output: play from a music_pipe object

Instead of passing individual buffers to audio_output_all_play(), pass
music_chunk objects.  Append all those chunks asynchronously to a
music_pipe instance.  All output threads may then read chunks from
this pipe.  This reduces MPD's internal latency by an order of
magnitude.

1 files changed, 182 insertions, 42 deletions

diff --git a/src/output_all.c b/src/output_all.c
index 7b308a428..c1353fa71 100644
--- a/src/output_all.c
+++ b/src/output_all.c
@@ -20,6 +20,12 @@
 #include "output_internal.h"
 #include "output_control.h"
 #include "conf.h"
+#include "pipe.h"
+#include "buffer.h"
+
+#ifndef NDEBUG
+#include "chunk.h"
+#endif
 
 #include <assert.h>
 #include <string.h>
@@ -32,6 +38,17 @@ static struct audio_format input_audio_format;
 static struct audio_output *audio_outputs;
 static unsigned int num_audio_outputs;
 
+/**
+ * The #music_buffer object where consumed chunks are returned.
+ */
+static struct music_buffer *g_music_buffer;
+
+/**
+ * The #music_pipe object which feeds all audio outputs.  It is filled
+ * by audio_output_all_play().
+ */
+static struct music_pipe *g_mp;
+
 unsigned int audio_output_count(void)
 {
 	return num_audio_outputs;
@@ -179,60 +196,63 @@ audio_output_all_update(void)
 
 	for (i = 0; i < num_audio_outputs; ++i)
 		ret = audio_output_update(&audio_outputs[i],
-					  &input_audio_format) || ret;
+					  &input_audio_format, g_mp) || ret;
 
 	return ret;
 }
 
 bool
-audio_output_all_play(const char *buffer, size_t length)
+audio_output_all_play(struct music_chunk *chunk)
 {
-	bool ret = false;
+	bool ret;
 	unsigned int i;
 
-	assert(length > 0);
-	/* no partial frames allowed */
-	assert((length % audio_format_frame_size(&input_audio_format)) == 0);
+	assert(g_music_buffer != NULL);
+	assert(g_mp != NULL);
+	assert(chunk != NULL);
+	assert(music_chunk_check_format(chunk, &input_audio_format));
 
-	audio_output_all_update();
+	ret = audio_output_all_update();
+	if (!ret)
+		return false;
+
+	music_pipe_push(g_mp, chunk);
 
 	for (i = 0; i < num_audio_outputs; ++i)
 		if (audio_output_is_open(&audio_outputs[i]))
-			audio_output_play(&audio_outputs[i],
-					  buffer, length);
-
-	while (true) {
-		bool finished = true;
-
-		for (i = 0; i < num_audio_outputs; ++i) {
-			struct audio_output *ao = &audio_outputs[i];
-
-			if (!audio_output_is_open(ao))
-				continue;
-
-			if (audio_output_command_is_finished(ao))
-				ret = true;
-			else {
-				finished = false;
-				audio_output_signal(ao);
-			}
-		}
-
-		if (finished)
-			break;
-
-		notify_wait(&audio_output_client_notify);
-	};
+			audio_output_play(&audio_outputs[i]);
 
 	return ret;
 }
 
 bool
-audio_output_all_open(const struct audio_format *audio_format)
+audio_output_all_open(const struct audio_format *audio_format,
+		      struct music_buffer *buffer)
 {
 	bool ret = false, enabled = false;
 	unsigned int i;
 
+	assert(buffer != NULL);
+	assert(g_music_buffer == NULL || g_music_buffer == buffer);
+	assert((g_mp == NULL) == (g_music_buffer == NULL));
+
+	g_music_buffer = buffer;
+
+	/* the audio format must be the same as existing chunks in the
+	   pipe */
+	assert(audio_format == NULL || g_mp == NULL ||
+	       music_pipe_check_format(g_mp, audio_format));
+
+	if (g_mp == NULL)
+		g_mp = music_pipe_new();
+	else
+		/* if the pipe hasn't been cleared, the the audio
+		   format must not have changed */
+		assert(music_pipe_size(g_mp) == 0 ||
+		       audio_format == NULL ||
+		       audio_format_equals(audio_format,
+					   &input_audio_format));
+
 	if (audio_format != NULL)
 		input_audio_format = *audio_format;
 
@@ -257,6 +277,121 @@ audio_output_all_open(const struct audio_format *audio_format)
 	return ret;
 }
 
+/**
+ * Has the specified audio output already consumed this chunk?
+ */
+static bool
+chunk_is_consumed_in(const struct audio_output *ao,
+		     const struct music_chunk *chunk)
+{
+	if (ao->chunk == NULL)
+		return false;
+
+	assert(chunk == ao->chunk || music_pipe_contains(g_mp, ao->chunk));
+
+	if (chunk != ao->chunk) {
+		assert(chunk->next != NULL);
+		return true;
+	}
+
+	return ao->chunk_finished && chunk->next == NULL;
+}
+
+/**
+ * Has this chunk been consumed by all audio outputs?
+ */
+static bool
+chunk_is_consumed(const struct music_chunk *chunk)
+{
+	for (unsigned i = 0; i < num_audio_outputs; ++i) {
+		const struct audio_output *ao = &audio_outputs[i];
+		bool consumed;
+
+		if (!ao->open)
+			continue;
+
+		g_mutex_lock(ao->mutex);
+		consumed = chunk_is_consumed_in(ao, chunk);
+		g_mutex_unlock(ao->mutex);
+
+		if (!consumed)
+			return false;
+	}
+
+	return true;
+}
+
+/**
+ * There's only one chunk left in the pipe (#g_mp), and all audio
+ * outputs have consumed it already.  Clear the reference.
+ */
+static void
+clear_tail_chunk(const struct music_chunk *chunk, bool *locked)
+{
+	assert(chunk->next == NULL);
+	assert(music_pipe_contains(g_mp, chunk));
+
+	for (unsigned i = 0; i < num_audio_outputs; ++i) {
+		struct audio_output *ao = &audio_outputs[i];
+
+		locked[i] = ao->open;
+
+		if (!locked[i])
+			continue;
+
+		/* this mutex will be unlocked by the caller when it's
+		   ready */
+		g_mutex_lock(ao->mutex);
+
+		assert(ao->chunk == chunk);
+		assert(ao->chunk_finished);
+		ao->chunk = NULL;
+	}
+}
+
+unsigned
+audio_output_all_check(void)
+{
+	const struct music_chunk *chunk;
+	bool is_tail;
+	struct music_chunk *shifted;
+	bool locked[num_audio_outputs];
+
+	assert(g_music_buffer != NULL);
+	assert(g_mp != NULL);
+
+	while ((chunk = music_pipe_peek(g_mp)) != NULL) {
+		assert(music_pipe_size(g_mp) > 0);
+
+		if (!chunk_is_consumed(chunk))
+			/* at least one output is not finished playing
+			   this chunk */
+			return music_pipe_size(g_mp) - 1;
+
+		is_tail = chunk->next == NULL;
+		if (is_tail)
+			/* this is the tail of the pipe - clear the
+			   chunk reference in all outputs */
+			clear_tail_chunk(chunk, locked);
+
+		/* remove the chunk from the pipe */
+		shifted = music_pipe_shift(g_mp);
+		assert(shifted == chunk);
+
+		if (is_tail)
+			/* unlock all audio outputs which were locked
+			   by clear_tail_chunk() */
+			for (unsigned i = 0; i < num_audio_outputs; ++i)
+				if (locked[i])
+					g_mutex_unlock(audio_outputs[i].mutex);
+
+		/* return the chunk to the buffer */
+		music_buffer_return(g_music_buffer, shifted);
+	}
+
+	return 0;
+}
+
 void
 audio_output_all_pause(void)
 {
@@ -278,12 +413,19 @@ audio_output_all_cancel(void)
 
 	audio_output_all_update();
 
+	/* send the cancel() command to all audio outputs */
+
 	for (i = 0; i < num_audio_outputs; ++i) {
 		if (audio_output_is_open(&audio_outputs[i]))
 			audio_output_cancel(&audio_outputs[i]);
 	}
 
 	audio_output_wait_all();
+
+	/* clear the music pipe and return all chunks to the buffer */
+
+	if (g_mp != NULL)
+		music_pipe_clear(g_mp, g_music_buffer);
 }
 
 void
@@ -293,16 +435,14 @@ audio_output_all_close(void)
 
 	for (i = 0; i < num_audio_outputs; ++i)
 		audio_output_close(&audio_outputs[i]);
-}
 
-void
-audio_output_all_tag(const struct tag *tag)
-{
-	unsigned int i;
+	if (g_mp != NULL) {
+		assert(g_music_buffer != NULL);
 
-	for (i = 0; i < num_audio_outputs; ++i)
-		if (audio_output_is_open(&audio_outputs[i]))
-			audio_output_send_tag(&audio_outputs[i], tag);
+		music_pipe_clear(g_mp, g_music_buffer);
+		music_pipe_free(g_mp);
+		g_mp = NULL;
+	}
 
-	audio_output_wait_all();
+	g_music_buffer = NULL;
 }