Auto merge of #16295 - jdm:transition-safety, r=nox

Root nodes for the duration of their CSS transitions

This ensures that we can pass a node address as part of the asynchronous
transition end notification, making it safe to fire the corresponding
DOM event on the node from the script thread. Without explicitly rooting
this node when the transition starts, we risk the node being GCed before
the transition is complete.

---
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix #14972
- [X] There are tests for these changes

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1 files changed, 17 insertions, 3 deletions

diff --git a/components/layout/animation.rs b/components/layout/animation.rs
index be6d5baf77c..d1673ac44a1 100644
--- a/components/layout/animation.rs
+++ b/components/layout/animation.rs
@@ -9,7 +9,9 @@ use flow::{self, Flow};
 use gfx::display_list::OpaqueNode;
 use ipc_channel::ipc::IpcSender;
 use msg::constellation_msg::PipelineId;
+use opaque_node::OpaqueNodeMethods;
 use script_traits::{AnimationState, ConstellationControlMsg, LayoutMsg as ConstellationMsg};
+use script_traits::UntrustedNodeAddress;
 use std::collections::HashMap;
 use std::sync::mpsc::Receiver;
 use style::animation::{Animation, update_style_for_animation};
@@ -24,6 +26,7 @@ pub fn update_animation_state(constellation_chan: &IpcSender<ConstellationMsg>,
                               script_chan: &IpcSender<ConstellationControlMsg>,
                               running_animations: &mut HashMap<OpaqueNode, Vec<Animation>>,
                               expired_animations: &mut HashMap<OpaqueNode, Vec<Animation>>,
+                              mut newly_transitioning_nodes: Option<&mut Vec<UntrustedNodeAddress>>,
                               new_animations_receiver: &Receiver<Animation>,
                               pipeline_id: PipelineId,
                               timer: &Timer) {
@@ -71,7 +74,7 @@ pub fn update_animation_state(constellation_chan: &IpcSender<ConstellationMsg>,
         let mut animations_still_running = vec![];
         for mut running_animation in running_animations.drain(..) {
             let still_running = !running_animation.is_expired() && match running_animation {
-                Animation::Transition(_, _, started_at, ref frame, _expired) => {
+                Animation::Transition(_, started_at, ref frame, _expired) => {
                     now < started_at + frame.duration
                 }
                 Animation::Keyframes(_, _, ref mut state) => {
@@ -86,8 +89,8 @@ pub fn update_animation_state(constellation_chan: &IpcSender<ConstellationMsg>,
                 continue
             }
 
-            if let Animation::Transition(_, unsafe_node, _, ref frame, _) = running_animation {
-                script_chan.send(ConstellationControlMsg::TransitionEnd(unsafe_node,
+            if let Animation::Transition(node, _, ref frame, _) = running_animation {
+                script_chan.send(ConstellationControlMsg::TransitionEnd(node.to_untrusted_node_address(),
                                                                         frame.property_animation
                                                                              .property_name().into(),
                                                                         frame.duration))
@@ -112,6 +115,17 @@ pub fn update_animation_state(constellation_chan: &IpcSender<ConstellationMsg>,
 
     // Add new running animations.
     for new_running_animation in new_running_animations {
+        if new_running_animation.is_transition() {
+            match newly_transitioning_nodes {
+                Some(ref mut nodes) => {
+                    nodes.push(new_running_animation.node().to_untrusted_node_address());
+                }
+                None => {
+                    warn!("New transition encountered from compositor-initiated layout.");
+                }
+            }
+        }
+
         running_animations.entry(*new_running_animation.node())
                           .or_insert_with(Vec::new)
                           .push(new_running_animation)