mptcpize: set GODEBUG=multipathtcp=1 env var (#316)

GO apps don't use the libC, so the LD_PRELOAD technique doesn't work
with these apps.

But since GO 1.21, MPTCP is natively supported, and can be forced by
simply setting GODEBUG=multipathtcp=1. So mptcpize can easily support GO
apps by also setting this env var.

Similar to the LD_PRELOAD env var, the GODEBUG one is appended with a
comma if it was already set.

Link: https://go.dev/doc/godebug
Link: https://pkg.go.dev/net#Dialer.SetMultipathTCP

Signed-off-by: Matthieu Baerts (NGI0) <[email protected]>

Author: matttbe

src/mptcpize.c

@@ -35,6 +35,8 @@
 #define SYSTEMCTL_SHOW		"systemctl show -p FragmentPath "
 #define PRELOAD_VAR		"LD_PRELOAD="
 #define MPTCPWRAP_ENV		PKGLIBDIR"/libmptcpwrap.so.0.0."LIBREVISION
+#define GODEBUG_VAR		"GODEBUG="
+#define MPTCPGO_ENV		"multipathtcp=1"
 
 /* Program documentation. */
 static char args_doc[] = "CMD";
@@ -62,7 +64,7 @@ static void help(void)
 
 static int run(int argc, char *av[])
 {
-	int i, nr = 0, ld, debug = 0;
+	int i, nr = 0, ld, go, debug = 0;
 	char **envp, **argv, *env;
 	size_t len;
 
@@ -81,18 +83,22 @@ static int run(int argc, char *av[])
 	// build environment, copying the current one ...
 	while (environ[nr])
 		nr++;
-	envp = calloc(nr + 3, sizeof(char *));
+	envp = calloc(nr + 4, sizeof(char *));
 	if (!envp)
 		error(1, errno, "can't allocate env list");
 
-	// ... filtering out any 'LD_PRELOAD' ...
+	// ... filtering out any 'LD_PRELOAD' and 'GODEBUG' ...
 	nr = 0;
 	i = 0;
 	ld = -1;
+	go = -1;
 	while (environ[nr]) {
 		if (strncmp(environ[nr], PRELOAD_VAR,
 			    strlen(PRELOAD_VAR)) == 0) {
 			ld = nr;
+		} else if (strncmp(environ[nr], GODEBUG_VAR,
+				   strlen(GODEBUG_VAR)) == 0) {
+			go = nr;
 		} else {
 			envp[i] = environ[nr];
 			i++;
@@ -110,6 +116,16 @@ static int run(int argc, char *av[])
 	}
 	envp[i++] = env;
 
+	// .. and GODEBUG=multipathtcp=1 ...
+	if (go >= 0) {
+		len = strlen(environ[go]) + strlen(MPTCPGO_ENV) + 2;
+		env = alloca(len);
+		snprintf(env, len, "%s,%s", environ[go], MPTCPGO_ENV);
+	} else {
+		env = GODEBUG_VAR MPTCPGO_ENV;
+	}
+	envp[i++] = env;
+
 	// ... and enable dbg if needed
 	if (debug)
 		envp[i++] = "MPTCPWRAP_DEBUG=1";