fetch-pack: refactor packet writing

Refactor write_fetch_command_and_capabilities() to a more
general-purpose function, write_command_and_capabilities(), enabling it
to serve both fetch and additional commands.

In this context, "command" refers to the "operations" supported by
Git's wire protocol https://git-scm.com/docs/protocol-v2, such as a Git
subcommand (e.g., git-fetch(1)) or a server-side operation like
"object-info" as implemented in commit a2ba162
(object-info: support for retrieving object info, 2021-04-20).

Furthermore, write_command_and_capabilities() is moved to connect.c,
making it accessible to additional commands in the future.

To move write_command_and_capabilities() to connect.c, we need to
adjust how `advertise_sid` is managed. Previously,
in fetch_pack.c, `advertise_sid` was a static variable, modified using
git_config_get_bool().

In connect.c, we now initialize `advertise_sid` at the beginning by
directly using git_config_get_bool(). This change is safe because:

In the original fetch-pack.c code, there are only two places that
write `advertise_sid` :
1. In function do_fetch_pack:
        if (!server_supports("session-id"))
                advertise_sid = 0;
2. In function fetch_pack_config():
        git_config_get_bool("transfer.advertisesid", &advertise_sid);

About 1, since do_fetch_pack() is only relevant for protocol v1, this
assignment can be ignored in our refactor, as
write_command_and_capabilities() is only used in protocol v2.

About 2, git_config_get_bool() is from config.h and it is an out-of-box
dependency of connect.c, so we can reuse it directly.

Helped-by: Jonathan Tan <[email protected]>
Helped-by: Christian Couder <[email protected]>
Signed-off-by: Calvin Wan <[email protected]>
Signed-off-by: Eric Ju  <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: calvin-wan-google

connect.c

@@ -688,6 +688,40 @@ int server_supports(const char *feature)
 	return !!server_feature_value(feature, NULL);
 }
 
+void write_command_and_capabilities(struct strbuf *req_buf, const char *command,
+				    const struct string_list *server_options)
+{
+	const char *hash_name;
+	int advertise_sid;
+
+	git_config_get_bool("transfer.advertisesid", &advertise_sid);
+
+	ensure_server_supports_v2(command);
+	packet_buf_write(req_buf, "command=%s", command);
+	if (server_supports_v2("agent"))
+		packet_buf_write(req_buf, "agent=%s", git_user_agent_sanitized());
+	if (advertise_sid && server_supports_v2("session-id"))
+		packet_buf_write(req_buf, "session-id=%s", trace2_session_id());
+	if (server_options && server_options->nr) {
+		ensure_server_supports_v2("server-option");
+		for (size_t i = 0; i < server_options->nr; i++)
+			packet_buf_write(req_buf, "server-option=%s",
+					 server_options->items[i].string);
+	}
+
+	if (server_feature_v2("object-format", &hash_name)) {
+		const int hash_algo = hash_algo_by_name(hash_name);
+		if (hash_algo_by_ptr(the_hash_algo) != hash_algo)
+			die(_("mismatched algorithms: client %s; server %s"),
+				the_hash_algo->name, hash_name);
+		packet_buf_write(req_buf, "object-format=%s", the_hash_algo->name);
+	} else if (hash_algo_by_ptr(the_hash_algo) != GIT_HASH_SHA1) {
+		die(_("the server does not support algorithm '%s'"),
+			the_hash_algo->name);
+	}
+	packet_buf_delim(req_buf);
+}
+
 enum protocol {
 	PROTO_LOCAL = 1,
 	PROTO_FILE,

connect.h

@@ -30,4 +30,12 @@ void check_stateless_delimiter(int stateless_rpc,
 			       struct packet_reader *reader,
 			       const char *error);
 
+/*
+ * Writes a command along with the requested
+ * server capabilities/features into a request buffer.
+ */
+struct string_list;
+void write_command_and_capabilities(struct strbuf *req_buf, const char *command,
+				    const struct string_list *server_options);
+
 #endif

fetch-pack.c

@@ -1319,37 +1319,6 @@ static int add_haves(struct fetch_negotiator *negotiator,
 	return haves_added;
 }
 
-static void write_fetch_command_and_capabilities(struct strbuf *req_buf,
-						 const struct string_list *server_options)
-{
-	const char *hash_name;
-
-	ensure_server_supports_v2("fetch");
-	packet_buf_write(req_buf, "command=fetch");
-	if (server_supports_v2("agent"))
-		packet_buf_write(req_buf, "agent=%s", git_user_agent_sanitized());
-	if (advertise_sid && server_supports_v2("session-id"))
-		packet_buf_write(req_buf, "session-id=%s", trace2_session_id());
-	if (server_options && server_options->nr) {
-		ensure_server_supports_v2("server-option");
-		for (size_t i = 0; i < server_options->nr; i++)
-			packet_buf_write(req_buf, "server-option=%s",
-					 server_options->items[i].string);
-	}
-
-	if (server_feature_v2("object-format", &hash_name)) {
-		int hash_algo = hash_algo_by_name(hash_name);
-		if (hash_algo_by_ptr(the_hash_algo) != hash_algo)
-			die(_("mismatched algorithms: client %s; server %s"),
-			    the_hash_algo->name, hash_name);
-		packet_buf_write(req_buf, "object-format=%s", the_hash_algo->name);
-	} else if (hash_algo_by_ptr(the_hash_algo) != GIT_HASH_SHA1) {
-		die(_("the server does not support algorithm '%s'"),
-		    the_hash_algo->name);
-	}
-	packet_buf_delim(req_buf);
-}
-
 static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 			      struct fetch_pack_args *args,
 			      const struct ref *wants, struct oidset *common,
@@ -1360,7 +1329,7 @@ static int send_fetch_request(struct fetch_negotiator *negotiator, int fd_out,
 	int done_sent = 0;
 	struct strbuf req_buf = STRBUF_INIT;
 
-	write_fetch_command_and_capabilities(&req_buf, args->server_options);
+	write_command_and_capabilities(&req_buf, "fetch", args->server_options);
 
 	if (args->use_thin_pack)
 		packet_buf_write(&req_buf, "thin-pack");
@@ -2188,7 +2157,7 @@ void negotiate_using_fetch(const struct oid_array *negotiation_tips,
 					   the_repository, "%d",
 					   negotiation_round);
 		strbuf_reset(&req_buf);
-		write_fetch_command_and_capabilities(&req_buf, server_options);
+		write_command_and_capabilities(&req_buf, "fetch", server_options);
 
 		packet_buf_write(&req_buf, "wait-for-done");