fs/StandardDirectory: add GetAppRuntimeDir()

src/Listen.cxx

@@ -86,13 +86,10 @@ ListenXdgRuntimeDir(ClientListener &listener) noexcept
 		   use $XDG_RUNTIME_DIR */
 		return false;
 
-	const auto xdg_runtime_dir = GetUserRuntimeDir();
-	if (xdg_runtime_dir.IsNull())
+	const auto mpd_runtime_dir = GetAppRuntimeDir();
+	if (mpd_runtime_dir.IsNull())
 		return false;
 
-	const auto mpd_runtime_dir = xdg_runtime_dir / Path::FromFS("mpd");
-	mkdir(mpd_runtime_dir.c_str(), 0700);
-
 	const auto socket_path = mpd_runtime_dir / Path::FromFS("socket");
 	unlink(socket_path.c_str());
 

src/fs/StandardDirectory.cxx

@@ -292,6 +292,20 @@ GetUserRuntimeDir() noexcept
 #endif
 }
 
+AllocatedPath
+GetAppRuntimeDir() noexcept
+{
+#ifdef USE_XDG
+	if (const auto user_dir = GetUserRuntimeDir(); !user_dir.IsNull()) {
+		auto dir = user_dir / Path::FromFS("mpd");
+		mkdir(dir.c_str(), 0700);
+		return dir;
+	}
+#endif
+
+	return nullptr;
+}
+
 #ifdef _WIN32
 
 AllocatedPath

src/fs/StandardDirectory.hxx

@@ -50,6 +50,13 @@ GetUserCacheDir() noexcept;
 AllocatedPath
 GetUserRuntimeDir() noexcept;
 
+/**
+ * Obtains the runtime directory for this application.
+ */
+[[gnu::const]]
+AllocatedPath
+GetAppRuntimeDir() noexcept;
+
 #ifdef _WIN32
 
 /**