output/Control: add LockEnableDisableAsync()

src/output/Control.hxx

@@ -360,6 +360,12 @@ public:
 	 * Caller must lock the mutex.
 	 */
 	void EnableDisableAsync();
+
+	void LockEnableDisableAsync() {
+		const std::lock_guard<Mutex> protect(mutex);
+		EnableDisableAsync();
+	}
+
 	void LockPauseAsync() noexcept;
 
 	void CloseWait() noexcept;

src/output/MultipleOutputs.cxx

@@ -147,10 +147,8 @@ MultipleOutputs::EnableDisable()
 {
 	/* parallel execution */
 
-	for (const auto &ao : outputs) {
-		const std::lock_guard<Mutex> lock(ao->mutex);
-		ao->EnableDisableAsync();
-	}
+	for (const auto &ao : outputs)
+		ao->LockEnableDisableAsync();
 
 	WaitAll();
 }