script: Do not run layout in a thread (#31346)

* script: Do not run layout in a thread

Instead of spawning a thread for layout that almost always runs
synchronously with script, simply run layout in the script thread.

This is a resurrection of #28708, taking just the bits that remove the
layout thread. It's a complex change and thus is just a first step
toward cleaning up the interface between script and layout. Messages are
still passed from script to layout via a `process()` method and script
proxies some messages to layout from other threads as well.

Big changes:

1. Layout is created in the script thread on Document load, thus every
   live document is guaranteed to have a layout. This isn't completely
   hidden in the interface, but we can safely `unwrap()` on a Document's
   layout.
2. Layout configuration is abstracted away into a LayoutConfig struct
   and the LayoutFactory is a struct passed around by the Constellation.
   This is to avoid having to monomorphize the entire script thread
   for each layout.
3. Instead of having the Constellation block on the layout thread to
   figure out the current epoch and whether there are pending web fonts
   loading, updates are sent synchronously to the Constellation when
   rendering to a screenshot. This practically only used by the WPT.

A couple tests start to fail, which is probably inevitable since removing
the layout thread has introduced timing changes in "exit after load" and
screenshot behavior.

Co-authored-by: Josh Matthews <josh@joshmatthews.net>

* Update test expectations

* Fix some issues found during review

* Clarify some comments

* Address review comments

---------

Co-authored-by: Josh Matthews <josh@joshmatthews.net>

1 files changed, 7 insertions, 9 deletions

diff --git a/components/metrics/lib.rs b/components/metrics/lib.rs
index f3ea4b26c51..971ad729441 100644
--- a/components/metrics/lib.rs
+++ b/components/metrics/lib.rs
@@ -261,7 +261,7 @@ impl ProgressiveWebMetric for InteractiveMetrics {
 // https://w3c.github.io/paint-timing/
 pub struct PaintTimeMetrics {
     pending_metrics: RefCell<HashMap<Epoch, (Option<TimerMetadata>, bool)>>,
-    navigation_start: Option<u64>,
+    navigation_start: u64,
     first_paint: Cell<Option<u64>>,
     first_contentful_paint: Cell<Option<u64>>,
     pipeline_id: PipelineId,
@@ -278,10 +278,11 @@ impl PaintTimeMetrics {
         constellation_chan: IpcSender<LayoutMsg>,
         script_chan: IpcSender<ConstellationControlMsg>,
         url: ServoUrl,
+        navigation_start: u64,
     ) -> PaintTimeMetrics {
         PaintTimeMetrics {
             pending_metrics: RefCell::new(HashMap::new()),
-            navigation_start: None,
+            navigation_start,
             first_paint: Cell::new(None),
             first_contentful_paint: Cell::new(None),
             pipeline_id,
@@ -342,11 +343,8 @@ impl PaintTimeMetrics {
     }
 
     pub fn maybe_set_metric(&self, epoch: Epoch, paint_time: u64) {
-        if self.first_paint.get().is_some() && self.first_contentful_paint.get().is_some() ||
-            self.navigation_start.is_none()
-        {
-            // If we already set all paint metrics or we have not set navigation start yet,
-            // we just bail out.
+        if self.first_paint.get().is_some() && self.first_contentful_paint.get().is_some() {
+            // If we already set all paint metrics we just bail out.
             return;
         }
 
@@ -387,11 +385,11 @@ impl PaintTimeMetrics {
 
 impl ProgressiveWebMetric for PaintTimeMetrics {
     fn get_navigation_start(&self) -> Option<u64> {
-        self.navigation_start
+        Some(self.navigation_start)
     }
 
     fn set_navigation_start(&mut self, time: u64) {
-        self.navigation_start = Some(time);
+        self.navigation_start = time;
     }
 
     fn send_queued_constellation_msg(&self, name: ProgressiveWebMetricType, time: u64) {