Make restart IPC command send a reply once restart completed (!) (#3743)

This is achieved by retaining the IPC connection which is sending the restart
command across the restart.

This is the cleaner fix for https://github.com/i3/go-i3/issues/3

fixes #3565

src/main.c

@@ -166,7 +166,7 @@ static void i3_exit(void) {
         fflush(stderr);
         shm_unlink(shmlogname);
     }
-    ipc_shutdown(SHUTDOWN_REASON_EXIT);
+    ipc_shutdown(SHUTDOWN_REASON_EXIT, -1);
     unlink(config.ipc_socket_path);
     xcb_disconnect(conn);
 
@@ -236,6 +236,20 @@ static void setup_term_handlers(void) {
     }
 }
 
+static int parse_restart_fd(void) {
+    const char *restart_fd = getenv("_I3_RESTART_FD");
+    if (restart_fd == NULL) {
+        return -1;
+    }
+
+    long int fd = -1;
+    if (!parse_long(restart_fd, &fd, 10)) {
+        ELOG("Malformed _I3_RESTART_FD \"%s\"\n", restart_fd);
+        return -1;
+    }
+    return fd;
+}
+
 int main(int argc, char *argv[]) {
     /* Keep a symbol pointing to the I3_VERSION string constant so that we have
      * it in gdb backtraces. */
@@ -847,6 +861,15 @@ int main(int argc, char *argv[]) {
         }
     }
 
+    {
+        const int restart_fd = parse_restart_fd();
+        if (restart_fd != -1) {
+            DLOG("serving restart fd %d", restart_fd);
+            ipc_client *client = ipc_new_client_on_fd(main_loop, restart_fd);
+            ipc_confirm_restart(client);
+        }
+    }
+
     /* Set up i3 specific atoms like I3_SOCKET_PATH and I3_CONFIG_PATH */
     x_set_i3_atoms();
     ewmh_update_workarea();