[PATCH 23/26] transport-helper: introduce connect-half-duplex

[Brandon Williams <bmwill@google.com>]

Introduce the transport-helper capability 'connect-half-duplex'.  This
capability indicates that the transport-helper can be requested to run
the 'connect-half-duplex' command which should attempt to make a
half-duplex connection with a remote end.  Once established, the
half-duplex connection can be used by the git client to communicate with
the remote end natively in a stateles-rpc manner as supported by
protocol v2.

If a half-duplex 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.

Signed-off-by: Brandon Williams <bmwill@google.com>
---
 transport-helper.c | 8 ++++++++
 transport.c        | 1 +
 transport.h        | 6 ++++++
 3 files changed, 15 insertions(+)

diff --git a/transport-helper.c b/transport-helper.c
index c032a2a87..d037609bc 100644
--- a/transport-helper.c
+++ b/transport-helper.c
@@ -26,6 +26,7 @@ struct helper_data {
 		option : 1,
 		push : 1,
 		connect : 1,
+		connect_half_duplex : 1,
 		signed_tags : 1,
 		check_connectivity : 1,
 		no_disconnect_req : 1,
@@ -188,6 +189,8 @@ 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, "connect-half-duplex")) {
+			data->connect_half_duplex = 1;
 		} else if (!strcmp(capname, "signed-tags")) {
 			data->signed_tags = 1;
 		} else if (skip_prefix(capname, "export-marks ", &arg)) {
@@ -612,6 +615,11 @@ 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->connect_half_duplex) {
+		strbuf_addf(&cmdbuf, "connect-half-duplex %s\n", name);
+		ret = run_connect(transport, &cmdbuf);
+		if (ret)
+			transport->stateless_rpc = 1;
 	}
 
 	strbuf_release(&cmdbuf);
diff --git a/transport.c b/transport.c
index 4fdbd9adc..aafb8fbb4 100644
--- a/transport.c
+++ b/transport.c
@@ -250,6 +250,7 @@ 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);
diff --git a/transport.h b/transport.h
index 4b656f315..9eac809ee 100644
--- a/transport.h
+++ b/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.
-- 
2.15.1.620.gb9897f4670-goog