output/Interface: add virtual method Interrupt()

This allows interrupting the output thread (for some plugins which
implement this method).  This way, operations can be canceled
properly, instead of waiting for some external entity.

src/output/Thread.cxx

@@ -18,6 +18,7 @@
  */
 
 #include "Control.hxx"
+#include "Error.hxx"
 #include "Filtered.hxx"
 #include "Client.hxx"
 #include "Domain.hxx"
@@ -135,6 +136,7 @@ AudioOutputControl::InternalOpen(const AudioFormat in_audio_format,
 
 	last_error = nullptr;
 	fail_timer.Reset();
+	caught_interrupted = false;
 	skip_delay = true;
 
 	AudioFormat f;
@@ -243,6 +245,9 @@ AudioOutputControl::PlayChunk(std::unique_lock<Mutex> &lock) noexcept
 		const ScopeUnlock unlock(mutex);
 		try {
 			output->SendTag(*tag);
+		} catch (AudioOutputInterrupted) {
+			caught_interrupted = true;
+			return false;
 		} catch (...) {
 			FormatError(std::current_exception(),
 				    "Failed to send tag to %s",
@@ -267,6 +272,9 @@ AudioOutputControl::PlayChunk(std::unique_lock<Mutex> &lock) noexcept
 			nbytes = output->Play(data.data, data.size);
 			assert(nbytes > 0);
 			assert(nbytes <= data.size);
+		} catch (AudioOutputInterrupted) {
+			caught_interrupted = true;
+			return false;
 		} catch (...) {
 			FormatError(std::current_exception(),
 				    "Failed to play on %s", GetLogName());
@@ -342,6 +350,7 @@ AudioOutputControl::InternalPause(std::unique_lock<Mutex> &lock) noexcept
 		try {
 			const ScopeUnlock unlock(mutex);
 			success = output->IteratePause();
+		} catch (AudioOutputInterrupted) {
 		} catch (...) {
 			FormatError(std::current_exception(),
 				    "Failed to pause %s",
@@ -425,7 +434,8 @@ AudioOutputControl::Task() noexcept
 			/* no pending command: play (or wait for a
 			   command) */
 
-			if (open && allow_play && InternalPlay(lock))
+			if (open && allow_play && !caught_interrupted &&
+			    InternalPlay(lock))
 				/* don't wait for an event if there
 				   are more chunks in the pipe */
 				continue;
@@ -463,6 +473,8 @@ AudioOutputControl::Task() noexcept
 				break;
 			}
 
+			caught_interrupted = false;
+
 			InternalPause(lock);
 			break;
 
@@ -475,6 +487,8 @@ AudioOutputControl::Task() noexcept
 				break;
 			}
 
+			caught_interrupted = false;
+
 			if (always_on) {
 				/* in "always_on" mode, the output is
 				   paused instead of being closed;
@@ -499,6 +513,8 @@ AudioOutputControl::Task() noexcept
 			break;
 
 		case Command::CANCEL:
+			caught_interrupted = false;
+
 			source.Cancel();
 
 			if (open) {