rendernode: Limit diff region

Limit the diff region to 30 rectangles (randomly chosen because it
looked big enough to not trigger by accident and small enough to not
cause performance issues).

If the diff region gets more complicated, we abort to the parent node
and use its bounds as the diff region instead and then continue diffing
the rest of the node tree.

Fixes: #4560
Fixes: #2396

diff --git a/gsk/gskrendernodeimpl.c b/gsk/gskrendernodeimpl.c
index 24bf09baac5dc2998892764f660fa56db1063a1a..dca0bbe585dd6bcaa85f71bae1a8661deb560202 100644 (file)
--- a/gsk/gskrendernodeimpl.c
+++ b/gsk/gskrendernodeimpl.c
@@ -33,6 +33,12 @@
 
 #include <hb-ot.h>
 
+/* maximal number of rectangles we keep in a diff region before we throw
+ * the towel and just use the bounding box of the parent node.
+ * Meant to avoid performance corner cases.
+ */
+#define MAX_RECTS_IN_DIFF 30
+
 static inline void
 gsk_cairo_rectangle (cairo_t               *cr,
                      const graphene_rect_t *rect)
@@ -2611,6 +2617,8 @@ static GskDiffResult
 gsk_container_node_keep_func (gconstpointer elem1, gconstpointer elem2, gpointer data)
 {
   gsk_render_node_diff ((GskRenderNode *) elem1, (GskRenderNode *) elem2, data);
+  if (cairo_region_num_rectangles (data) > MAX_RECTS_IN_DIFF)
+    return GSK_DIFF_ABORTED;
 
   return GSK_DIFF_OK;
 }
@@ -2624,6 +2632,8 @@ gsk_container_node_change_func (gconstpointer elem, gsize idx, gpointer data)
 
   rectangle_init_from_graphene (&rect, &node->bounds);
   cairo_region_union_rectangle (region, &rect);
+  if (cairo_region_num_rectangles (region) > MAX_RECTS_IN_DIFF)
+    return GSK_DIFF_ABORTED;
 
   return GSK_DIFF_OK;
 }