output/Control: Support always_off outputs that are never used for playback

Add an `always_off` option to outputs that causes them to never start
playback even if they're enabled.

This allows placeholder `null` outputs to be defined for the purpose
of having an external client react to the enabled state without the
side effects of real outputs. Like an external mixer, the client can
perform some action when an output is enabled.

Normally `null` outputs can be used for playback so it's possible for
MPD to continue playback silently if a problem occurs with all the real
outputs (or there are none enabled).

src/output/Control.cxx

@@ -25,6 +25,7 @@ AudioOutputControl::AudioOutputControl(std::unique_ptr<FilteredAudioOutput> _out
 	 thread(BIND_THIS_METHOD(Task)),
 	 tags(block.GetBlockValue("tags", true)),
 	 always_on(block.GetBlockValue("always_on", false)),
+	 always_off(block.GetBlockValue("always_off", false)),
 	 enabled(block.GetBlockValue("enabled", true))
 {
 }
@@ -36,7 +37,8 @@ AudioOutputControl::AudioOutputControl(AudioOutputControl &&src,
 	 client(_client),
 	 thread(BIND_THIS_METHOD(Task)),
 	 tags(src.tags),
-	 always_on(src.always_on)
+	 always_on(src.always_on),
+	 always_off(src.always_off)
 {
 }
 
@@ -176,6 +178,9 @@ AudioOutputControl::EnableAsync()
 	if (!output)
 		return;
 
+	if (always_off)
+		return;
+
 	if (!thread.IsDefined()) {
 		if (!output->SupportsEnableDisable()) {
 			/* don't bother to start the thread now if the

src/output/Control.hxx

@@ -143,6 +143,11 @@ class AudioOutputControl {
 	 */
 	const bool always_on;
 
+	/**
+	 * Should this output never play anything, even when enabled?
+	 */
+	const bool always_off;
+
 	/**
 	 * Has the user enabled this device?
 	 */
@@ -274,6 +279,10 @@ public:
 		return !output;
 	}
 
+	bool AlwaysOff() const noexcept {
+		return always_off;
+	}
+
 	/**
 	 * Caller must lock the mutex.
 	 */

src/output/MultipleOutputs.cxx

@@ -215,6 +215,10 @@ MultipleOutputs::Open(const AudioFormat audio_format)
 	for (const auto &ao : outputs) {
 		const std::scoped_lock<Mutex> lock(ao->mutex);
 
+		/* can't play on this device even if it's enabled */
+		if (ao->AlwaysOff())
+			continue;
+
 		if (ao->IsEnabled())
 			enabled = true;