timeline-tree: make sure the layer priority refers to an existing layer

If a layer priority sits between the priorities of two layers in the
timeline, i.e. it references a gap in the timeline's layers, then
ges_timeline_append_layer will never fill this gap and create the
desired layer, so the edit in timeline-tree would loop forever. So a
check was added to avoid this.

This would be a usage error, but a user can reasonably end up with a gap
in their layers if they remove a layer from the timeline.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-editing-services/-/merge_requests/177>

ges/ges-internal.h

@@ -121,6 +121,9 @@ ges_timeline_edit (GESTimeline * timeline, GESTimelineElement * element,
     gint64 new_layer_priority, GESEditMode mode, GESEdge edge,
     guint64 position, GError ** error);
 
+G_GNUC_INTERNAL gboolean
+ges_timeline_layer_priority_in_gap (GESTimeline * timeline, guint layer_priority);
+
 G_GNUC_INTERNAL void
 ges_timeline_set_track_selection_error  (GESTimeline * timeline,
                                          gboolean was_error,

ges/ges-timeline-tree.c

@@ -962,6 +962,13 @@ set_layer_priority (GESTimelineElement * element, EditData * data,
 
   data->layer_priority = (guint32) (layer_prio - (gint64) layer_offset);
 
+  if (ges_timeline_layer_priority_in_gap (element->timeline,
+          data->layer_priority)) {
+    GST_ERROR_OBJECT (element, "Edit layer %" G_GUINT32_FORMAT " would "
+        "be within a gap in the timeline layers", data->layer_priority);
+    return FALSE;
+  }
+
   GST_INFO_OBJECT (element, "%s will move to layer %" G_GUINT32_FORMAT,
       element->name, data->layer_priority);
 
@@ -1814,6 +1821,13 @@ perform_element_edit (GESTimelineElement * element, EditData * edit)
         edit->layer_priority);
 
     if (layer == NULL) {
+      /* make sure we won't loop forever */
+      if (ges_timeline_layer_priority_in_gap (timeline, edit->layer_priority)) {
+        GST_ERROR_OBJECT (element, "Requested layer %" G_GUINT32_FORMAT
+            " is within a gap in the timeline layers", edit->layer_priority);
+        goto done;
+      }
+
       do {
         layer = ges_timeline_append_layer (timeline);
       } while (ges_layer_get_priority (layer) < edit->layer_priority);

ges/ges-timeline.c

@@ -3074,6 +3074,26 @@ ges_timeline_get_layer (GESTimeline * timeline, guint priority)
   return layer;
 }
 
+gboolean
+ges_timeline_layer_priority_in_gap (GESTimeline * timeline, guint priority)
+{
+  GList *tmp;
+
+  CHECK_THREAD (timeline);
+
+  for (tmp = timeline->layers; tmp; tmp = tmp->next) {
+    GESLayer *layer = GES_LAYER (tmp->data);
+    guint tmp_priority = ges_layer_get_priority (layer);
+
+    if (tmp_priority == priority)
+      return FALSE;
+    else if (tmp_priority > priority)
+      return TRUE;
+  }
+
+  return FALSE;
+}
+
 /**
  * ges_timeline_paste_element:
  * @timeline: The #GESTimeline onto which @element should be pasted