enable SO_KEEPALIVE for connected TCP sockets

Sockets may never receive notification of some link errors,
causing "git fetch" or similar processes to hang forever.
Enabling keepalive messages allows hung processes to error out
after a few minutes/hours depending on the keepalive settings of
the system.

This is a problem noticed when running non-interactive
cronjobs to mirror repositories using "git fetch".

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

connect.c

@@ -175,6 +175,15 @@ static void get_host_and_port(char **host, const char **port)
 	}
 }
 
+static void enable_keepalive(int sockfd)
+{
+	int ka = 1;
+
+	if (setsockopt(sockfd, SOL_SOCKET, SO_KEEPALIVE, &ka, sizeof(ka)) < 0)
+		fprintf(stderr, "unable to set SO_KEEPALIVE on socket: %s\n",
+			strerror(errno));
+}
+
 #ifndef NO_IPV6
 
 static const char *ai_name(const struct addrinfo *ai)
@@ -239,6 +248,8 @@ static int git_tcp_connect_sock(char *host, int flags)
 	if (sockfd < 0)
 		die("unable to connect to %s:\n%s", host, error_message.buf);
 
+	enable_keepalive(sockfd);
+
 	if (flags & CONNECT_VERBOSE)
 		fprintf(stderr, "done.\n");
 
@@ -315,6 +326,8 @@ static int git_tcp_connect_sock(char *host, int flags)
 	if (sockfd < 0)
 		die("unable to connect a socket (%s)", strerror(saved_errno));
 
+	enable_keepalive(sockfd);
+
 	if (flags & CONNECT_VERBOSE)
 		fprintf(stderr, "done.\n");