output/Internal: remember the most recent error

src/output/Internal.hxx

@@ -29,6 +29,8 @@
 #include "thread/Thread.hxx"
 #include "system/PeriodClock.hxx"
 
+#include <exception>
+
 class PreparedFilter;
 class MusicPipe;
 class EventLoop;
@@ -256,6 +258,14 @@ struct AudioOutput {
 	AudioOutputSource source;
 
 	/**
+	 * The error that occurred in the output thread.  It is
+	 * cleared whenever the output is opened successfully.
+	 *
+	 * Protected by #mutex.
+	 */
+	std::exception_ptr last_error;
+
+	/**
 	 * Throws #std::runtime_error on error.
 	 */
 	AudioOutput(const AudioOutputPlugin &_plugin,

src/output/OutputThread.cxx

@@ -405,11 +405,14 @@ AudioOutput::Task()
 			break;
 
 		case Command::ENABLE:
+			last_error = nullptr;
+
 			try {
 				Enable();
 			} catch (const std::runtime_error &e) {
 				LogError(e);
 				fail_timer.Update();
+				last_error = std::current_exception();
 			}
 
 			CommandFinished();
@@ -421,11 +424,14 @@ AudioOutput::Task()
 			break;
 
 		case Command::OPEN:
+			last_error = nullptr;
+
 			try {
 				Open();
 			} catch (const std::runtime_error &e) {
 				LogError(e);
 				fail_timer.Update();
+				last_error = std::current_exception();
 			}
 
 			CommandFinished();