output/Internal: add method IsBusy()

src/output/Internal.hxx

@@ -316,6 +316,13 @@ public:
 	/**
 	 * Caller must lock the mutex.
 	 */
+	bool IsBusy() const {
+		return IsOpen() && !IsCommandFinished();
+	}
+
+	/**
+	 * Caller must lock the mutex.
+	 */
 	const std::exception_ptr &GetLastError() const {
 		return last_error;
 	}

src/output/MultipleOutputs.cxx

@@ -138,7 +138,7 @@ MultipleOutputs::AllFinished() const
 {
 	for (auto ao : outputs) {
 		const std::lock_guard<Mutex> protect(ao->mutex);
-		if (ao->IsOpen() && !ao->IsCommandFinished())
+		if (ao->IsBusy())
 			return false;
 	}