Use XDG_RUNTIME_DIR when available

XDG_RUNTIME_DIR is the volatile runtime data dir provided by modern
session manager such as systemd

src/main.c

@@ -260,12 +260,13 @@ int main(int argc, char *argv[]) {
         exit(0);
     }
 
-    if (config.ipc_socket_path == NULL)
+    if (config.ipc_socket_path == NULL) {
-        config.ipc_socket_path = getenv("I3SOCK");
-
         /* Fall back to a file name in /tmp/ based on the PID */
-    if (config.ipc_socket_path == NULL)
+        if ((config.ipc_socket_path = getenv("I3SOCK")) == NULL)
-        config.ipc_socket_path = get_process_filename("i3-ipc-socket");
+            config.ipc_socket_path = get_process_filename("ipc-socket");
+        else
+            config.ipc_socket_path = sstrdup(config.ipc_socket_path);
+    }
 
     uint32_t mask = XCB_CW_EVENT_MASK;
     uint32_t values[] = { XCB_EVENT_MASK_SUBSTRUCTURE_REDIRECT |
@@ -389,6 +390,7 @@ int main(int argc, char *argv[]) {
     if (ipc_socket == -1) {
         ELOG("Could not create the IPC socket, IPC disabled\n");
     } else {
+        free(config.ipc_socket_path);
         struct ev_io *ipc_io = scalloc(sizeof(struct ev_io));
         ev_io_init(ipc_io, ipc_new_client, ipc_socket, EV_READ);
         ev_io_start(loop, ipc_io);

src/util.c

@@ -244,15 +244,35 @@ static char **append_argument(char **original, char *argument) {
  *
  */
 char *get_process_filename(const char *prefix) {
+    char *dir = getenv("XDG_RUNTIME_DIR");
+    if (dir == NULL) {
         struct passwd *pw = getpwuid(getuid());
         const char *username = pw ? pw->pw_name : "unknown";
-    char *filename;
+        if (asprintf(&dir, "/tmp/i3-%s", username) == -1) {
-    int res = asprintf(&filename, "/tmp/%s-%s.%d", prefix, username, getpid());
-    if (res == -1) {
             perror("asprintf()");
             return NULL;
         }
+    } else {
+        char *tmp;
+        if (asprintf(&tmp, "%s/i3", dir) == -1) {
+            perror("asprintf()");
+            return NULL;
+        }
+        dir = tmp;
+    }
+    if (!path_exists(dir)) {
+        if (mkdir(dir, 0700) == -1) {
+            perror("mkdir()");
+            return NULL;
+        }
+    }
+    char *filename;
+    if (asprintf(&filename, "%s/%s.%d", dir, prefix, getpid()) == -1) {
+        perror("asprintf()");
+        filename = NULL;
+    }
 
+    free(dir);
     return filename;
 }
 
@@ -275,7 +295,7 @@ char *store_restart_layout() {
      * resolve the tildes in the specified path */
     char *filename;
     if (config.restart_state_path == NULL) {
-        filename = get_process_filename("i3-restart-state");
+        filename = get_process_filename("restart-state");
         if (!filename)
             return NULL;
     } else {