nlecomposition: Optimize prerolling when using nested compositions

When a composition is nested into anotherone, we *know* that the
toplevel composition is going to send a stack initializing seek,
we can thus avoid sending it on the subcomposition itself when
prerolling. This avoid seeking roundtrips and we now have one and
only one seek travelling in the overall pipeline (after it has
prerolled).

plugins/nle/nlecomposition.c

@@ -1642,7 +1642,29 @@ get_new_seek_event (NleComposition * comp, gboolean initial,
       priv->segment->format, flags, starttype, start, GST_SEEK_TYPE_SET, stop);
 }
 
-/* OBJECTS LOCK must be taken when calling this ! */
+static gboolean
+nle_composition_needs_topelevel_initializing_seek (NleComposition * comp)
+{
+  GstObject *parent;
+
+  parent = gst_object_get_parent (GST_OBJECT (comp));
+  while (parent) {
+    if (NLE_IS_COMPOSITION (parent)
+        && NLE_COMPOSITION (parent)->priv->stack_initialization_seek) {
+      gst_object_unref (parent);
+      GST_INFO_OBJECT (comp,
+          "Not sending an initializing seek as %" GST_PTR_FORMAT
+          "is gonna seek anyway!", parent);
+      return FALSE;
+    }
+
+    gst_object_unref (parent);
+    parent = gst_object_get_parent (parent);
+  }
+
+  return TRUE;
+}
+
 static GstClockTime
 get_current_position (NleComposition * comp)
 {
@@ -3099,7 +3121,15 @@ _activate_new_stack (NleComposition * comp, GstEvent * toplevel_seek)
   /* The stack is entirely ready, stack initializing seek once ready */
   GST_INFO_OBJECT (comp, "Activating stack with seek: %" GST_PTR_FORMAT,
       toplevel_seek);
-  comp->priv->stack_initialization_seek = toplevel_seek;
+
+  if (!toplevel_seek) {
+    GST_INFO_OBJECT (comp,
+        "This is a sub composition, not seeking to initialize stack");
+    g_atomic_int_set (&priv->send_stream_start, TRUE);
+  } else {
+    GST_INFO_OBJECT (comp, "Needs seeking to initialize stack");
+    comp->priv->stack_initialization_seek = toplevel_seek;
+  }
 
   topelement = GST_ELEMENT (priv->current->data);
   /* Get toplevel object source pad */
@@ -3113,7 +3143,8 @@ _activate_new_stack (NleComposition * comp, GstEvent * toplevel_seek)
   GST_DEBUG_OBJECT (comp, "New stack activated!");
 
 resync_state:
-  g_atomic_int_set (&priv->stack_initialization_seek_sent, FALSE);
+  if (toplevel_seek)
+    g_atomic_int_set (&priv->stack_initialization_seek_sent, FALSE);
   gst_element_set_locked_state (priv->current_bin, FALSE);
 
   GST_DEBUG ("going back to parent state");
@@ -3365,9 +3396,23 @@ update_pipeline (NleComposition * comp, GstClockTime currenttime, gint32 seqnum,
     comp->priv->updating_reason = update_reason;
     comp->priv->seqnum_to_restart_task = seqnum;
 
-    if (!_pause_task (comp)) {
+    /* Subcomposition can preroll without sending initializing seeks
+     * as the toplevel composition will send it anyway.
+     *
+     * This avoid seeking round trips (otherwise we get 1 extra seek
+     * per level of nesting)
+     */
+
+    if (tear_down && !nle_composition_needs_topelevel_initializing_seek (comp))
+      gst_clear_event (&toplevel_seek);
+
+    if (toplevel_seek) {
+      if (!_pause_task (comp)) {
         gst_event_unref (toplevel_seek);
         return FALSE;
+      }
+    } else {
+      GST_INFO_OBJECT (comp, "Not pausing composition when first initializing");
     }
   }