capname, "signed-tags")) {
 			data->signed_tags = 1;
 		} else if (skip_prefix(capname, "export-marks ", &arg)) {
@@ -612,6 +616,13 @@ static int process_connect_service(struct transport *transport,
 	if (data->connect) {
 		strbuf_addf(&cmdbuf, "connect %s\n", name);
 		ret = run_connect(transport, &cmdbuf);
+	} else if (data->stateless_connect &&
+		   (get_protocol_version_config() == protocol_v2) &&
+		   !strcmp("git-upload-pack", name)) {
+		strbuf_addf(&cmdbuf, "stateless-connect %s\n", name);
+		ret = run_connect(transport, &cmdbuf);
+		if (ret)
+			transport->stateless_rpc = 1;
 	}
 
 	strbuf_release(&cmdbuf);

transport.c

@@ -252,6 +252,7 @@ static int fetch_refs_via_pack(struct transport *transport,
 		data->options.check_self_contained_and_connected;
 	args.cloning = transport->cloning;
 	args.update_shallow = data->options.update_shallow;
+	args.stateless_rpc = transport->stateless_rpc;
 
 	if (!data->got_remote_heads)
 		refs_tmp = get_refs_via_connect(transport, 0, NULL);

transport.h

@@ -55,6 +55,12 @@ struct transport {
 	 */
 	unsigned cloning : 1;
 
+	/*
+	 * Indicates that the transport is connected via a half-duplex
+	 * connection and should operate in stateless-rpc mode.
+	 */
+	unsigned stateless_rpc : 1;
+
 	/*
 	 * These strings will be passed to the {pre, post}-receive hook,
 	 * on the remote side, if both sides support the push options capability.