serve: advertise object-info feature

In order for a client to know what object-info components a server can
provide, advertise supported object-info features. This allows a client
to decide whether to query the server for object-info or fetch as a
fallback.

While at it, update the object-info section in 'gitprotocol-v2.adoc':
- Require full obj-oid explicitly.
- Fix parentheses.
- Define obj-size explicitly.
- Make obj-size optional in obj-info and document the behavior
  for unrecognized object IDs.

Helped-by: Jonathan Tan <jonathantanmy@google.com>
Helped-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Calvin Wan <calvinwan@google.com>
Signed-off-by: Eric Ju <eric.peijian@gmail.com>
Signed-off-by: Pablo Sabater <pabloosabaterr@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: calvin-wan-google

Documentation/gitprotocol-v2.adoc

@@ -568,21 +568,26 @@ An `object-info` request takes the following arguments:
 
 	oid <oid>
 	Indicates to the server an object which the client wants to obtain
-	information for.
+	information for. They must be full object IDs.
 
 The response of `object-info` is a list of the requested object ids
 and associated requested information, each separated by a single space.
 
 	output = info flush-pkt
 
-	info = PKT-LINE(attrs) LF)
+	info = PKT-LINE(attrs LF)
 		*PKT-LINE(obj-info LF)
 
 	attrs = attr | attrs SP attrs
 
+	obj-size = 1*DIGIT
+
 	attr = "size"
 
-	obj-info = obj-id SP obj-size
+	obj-info = obj-id SP [obj-size]
+
+	If the server does not recognize the object id, the response will be
+	`obj-id SP` regardless of the number of attributes requested.
 
 bundle-uri
 ~~~~~~~~~~

serve.c

@@ -89,14 +89,17 @@ static void session_id_receive(struct repository *r UNUSED,
 	trace2_data_string("transfer", NULL, "client-sid", client_sid);
 }
 
-static int object_info_advertise(struct repository *r, struct strbuf *value UNUSED)
+static int object_info_advertise(struct repository *r, struct strbuf *value)
 {
 	if (advertise_object_info == -1 &&
 	    repo_config_get_bool(r, "transfer.advertiseobjectinfo",
 				 &advertise_object_info)) {
 		/* disabled by default */
 		advertise_object_info = 0;
 	}
+	/* Currently only size is supported */
+	if (value && advertise_object_info)
+		strbuf_addstr(value, "size");
 	return advertise_object_info;
 }