[alsa-devel] [PATCH - IOPLUG DRAIN 1/1] ioplug: drain: Wait with pollwhen EAGAIN in blocking mode

[twischer at de.adit-jv.com]

From: Timo Wischer <twischer at de.adit-jv.com>

With this changes an IO plugin should not block by it self in the
its own snd_pcm_drain() implementation. It should return -EAGAIN and
inform the ALSA library via the poll_descriptors that draining is
done.

In non-blocking mode this implementation expects that the user blocks on
the poll_descriptors whenever snd_pcm_drain() returns -EAGAIN. In
addition the user has to call snd_pcm_drain() again to check if draining
is really done.

This change will not harm existing IO plugins which are blocking in its
snd_pcm_drain() callback.

Signed-off-by: Timo Wischer <twischer at de.adit-jv.com>

diff --git a/include/pcm_ioplug.h b/include/pcm_ioplug.h
index c1310e3..aa08962 100644
--- a/include/pcm_ioplug.h
+++ b/include/pcm_ioplug.h
@@ -169,6 +169,12 @@ struct snd_pcm_ioplug_callback {
 	int (*prepare)(snd_pcm_ioplug_t *io);
 	/**
 	 * drain; optional
+	 * This function should never block. It should return -EAGAIN in case of
+	 * it is not yet done. In this case it will be called again by the ALSA
+	 * library (in blocking mode) or by the user (in non blocking mode).
+	 * In case of -EAGAIN the poll_descriptors have to be used to inform the
+	 * ALSA library that draining is possibly done and this callback has to
+	 * be called again.
 	 */
 	int (*drain)(snd_pcm_ioplug_t *io);
 	/**
diff --git a/src/pcm/pcm_ioplug.c b/src/pcm/pcm_ioplug.c
index 8c0ed48..7682dcd 100644
--- a/src/pcm/pcm_ioplug.c
+++ b/src/pcm/pcm_ioplug.c
@@ -492,17 +492,37 @@ static int snd_pcm_ioplug_drop(snd_pcm_t *pcm)
 static int snd_pcm_ioplug_drain(snd_pcm_t *pcm)
 {
 	ioplug_priv_t *io = pcm->private_data;
-	int err;
+	int err = 0;
 
 	if (io->data->state == SND_PCM_STATE_OPEN)
 		return -EBADFD;
 
 	io->data->state = SND_PCM_STATE_DRAINING;
-	if (io->data->callback->drain)
-		io->data->callback->drain(io->data);
-	snd_pcm_lock(pcm);
-	err = snd_pcm_ioplug_drop(pcm);
-	snd_pcm_unlock(pcm);
+	if (io->data->callback->drain) {
+		err = io->data->callback->drain(io->data);
+
+		/* in blocking mode wait unti draining is done or
+		 * an issue was detected.
+		 */
+		if (!io->data->nonblock) {
+			while (err == -EAGAIN) {
+				snd_pcm_lock(pcm);
+				err = snd_pcm_wait_nocheck(pcm, -1);
+				snd_pcm_unlock(pcm);
+				if (err  < 0)
+					break;
+				err = io->data->callback->drain(io->data);
+			}
+		}
+	}
+
+	/* only drop if draining is done or there was an issue */
+	if (err != -EAGAIN) {
+		snd_pcm_lock(pcm);
+		err = snd_pcm_ioplug_drop(pcm);
+		snd_pcm_unlock(pcm);
+	}
+
 	return err;
 }
 
-- 
2.7.4