receive-pack.c: add protocol support to negotiate atomic-push

This adds support to the protocol between send-pack and receive-pack to
* allow receive-pack to inform the client that it has atomic push capability
* allow send-pack to request atomic push back.

There is currently no setting in send-pack to actually request that atomic
pushes are to be used yet. This only adds protocol capability not ability
for the user to activate it.

Signed-off-by: Ronnie Sahlberg <sahlberg@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

3 files changed, 21 insertions, 3 deletions

diff --git a/Documentation/technical/protocol-capabilities.txt b/Documentation/technical/protocol-capabilities.txt
index 0c92deebcc..26bc5b1cbb 100644
--- a/Documentation/technical/protocol-capabilities.txt
+++ b/Documentation/technical/protocol-capabilities.txt
@@ -18,8 +18,9 @@ was sent.  Server MUST NOT ignore capabilities that client requested
 and server advertised.  As a consequence of these rules, server MUST
 NOT advertise capabilities it does not understand.
 
-The 'report-status', 'delete-refs', 'quiet', and 'push-cert' capabilities
-are sent and recognized by the receive-pack (push to server) process.
+The 'atomic-push', 'report-status', 'delete-refs', 'quiet', and 'push-cert'
+capabilities are sent and recognized by the receive-pack (push to server)
+process.
 
 The 'ofs-delta' and 'side-band-64k' capabilities are sent and recognized
 by both upload-pack and receive-pack protocols.  The 'agent' capability
@@ -244,6 +245,13 @@ respond with the 'quiet' capability to suppress server-side progress
 reporting if the local progress reporting is also being suppressed
 (e.g., via `push -q`, or if stderr does not go to a tty).
 
+atomic-push
+-----------
+
+If the server sends the 'atomic-push' capability, it means it is
+capable of accepting atomic pushes. If the pushing client requests this
+capability, the server will update the refs in one single atomic transaction.
+
 allow-tip-sha1-in-want
 ----------------------
 
diff --git a/builtin/receive-pack.c b/builtin/receive-pack.c
index 7ddd75620d..b8ffd9eba1 100644
--- a/builtin/receive-pack.c
+++ b/builtin/receive-pack.c
@@ -40,6 +40,7 @@ static int transfer_unpack_limit = -1;
 static int unpack_limit = 100;
 static int report_status;
 static int use_sideband;
+static int use_atomic_push;
 static int quiet;
 static int prefer_ofs_delta = 1;
 static int auto_update_server_info;
@@ -171,7 +172,8 @@ static void show_ref(const char *path, const unsigned char *sha1)
 		struct strbuf cap = STRBUF_INIT;
 
 		strbuf_addstr(&cap,
-			      "report-status delete-refs side-band-64k quiet");
+			      "report-status delete-refs side-band-64k quiet "
+			      "atomic-push");
 		if (prefer_ofs_delta)
 			strbuf_addstr(&cap, " ofs-delta");
 		if (push_cert_nonce)
@@ -1173,6 +1175,8 @@ static struct command *read_head_info(struct sha1_array *shallow)
 				use_sideband = LARGE_PACKET_MAX;
 			if (parse_feature_request(feature_list, "quiet"))
 				quiet = 1;
+			if (parse_feature_request(feature_list, "atomic-push"))
+				use_atomic_push = 1;
 		}
 
 		if (!strcmp(line, "push-cert")) {
diff --git a/send-pack.c b/send-pack.c
index 949cb61aa0..1ccc84c889 100644
--- a/send-pack.c
+++ b/send-pack.c
@@ -294,6 +294,8 @@ int send_pack(struct send_pack_args *args,
 	int use_sideband = 0;
 	int quiet_supported = 0;
 	int agent_supported = 0;
+	int atomic_push_supported = 0;
+	int atomic_push = 0;
 	unsigned cmds_sent = 0;
 	int ret;
 	struct async demux;
@@ -314,6 +316,8 @@ int send_pack(struct send_pack_args *args,
 		agent_supported = 1;
 	if (server_supports("no-thin"))
 		args->use_thin_pack = 0;
+	if (server_supports("atomic-push"))
+		atomic_push_supported = 1;
 	if (args->push_cert) {
 		int len;
 
@@ -335,6 +339,8 @@ int send_pack(struct send_pack_args *args,
 		strbuf_addstr(&cap_buf, " side-band-64k");
 	if (quiet_supported && (args->quiet || !args->progress))
 		strbuf_addstr(&cap_buf, " quiet");
+	if (atomic_push)
+		strbuf_addstr(&cap_buf, " atomic-push");
 	if (agent_supported)
 		strbuf_addf(&cap_buf, " agent=%s", git_user_agent_sanitized());