Improve PCM plugin drain timeout calculation

Also adds additional debug logging to the PCM plugin drain function.

src/asound/bluealsa-pcm.c

@@ -692,29 +692,44 @@ static int bluealsa_drain(snd_pcm_ioplug_t *io) {
 	snd_pcm_sframes_t hw_ptr;
 	while ((hw_ptr = bluealsa_pointer(io)) >= 0 && io->state == SND_PCM_STATE_DRAINING) {
 
-		/* We set a timeout to ensure that the plugin cannot block forever in
-		 * case the server has stopped reading from the FIFO. There may be a
-		 * wait of up to one period before the IO thread next writes to the
-		 * FIFO, so we allow for that in setting the maximum wait time. If the
-		 * wait is re-started after being interrupted by a signal then we must
-		 * re-calculate the maximum waiting time that remains. */
 		snd_pcm_uframes_t avail = snd_pcm_ioplug_hw_avail(io, hw_ptr, io->appl_ptr);
-		int timeout = (avail + io->period_size) * 1000 / io->rate;
+
+		/* If the buffer is empty then the local drain is complete. */
+		if (avail == 0)
+			break;
+
+		/* We set a timeout to ensure that the plugin cannot block forever in
+		 * case the server has stopped reading from the FIFO. Allow enough time
+		 * to drain the available frames as full periods, plus 100 ms:
+		 * e.g. one or less periods in buffer, allow 100ms + one period,
+		 *      more than one but not more than two, allow 100ms + two periods,
+		 * etc. If the wait is re-started after being interrupted by a signal
+		 * then we must re-calculate the maximum waiting time that remains. */
+		int timeout = 100 + (((avail - 1) / io->period_size) + 1) * io->period_size * 1000 / io->rate;
 
 		int nready = poll(&pfd, 1, timeout);
 		if (nready == -1) {
-			/* It is not well documented by ALSA, but if the application
-			 * has requested that the PCM should be aborted by a signal
-			 * then the ioplug nonblock flag is set to the special value 2. */
-			if (errno == EINTR && io->nonblock != 2)
-				continue;
-			/* Application has aborted the drain. */
-			aborted = true;
+			if (errno == EINTR) {
+				/* It is not well documented by ALSA, but if the application
+				 * has requested that the PCM should be aborted by a signal
+				 * then the ioplug nonblock flag is set to the special value 2
+				 */
+				if (io->nonblock != 2)
+					continue;
+				/* Application has aborted the drain. */
+				debug2("Drain aborted by signal");
+				aborted = true;
+			}
+			else
+				debug2("Drain poll error - %s", strerror(errno));
+
 			break;
 		}
-		if (nready == 0)
+		if (nready == 0) {
 			/* Timeout - do not wait any longer. */
+			SNDERR("Drain timed out - possible Bluetooth transport failure");
 			break;
+		}
 
 		if (pfd.revents & POLLIN) {
 			eventfd_t event;