Bug 1591524 - Clean up native compositor surfaces that are not retained. r=kvark

Differential Revision: https://phabricator.services.mozilla.com/D50773

--HG--
extra : moz-landing-system : lando

gfx/wr/webrender/src/composite.rs

@@ -233,7 +233,12 @@ pub trait Compositor {
         size: DeviceIntSize,
     );
 
-    /// Destroy the surface with the specified id.
+    /// Destroy the surface with the specified id. WR may call this
+    /// at any time the surface is no longer required (including during
+    /// renderer deinit). It's the responsibility of the embedder
+    /// to ensure that the surface is only freed once the GPU is
+    /// no longer using the surface (if this isn't already handled
+    /// by the operating system).
     fn destroy_surface(
         &mut self,
         id: NativeSurfaceId,

gfx/wr/webrender/src/frame_builder.rs

@@ -349,6 +349,22 @@ impl FrameBuilder {
                 &visibility_context,
                 &mut visibility_state,
             );
+
+            // When a new display list is processed by WR, the existing tiles from
+            // any picture cache are stored in the `retained_tiles` field above. This
+            // allows the first frame of a new display list to reuse any existing tiles
+            // and surfaces that match. Once the `update_visibility` call above is
+            // complete, any tiles that are left remaining in the `retained_tiles`
+            // map are not needed and will be dropped. For simple compositing mode,
+            // this is fine, since texture cache handles are garbage collected at
+            // the end of each frame. However, if we're in native compositor mode,
+            // we need to manually clean up any native compositor surfaces that were
+            // allocated by these tiles.
+            for (_, cache_state) in visibility_state.retained_tiles.caches.drain() {
+                visibility_state.composite_state.destroy_native_surfaces(
+                    cache_state.tiles.values(),
+                );
+            }
         }
 
         let mut frame_state = FrameBuildingState {

gfx/wr/webrender/src/picture.rs

@@ -4656,7 +4656,7 @@ impl TileNode {
 
 impl CompositeState {
     // A helper function to destroy all native surfaces for a given list of tiles
-    fn destroy_native_surfaces<'a, I: Iterator<Item = &'a Tile>>(
+    pub fn destroy_native_surfaces<'a, I: Iterator<Item = &'a Tile>>(
         &mut self,
         tiles_iter: I,
     ) {