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.

output/alsa: check PCM state before calling snd_pcm_drain()
543776d9 · Max Kellermann · 8bf3f9b8 · 543776d9 · 543776d9
Commit 543776d9 authored Jun 28, 2019 by Max Kellermann
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NEWS NEWS +1 -0

AlsaOutputPlugin.cxx src/output/plugins/AlsaOutputPlugin.cxx +18 -0

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--- a/NEWS
+++ b/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

--- a/src/output/plugins/AlsaOutputPlugin.cxx
+++ b/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;