output/alsa: check PCM state before calling snd_pcm_drain()

Apparently, if snd_pcm_drain() returns EAGAIN, it does not actually
want to be called again; the next call will snd_pcm_drain() will also
return EAGAIN, forever, even though the PCM state has meanwhile
switched to SND_PCM_STATE_SETUP.  This causes a busy loop; to fix
this, we should always check snd_pcm_state() to see if draining is
really required.

NEWS

@@ -4,6 +4,7 @@ ver 0.21.11 (not yet released)
 * decoder
   - wildmidi: log error if library initialization fails
 * output
+  - alsa: fix busy loop while draining
   - alsa, osx: fix distortions with DSD_U32 and DoP on 32 bit CPUs
 * protocol
   - fix "list" with multiple "group" levels

src/output/plugins/AlsaOutputPlugin.cxx

@@ -774,6 +774,24 @@ AlsaOutput::DrainInternal()
 		   don't need to drain it */
 		return true;
 
+	switch (snd_pcm_state(pcm)) {
+	case SND_PCM_STATE_PREPARED:
+	case SND_PCM_STATE_RUNNING:
+		/* these states require a call to snd_pcm_drain() */
+		break;
+
+	case SND_PCM_STATE_DRAINING:
+		/* already draining, but not yet finished; this is
+		   probably a spurious epoll event, and we should wait
+		   for the next one */
+		return false;
+
+	default:
+		/* all other states cannot be drained, and we're
+		   done */
+		return true;
+	}
+
 	/* .. and finally drain the ALSA hardware buffer */
 
 	int result;