appsink: don't signal the GCond if no one is waiting on it

Performance optimisation: Keep track whenever the streaming
thread or the application thread are waiting on the GCond
for more space or new data, and only signal on the GCond if
someone is actually waiting. Avoids unnecessary syscalls and
thus context switches.

gst-libs/gst/app/gstappsink.c

@@ -73,6 +73,13 @@
 
 #include "gstappsink.h"
 
+typedef enum
+{
+  NOONE_WAITING,
+  STREAM_WAITING,               /* streaming thread is waiting for application thread */
+  APP_WAITING,                  /* application thread is waiting for streaming thread */
+} GstAppSinkWaitStatus;
+
 struct _GstAppSinkPrivate
 {
   GstCaps *caps;
@@ -81,6 +88,7 @@ struct _GstAppSinkPrivate
   guint max_buffers;
   gboolean drop;
   gboolean wait_on_eos;
+  GstAppSinkWaitStatus wait_status;
 
   GCond cond;
   GMutex mutex;
@@ -459,6 +467,7 @@ gst_app_sink_init (GstAppSink * appsink)
   priv->drop = DEFAULT_PROP_DROP;
   priv->wait_on_eos = DEFAULT_PROP_WAIT_ON_EOS;
   priv->buffer_lists_supported = DEFAULT_PROP_BUFFER_LIST;
+  priv->wait_status = NOONE_WAITING;
 }
 
 static void
@@ -632,6 +641,7 @@ gst_app_sink_start (GstBaseSink * psink)
 
   g_mutex_lock (&priv->mutex);
   GST_DEBUG_OBJECT (appsink, "starting");
+  priv->wait_status = NOONE_WAITING;
   priv->flushing = FALSE;
   priv->started = TRUE;
   gst_segment_init (&priv->preroll_segment, GST_FORMAT_TIME);
@@ -651,6 +661,7 @@ gst_app_sink_stop (GstBaseSink * psink)
   GST_DEBUG_OBJECT (appsink, "stopping");
   priv->flushing = TRUE;
   priv->started = FALSE;
+  priv->wait_status = NOONE_WAITING;
   gst_app_sink_flush_unlocked (appsink);
   gst_buffer_replace (&priv->preroll_buffer, NULL);
   gst_caps_replace (&priv->preroll_caps, NULL);
@@ -707,8 +718,11 @@ gst_app_sink_event (GstBaseSink * sink, GstEvent * event)
        * Otherwise we might signal EOS before all buffers are
        * consumed, which is a bit confusing for the application
        */
-      while (priv->num_buffers > 0 && !priv->flushing && priv->wait_on_eos)
+      while (priv->num_buffers > 0 && !priv->flushing && priv->wait_on_eos) {
+        priv->wait_status = STREAM_WAITING;
         g_cond_wait (&priv->cond, &priv->mutex);
+        priv->wait_status = NOONE_WAITING;
+      }
       if (priv->flushing)
         emit = FALSE;
       g_mutex_unlock (&priv->mutex);
@@ -755,7 +769,9 @@ gst_app_sink_preroll (GstBaseSink * psink, GstBuffer * buffer)
   GST_DEBUG_OBJECT (appsink, "setting preroll buffer %p", buffer);
   gst_buffer_replace (&priv->preroll_buffer, buffer);
 
-  g_cond_signal (&priv->cond);
+  if (priv->wait_status == APP_WAITING)
+    g_cond_signal (&priv->cond);
+
   emit = priv->emit_signals;
   g_mutex_unlock (&priv->mutex);
 
@@ -869,7 +885,10 @@ restart:
       }
 
       /* wait for a buffer to be removed or flush */
+      priv->wait_status = STREAM_WAITING;
       g_cond_wait (&priv->cond, &priv->mutex);
+      priv->wait_status = NOONE_WAITING;
+
       if (priv->flushing)
         goto flushing;
     }
@@ -877,7 +896,10 @@ restart:
   /* we need to ref the buffer/list when pushing it in the queue */
   gst_queue_array_push_tail (priv->queue, gst_mini_object_ref (data));
   priv->num_buffers++;
-  g_cond_signal (&priv->cond);
+
+  if (priv->wait_status == APP_WAITING)
+    g_cond_signal (&priv->cond);
+
   emit = priv->emit_signals;
   g_mutex_unlock (&priv->mutex);
 
@@ -972,8 +994,11 @@ gst_app_sink_query (GstBaseSink * bsink, GstQuery * query)
     {
       g_mutex_lock (&priv->mutex);
       GST_DEBUG_OBJECT (appsink, "waiting buffers to be consumed");
-      while (priv->num_buffers > 0 || priv->preroll_buffer)
+      while (priv->num_buffers > 0 || priv->preroll_buffer) {
+        priv->wait_status = STREAM_WAITING;
         g_cond_wait (&priv->cond, &priv->mutex);
+        priv->wait_status = NOONE_WAITING;
+      }
       g_mutex_unlock (&priv->mutex);
       ret = GST_BASE_SINK_CLASS (parent_class)->query (bsink, query);
       break;
@@ -1486,12 +1511,14 @@ gst_app_sink_try_pull_preroll (GstAppSink * appsink, GstClockTime timeout)
 
     /* nothing to return, wait */
     GST_DEBUG_OBJECT (appsink, "waiting for the preroll buffer");
+    priv->wait_status = APP_WAITING;
     if (timeout_valid) {
       if (!g_cond_wait_until (&priv->cond, &priv->mutex, end_time))
         goto expired;
     } else {
       g_cond_wait (&priv->cond, &priv->mutex);
     }
+    priv->wait_status = NOONE_WAITING;
   }
   sample =
       gst_sample_new (priv->preroll_buffer, priv->preroll_caps,
@@ -1506,6 +1533,7 @@ gst_app_sink_try_pull_preroll (GstAppSink * appsink, GstClockTime timeout)
 expired:
   {
     GST_DEBUG_OBJECT (appsink, "timeout expired, return NULL");
+    priv->wait_status = NOONE_WAITING;
     g_mutex_unlock (&priv->mutex);
     return NULL;
   }
@@ -1581,12 +1609,14 @@ gst_app_sink_try_pull_sample (GstAppSink * appsink, GstClockTime timeout)
 
     /* nothing to return, wait */
     GST_DEBUG_OBJECT (appsink, "waiting for a buffer");
+    priv->wait_status = APP_WAITING;
     if (timeout_valid) {
       if (!g_cond_wait_until (&priv->cond, &priv->mutex, end_time))
         goto expired;
     } else {
       g_cond_wait (&priv->cond, &priv->mutex);
     }
+    priv->wait_status = NOONE_WAITING;
   }
 
   obj = dequeue_buffer (appsink);
@@ -1601,7 +1631,9 @@ gst_app_sink_try_pull_sample (GstAppSink * appsink, GstClockTime timeout)
   }
   gst_mini_object_unref (obj);
 
-  g_cond_signal (&priv->cond);
+  if (priv->wait_status == STREAM_WAITING)
+    g_cond_signal (&priv->cond);
+
   g_mutex_unlock (&priv->mutex);
 
   return sample;
@@ -1610,6 +1642,7 @@ gst_app_sink_try_pull_sample (GstAppSink * appsink, GstClockTime timeout)
 expired:
   {
     GST_DEBUG_OBJECT (appsink, "timeout expired, return NULL");
+    priv->wait_status = NOONE_WAITING;
     g_mutex_unlock (&priv->mutex);
     return NULL;
   }