output/alsa: add a new flag "waiting" for xrun management

In DispatchSockets(), when there was not enough data, but enough for
current playback, the method would disable the "active" flag so the
next Play() call would re-enable the MultiSocketMonitor.

This was an abuse of the flag which could result in a crash
in Cancel(), because that method asserts that the period_buffer is
empty, which it may be not.

The solution is to add anther flag called "waiting" which shares some
behavior with the old flag.

src/output/plugins/AlsaOutputPlugin.cxx

@@ -135,6 +135,13 @@ class AlsaOutput final
 	 */
 	bool active;
 
+	/**
+	 * Is this output waiting for more data?
+	 *
+	 * Protected by #mutex.
+	 */
+	bool waiting;
+
 	/**
 	 * Do we need to call snd_pcm_prepare() before the next write?
 	 * It means that we put the device to SND_PCM_STATE_SETUP by
@@ -176,7 +183,7 @@ class AlsaOutput final
 	Alsa::PeriodBuffer period_buffer;
 
 	/**
-	 * Protects #cond, #error, #active, #drain.
+	 * Protects #cond, #error, #active, #waiting, #drain.
 	 */
 	mutable Mutex mutex;
 
@@ -248,6 +255,12 @@ private:
 		return active;
 	}
 
+	gcc_pure
+	bool LockIsActiveAndNotWaiting() const noexcept {
+		const std::lock_guard<Mutex> lock(mutex);
+		return active && !waiting;
+	}
+
 	/**
 	 * Activate the output by registering the sockets in the
 	 * #EventLoop.  Before calling this, filling the ring buffer
@@ -260,10 +273,11 @@ private:
 	 * was never unlocked
 	 */
 	bool Activate() noexcept {
-		if (active)
+		if (active && !waiting)
 			return false;
 
 		active = true;
+		waiting = false;
 
 		const ScopeUnlock unlock(mutex);
 		defer_invalidate_sockets.Schedule();
@@ -330,6 +344,7 @@ private:
 		const std::lock_guard<Mutex> lock(mutex);
 		error = std::current_exception();
 		active = false;
+		waiting = false;
 		cond.signal();
 	}
 
@@ -684,6 +699,7 @@ AlsaOutput::Open(AudioFormat &audio_format)
 	period_buffer.Allocate(period_frames, out_frame_size);
 
 	active = false;
+	waiting = false;
 	must_prepare = false;
 	written = false;
 	error = {};
@@ -845,6 +861,7 @@ AlsaOutput::CancelInternal() noexcept
 	ring_buffer->reset();
 
 	active = false;
+	waiting = false;
 
 	MultiSocketMonitor::Reset();
 	defer_invalidate_sockets.Cancel();
@@ -930,7 +947,7 @@ AlsaOutput::Play(const void *chunk, size_t size)
 std::chrono::steady_clock::duration
 AlsaOutput::PrepareSockets() noexcept
 {
-	if (!LockIsActive()) {
+	if (!LockIsActiveAndNotWaiting()) {
 		ClearSocketList();
 		return std::chrono::steady_clock::duration(-1);
 	}
@@ -1001,13 +1018,13 @@ try {
 
 			{
 				const std::lock_guard<Mutex> lock(mutex);
-				active = false;
+				waiting = true;
 				cond.signal();
 			}
 
 			/* avoid race condition: see if data has
 			   arrived meanwhile before disabling the
-			   event (but after clearing the "active"
+			   event (but after setting the "waiting"
 			   flag) */
 			if (!CopyRingToPeriodBuffer()) {
 				MultiSocketMonitor::Reset();