receive-pack.c: negotiate atomic push support

This adds the atomic protocol option to allow
receive-pack to inform the client that it has
atomic push capability.

This commit makes the functionality introduced
in the previous commits go live for the serving
side. The changes in documentation reflect the
protocol capabilities of the server.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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', '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 +245,14 @@ 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
+------
+
+If the server sends the 'atomic' capability it is capable of accepting
+atomic pushes. If the pushing client requests this capability, the server
+will update the refs in one atomic transaction. Either all refs are
+updated or none.
+
 allow-tip-sha1-in-want
 ----------------------
 

builtin/receive-pack.c

@@ -37,6 +37,7 @@ static int receive_fsck_objects = -1;
 static int transfer_fsck_objects = -1;
 static int receive_unpack_limit = -1;
 static int transfer_unpack_limit = -1;
+static int advertise_atomic_push = 1;
 static int unpack_limit = 100;
 static int report_status;
 static int use_sideband;
@@ -159,6 +160,11 @@ static int receive_pack_config(const char *var, const char *value, void *cb)
 		return 0;
 	}
 
+	if (strcmp(var, "receive.advertiseatomic") == 0) {
+		advertise_atomic_push = git_config_bool(var, value);
+		return 0;
+	}
+
 	return git_default_config(var, value, cb);
 }
 
@@ -174,6 +180,8 @@ static void show_ref(const char *path, const unsigned char *sha1)
 
 		strbuf_addstr(&cap,
 			      "report-status delete-refs side-band-64k quiet");
+		if (advertise_atomic_push)
+			strbuf_addstr(&cap, " atomic");
 		if (prefer_ofs_delta)
 			strbuf_addstr(&cap, " ofs-delta");
 		if (push_cert_nonce)
@@ -1263,6 +1271,9 @@ 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 (advertise_atomic_push
+			    && parse_feature_request(feature_list, "atomic"))
+				use_atomic = 1;
 		}
 
 		if (!strcmp(line, "push-cert")) {