partial support for non-blocking ALSA drain

Because the BlueALSA server does not have a non-blocking drain
operation, we implement non-blocking drain by omitting the server
drain request. This means that a small amount of audio at the end
of a stream may be lost. Most, possibly all, applications clear the
PCM non-block flag before calling snd_pcm_drain(), so this
incomplete drain is unlikely to impact any real use cases.

src/asound/bluealsa-pcm.c

@@ -692,6 +692,10 @@ static int bluealsa_drain(snd_pcm_ioplug_t *io) {
 		return -EIO;
 	}
 
+	/* For a non-blocking drain, we do not wait for the drain to complete. */
+	if (io->nonblock == 1)
+		return -EAGAIN;
+
 	bool aborted = false;
 
 	struct pollfd pfd = { pcm->event_fd, POLLIN, 0 };
@@ -1002,7 +1006,8 @@ static int bluealsa_poll_revents(snd_pcm_ioplug_t *io, struct pollfd *pfd,
 			goto fail;
 
 		/* This call synchronizes the ring buffer pointers and updates the
-		 * ioplug state. */
+		 * ioplug state. For non-blocking drains it also causes ioplug to drop
+		 * the stream when the buffer is empty. */
 		snd_pcm_sframes_t avail = snd_pcm_avail(io->pcm);
 
 		/* ALSA expects that the event will match stream direction, e.g.
@@ -1015,8 +1020,13 @@ static int bluealsa_poll_revents(snd_pcm_ioplug_t *io, struct pollfd *pfd,
 
 		switch (io->state) {
 			case SND_PCM_STATE_SETUP:
+				/* To support non-blocking drain we must report a POLLOUT event
+				 * for playback PCMs here, because the above call to
+				 * snd_pcm_avail() may have changed the state to
+				 * SND_PCM_STATE_SETUP. */
+				if (io->stream == SND_PCM_STREAM_CAPTURE)
+					*revents = 0;
 				ready = false;
-				*revents = 0;
 				break;
 			case SND_PCM_STATE_PREPARED:
 				/* capture poll should block forever */