Do not attempt to connect to the socket as a client if systemd is active, from

Julien Moutinho in GitHub issue 3345.

client.c

@@ -284,6 +284,12 @@ client_main(struct event_base *base, int argc, char **argv, uint64_t flags,
 	log_debug("flags are %#llx", (unsigned long long)client_flags);
 
 	/* Initialize the client socket and start the server. */
+#ifdef HAVE_SYSTEMD
+	if (systemd_activated()) {
+		/* socket-based activation, do not even try to be a client. */
+		fd = server_start(client_proc, flags, base, 0, NULL);
+	} else
+#endif
 	fd = client_connect(base, socket_path, client_flags);
 	if (fd == -1) {
 		if (errno == ECONNREFUSED) {

compat.h

@@ -423,6 +423,7 @@ void		*recallocarray(void *, size_t, size_t, size_t);
 
 #ifdef HAVE_SYSTEMD
 /* systemd.c */
+int		 systemd_activated(void);
 int		 systemd_create_socket(int, char **);
 #endif
 

compat/systemd.c

@@ -25,6 +25,12 @@
 
 #include "tmux.h"
 
+int
+systemd_activated(void)
+{
+	return (sd_listen_fds(0) >= 1);
+}
+
 int
 systemd_create_socket(int flags, char **cause)
 {