output_thread: reimplement CANCEL synchronization

The output thread could hang indefinitely after finishing CANCEL,
because it could have missed the signal while the output was not
unlocked in ao_command_finished().

This patch removes the wait() call after CANCEL, and adds the flag
"allow_play" instead.  While this flag is set, playback is skipped.
With this flag, there will not be any excess wait() call after the
pipe has been cleared.

This patch fixes a bug that causes mpd to discontinue playback after
seeking, due to the race condition described above.

1 files changed, 13 insertions, 5 deletions

diff --git a/src/output_control.c b/src/output_control.c
index 0823b667b..14976dbfb 100644
--- a/src/output_control.c
+++ b/src/output_control.c
@@ -115,6 +115,8 @@ audio_output_open(struct audio_output *ao,
 {
 	bool open;
 
+	assert(ao != NULL);
+	assert(ao->allow_play);
 	assert(audio_format_valid(audio_format));
 	assert(mp != NULL);
 
@@ -140,10 +142,6 @@ audio_output_open(struct audio_output *ao,
 			/* we're not using audio_output_cancel() here,
 			   because that function is asynchronous */
 			ao_command(ao, AO_COMMAND_CANCEL);
-
-			/* the audio output is now waiting for a
-			   signal; wake it up immediately */
-			g_cond_signal(ao->cond);
 		}
 
 		return true;
@@ -181,6 +179,7 @@ static void
 audio_output_close_locked(struct audio_output *ao)
 {
 	assert(ao != NULL);
+	assert(ao->allow_play);
 
 	if (ao->mixer != NULL)
 		mixer_auto_close(ao->mixer);
@@ -223,6 +222,8 @@ audio_output_play(struct audio_output *ao)
 {
 	g_mutex_lock(ao->mutex);
 
+	assert(ao->allow_play);
+
 	if (audio_output_is_open(ao))
 		g_cond_signal(ao->cond);
 
@@ -238,6 +239,7 @@ void audio_output_pause(struct audio_output *ao)
 		mixer_auto_close(ao->mixer);
 
 	g_mutex_lock(ao->mutex);
+	assert(ao->allow_play);
 	if (audio_output_is_open(ao))
 		ao_command_async(ao, AO_COMMAND_PAUSE);
 	g_mutex_unlock(ao->mutex);
@@ -247,6 +249,7 @@ void
 audio_output_drain_async(struct audio_output *ao)
 {
 	g_mutex_lock(ao->mutex);
+	assert(ao->allow_play);
 	if (audio_output_is_open(ao))
 		ao_command_async(ao, AO_COMMAND_DRAIN);
 	g_mutex_unlock(ao->mutex);
@@ -255,8 +258,12 @@ audio_output_drain_async(struct audio_output *ao)
 void audio_output_cancel(struct audio_output *ao)
 {
 	g_mutex_lock(ao->mutex);
-	if (audio_output_is_open(ao))
+
+	if (audio_output_is_open(ao)) {
+		ao->allow_play = false;
 		ao_command_async(ao, AO_COMMAND_CANCEL);
+	}
+
 	g_mutex_unlock(ao->mutex);
 }
 
@@ -287,6 +294,7 @@ void audio_output_finish(struct audio_output *ao)
 
 	if (ao->thread != NULL) {
 		g_mutex_lock(ao->mutex);
+		assert(ao->allow_play);
 		ao_command(ao, AO_COMMAND_KILL);
 		g_mutex_unlock(ao->mutex);
 		g_thread_join(ao->thread);