Re: Small patch to add support for MPTCP on Linux

From

Muhammad Nuzaihan <zaihan@unrealasia.net>

Date

May 20, 2025, 04:32 UTC

Message-ID

<ZXLJWS.WPQLCXFNN8TH@unrealasia.net>

In-Reply-To

<xmqqo6vokvpv.fsf@gitster.g>

> Phillip Wood <phillip.wood123@gmail.com> writes:
> 
>>  As brian has already said I think it would be better to have a
>>  Makefile knob to control this which defaults to being on for
>>  linux. Take a look at the various USE_xxx definitions in the 
>> Makefile
>>  and config.mak.uname for setting default compile flags for different
>>  operating systems.
>> 
>>>  Also another check if a socket is supported by looking for a return
>>>  value of
>>>  "EAI_SOCKTYPE" (not EINVAL) and fallback to regular TCP if that is
>>>  returned.
>>>  EAI_SOCKTYPE should work across different UNIX systems as this is a
>>>  posix error code.
>> 
>>  That error is not mentioned in the documentation for MCTCP on Linux
>>  [1]. Please make sure your code checks for the errno values 
>> described
>>  in the documentation.
> 
> Also according to RFC 6897, "MPTCP is designed to be totally
> backward compatible to applications".  I understand that this is
> quite unlike introducing IPv6 into IPv4-only world.  You can tell
> the system that supports MPTCP to use it in specific ways by
> updating your application, but your system's local policy may
> allow MPTCP to automatically set up multiple subflows even your
> application is not quite aware of MPTCP.
> 
> So, ... I somehow would be mildly surprised if Git were a kind of
> application that needs to take advantage of "several additional
> degrees of freedom that applications may wish to exploit" by using
> API that is "a simple extension of TCP's interface for MPTCP-aware
> applications".  Requiring a simple application like ours to tweak
> and rebuild in today's world does not sound like a winning strategy
> to promote a technology that "is designed to be totally backward
> compatible to applications", at least to me.
> 

I think that's the relevant use-case for a laptop user working with git.

git config --global core.mptcp true (or a single repo without --global)

> 

diff --git a/cache.h b/cache.h
index eba12487b9..6839b3acbc 100644
--- a/cache.h
+++ b/cache.h
@@ -944,6 +944,7 @@ extern int verify_ce_order;
 
 /* Environment bits from configuration mechanism */
 extern int trust_executable_bit;
+extern int enable_mptcp;
 extern int trust_ctime;
 extern int check_stat;
 extern int quote_path_fully;
diff --git a/config.c b/config.c
index 2317a76696..833396aa4b 100644
--- a/config.c
+++ b/config.c
@@ -1464,6 +1464,11 @@ static int git_default_core_config(const char *var, const char *value, void *cb)
 	if (!strcmp(var, "core.editor"))
 		return git_config_string(&editor_program, var, value);
 
+	if (!strcmp(var, "core.mptcp")) {
+		enable_mptcp = git_config_bool(var, value);
+		return 0;
+	}
+
 	if (!strcmp(var, "core.commentchar")) {
 		if (!value)
 			return config_error_nonbool(var);
diff --git a/connect.c b/connect.c
index eaf7d6d261..ebeac99bd6 100644
--- a/connect.c
+++ b/connect.c
@@ -721,6 +721,16 @@ static void enable_keepalive(int sockfd)
 		error_errno(_("unable to set SO_KEEPALIVE on socket"));
 }
 
+static const int needs_mptcp(void)
+{
+	int mptcp = 0;
+
+	if (git_config_get_bool("core.mptcp", &mptcp))
+		return mptcp;
+
+	return mptcp;
+}
+
 #ifndef NO_IPV6
 
 static const char *ai_name(const struct addrinfo *ai)
@@ -770,7 +780,11 @@ static int git_tcp_connect_sock(char *host, int flags)
 
 	for (ai0 = ai; ai; ai = ai->ai_next, cnt++) {
 		sockfd = socket(ai->ai_family,
-				ai->ai_socktype, ai->ai_protocol);
+				ai->ai_socktype,
+#ifdef IPPROTO_MPTCP
+				needs_mptcp() ? IPPROTO_MPTCP :
+#endif
+				ai->ai_protocol);
 		if ((sockfd < 0) ||
 		    (connect(sockfd, ai->ai_addr, ai->ai_addrlen) < 0)) {
 			strbuf_addf(&error_message, "%s[%d: %s]: errno=%s\n",
@@ -817,6 +831,7 @@ static int git_tcp_connect_sock(char *host, int flags)
 	char **ap;
 	unsigned int nport;
 	int cnt;
+	const int needs_mptcp;
 
 	get_host_and_port(&host, &port);
 
diff --git a/daemon.c b/daemon.c
index b1fcbe0d6f..08a16ccf03 100644
--- a/daemon.c
+++ b/daemon.c
@@ -17,6 +17,7 @@ static enum log_destination {
 } log_destination = LOG_DESTINATION_UNSET;
 static int verbose;
 static int reuseaddr;
+static int mptcp;
 static int informative_errors;
 
 static const char daemon_usage[] =
@@ -1007,6 +1008,10 @@ static int setup_named_sock(char *listen_addr, int listen_port, struct socketlis
 	for (ai = ai0; ai; ai = ai->ai_next) {
 		int sockfd;
 
+#if defined(__linux__) && defined(IPPROTO_MPTCP)
+		sockfd = socket(ai->ai_family, ai->ai_socktype, IPPROTO_MPTCP);
+		if (sockfd < 0)
+#endif
 		sockfd = socket(ai->ai_family, ai->ai_socktype, ai->ai_protocol);
 		if (sockfd < 0)
 			continue;
@@ -1360,6 +1365,10 @@ int cmd_main(int argc, const char **argv)
 			reuseaddr = 1;
 			continue;
 		}
+		if (!strcmp(arg, "--mptcp")) {
+			mptcp = 1;
+			continue;
+		}
 		if (!strcmp(arg, "--user-path")) {
 			user_path = "";
 			continue;
diff --git a/environment.c b/environment.c
index 9da7f3c1a1..72f1adef6c 100644
--- a/environment.c
+++ b/environment.c
@@ -33,6 +33,7 @@ int warn_ambiguous_refs = 1;
 int warn_on_object_refname_ambiguity = 1;
 int repository_format_precious_objects;
 int repository_format_worktree_config;
+int enable_mptcp;
 const char *git_commit_encoding;
 const char *git_log_output_encoding;
 char *apply_default_whitespace;