Fixes a number of race conditions and reliability issues with reftests and compositor.

The basic idea is it's safe to output an image for reftest by testing:
 - That the compositor doesn't have any animations active.
 - That the compositor is not waiting on any outstanding paint messages to arrive.
 - That the script tasks are "idle" and therefore won't cause reflow.
    - This currently means page loaded, onload fired, reftest-wait not active, first reflow triggered.
    - It could easily be expanded to handle pending timers etc.
 - That the "epoch" that the layout tasks have last laid out after script went idle, is reflected by the compositor in all visible layers for that pipeline.

1 files changed, 4 insertions, 0 deletions

diff --git a/components/script/layout_interface.rs b/components/script/layout_interface.rs
index 6fde5c4312f..ad474a0daed 100644
--- a/components/script/layout_interface.rs
+++ b/components/script/layout_interface.rs
@@ -12,6 +12,7 @@ use geom::point::Point2D;
 use geom::rect::Rect;
 use libc::uintptr_t;
 use msg::constellation_msg::{PipelineExitType, WindowSizeData};
+use msg::compositor_msg::Epoch;
 use net_traits::PendingAsyncLoad;
 use profile_traits::mem::{Reporter, ReportsChan};
 use script_traits::{ScriptControlChan, OpaqueScriptLayoutChannel, UntrustedNodeAddress};
@@ -63,6 +64,9 @@ pub enum Msg {
     /// Requests that the layout task immediately shut down. There must be no more nodes left after
     /// this, or layout will crash.
     ExitNow(PipelineExitType),
+
+    /// Get the last epoch counter for this layout task.
+    GetCurrentEpoch(Sender<Epoch>)
 }
 
 /// Synchronous messages that script can send to layout.