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.

components/script/script_task.rs

@@ -58,9 +58,8 @@ use script_traits::CompositorEvent::{MouseDownEvent, MouseUpEvent};
 use script_traits::CompositorEvent::{MouseMoveEvent, KeyEvent};
 use script_traits::{NewLayoutInfo, OpaqueScriptLayoutChannel};
 use script_traits::{ConstellationControlMsg, ScriptControlChan};
-use script_traits::ScriptTaskFactory;
+use script_traits::{ScriptState, ScriptTaskFactory};
 use webdriver_traits::WebDriverScriptCommand;
-use msg::compositor_msg::ReadyState::{FinishedLoading, Loading, PerformingLayout};
 use msg::compositor_msg::{LayerId, ScriptListener};
 use msg::constellation_msg::{ConstellationChan, FocusType};
 use msg::constellation_msg::{LoadData, PipelineId, SubpageId, MozBrowserEvent, WorkerId};
@@ -735,6 +734,10 @@ impl ScriptTask {
                 responder.respond();
                 self.handle_resource_loaded(id, LoadType::Stylesheet(url));
             }
+            ConstellationControlMsg::GetCurrentState(sender, pipeline_id) => {
+                let state = self.handle_get_current_state(pipeline_id);
+                sender.send(state).unwrap();
+            }
         }
     }
 
@@ -859,6 +862,40 @@ impl ScriptTask {
         doc.r().finish_load(load);
     }
 
+    /// Get the current state of a given pipeline.
+    fn handle_get_current_state(&self, pipeline_id: PipelineId) -> ScriptState {
+        // Check if the main page load is still pending
+        let loads = self.incomplete_loads.borrow();
+        if let Some(_) = loads.iter().find(|load| load.pipeline_id == pipeline_id) {
+            return ScriptState::DocumentLoading;
+        }
+
+        // If not in pending loads, the page should exist by now.
+        let page = self.root_page();
+        let page = page.find(pipeline_id).expect("GetCurrentState sent to nonexistent pipeline");
+        let doc = page.document().root();
+
+        // Check if document load event has fired. If the document load
+        // event has fired, this also guarantees that the first reflow
+        // has been kicked off. Since the script task does a join with
+        // layout, this ensures there are no race conditions that can occur
+        // between load completing and the first layout completing.
+        let load_pending = doc.r().ReadyState() != DocumentReadyState::Complete;
+        if load_pending {
+            return ScriptState::DocumentLoading;
+        }
+
+        // Checks if the html element has reftest-wait attribute present.
+        // See http://testthewebforward.org/docs/reftests.html
+        let html_element = doc.r().GetDocumentElement().root();
+        let reftest_wait = html_element.r().map_or(false, |elem| elem.has_class(&Atom::from_slice("reftest-wait")));
+        if reftest_wait {
+            return ScriptState::DocumentLoading;
+        }
+
+        return ScriptState::DocumentLoaded;
+    }
+
     fn handle_new_layout(&self, new_layout_info: NewLayoutInfo) {
         let NewLayoutInfo {
             containing_pipeline_id,
@@ -993,14 +1030,6 @@ impl ScriptTask {
              with this script task. This is a bug.");
         let window = page.window().root();
         window.r().handle_reflow_complete_msg(reflow_id);
-
-        let doc = page.document().root();
-        let html_element = doc.r().GetDocumentElement().root();
-        let reftest_wait = html_element.r().map_or(false, |elem| elem.has_class(&Atom::from_slice("reftest-wait")));
-
-        if !reftest_wait {
-            self.compositor.borrow_mut().set_ready_state(pipeline_id, FinishedLoading);
-        }
     }
 
     /// Window was resized, but this script was not active, so don't reflow yet
@@ -1102,8 +1131,6 @@ impl ScriptTask {
             })
         }).root();
 
-        self.compositor.borrow_mut().set_ready_state(incomplete.pipeline_id, Loading);
-
         // Create a new frame tree entry.
         let page = Rc::new(Page::new(incomplete.pipeline_id, final_url.clone()));
         if !root_page_exists {
@@ -1462,7 +1489,6 @@ impl ScriptTask {
         let final_url = document.r().url();
 
         document.r().set_ready_state(DocumentReadyState::Interactive);
-        self.compositor.borrow_mut().set_ready_state(id, PerformingLayout);
 
         // Kick off the initial reflow of the page.
         debug!("kicking off initial reflow of {:?}", final_url);