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

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* 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>
This commit is contained in:
Martin Robinson 2024-02-23 09:14:10 +01:00 • committed by GitHub
parent 4849ba901e
commit 9c0561536d
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44 changed files with 636 additions and 1253 deletions
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49
components/script/dom/document.rs
View file

@ -320,7 +320,7 @@ pub struct Document {
/// This field is set to the document itself for inert documents.
/// <https://html.spec.whatwg.org/multipage/#appropriate-template-contents-owner-document>
appropriate_template_contents_owner_document: MutNullableDom<Document>,
/// Information on elements needing restyle to ship over to the layout thread when the
/// Information on elements needing restyle to ship over to layout when the
/// time comes.
pending_restyles: DomRefCell<HashMap<Dom<Element>, NoTrace<PendingRestyle>>>,
/// This flag will be true if layout suppressed a reflow attempt that was
@ -367,7 +367,7 @@ pub struct Document {
spurious_animation_frames: Cell<u8>,
/// Track the total number of elements in this DOM's tree.
/// This is sent to the layout thread every time a reflow is done;
/// This is sent to layout every time a reflow is done;
/// layout uses this to determine if the gains from parallel layout will be worth the overhead.
///
/// See also: <https://github.com/servo/servo/issues/10110>
@ -827,10 +827,9 @@ impl Document {
let old_mode = self.quirks_mode.replace(new_mode);
if old_mode != new_mode {
match self.window.layout_chan() {
Some(chan) => chan.send(Msg::SetQuirksMode(new_mode)).unwrap(),
None => warn!("Layout channel unavailable"),
}
let _ = self
.window
.with_layout(move |layout| layout.process(Msg::SetQuirksMode(new_mode)));
}
}
@ -2092,7 +2091,7 @@ impl Document {
},
LoadType::PageSource(_) => {
if self.has_browsing_context && self.is_fully_active() {
// Note: if the document is not fully active, the layout thread will have exited already.
// Note: if the document is not fully active, layout will have exited already.
// The underlying problem might actually be that layout exits while it should be kept alive.
// See https://github.com/servo/servo/issues/22507
@ -2264,7 +2263,7 @@ impl Document {
None => false,
};
// Note: if the document is not fully active, the layout thread will have exited already,
// Note: if the document is not fully active, layout will have exited already,
// and this method will panic.
// The underlying problem might actually be that layout exits while it should be kept alive.
// See https://github.com/servo/servo/issues/22507
@ -3441,12 +3440,11 @@ impl Document {
/// Flushes the stylesheet list, and returns whether any stylesheet changed.
pub fn flush_stylesheets_for_reflow(&self) -> bool {
// NOTE(emilio): The invalidation machinery is used on the replicated
// list on the layout thread.
// list in layout.
//
// FIXME(emilio): This really should differentiate between CSSOM changes
// and normal stylesheets additions / removals, because in the last case
// the layout thread already has that information and we could avoid
// dirtying the whole thing.
// layout already has that information and we could avoid dirtying the whole thing.
let mut stylesheets = self.stylesheets.borrow_mut();
let have_changed = stylesheets.has_changed();
stylesheets.flush_without_invalidation();
@ -3830,15 +3828,14 @@ impl Document {
})
.cloned();
match self.window.layout_chan() {
Some(chan) => chan
.send(Msg::AddStylesheet(
sheet.clone(),
insertion_point.as_ref().map(|s| s.sheet.clone()),
))
.unwrap(),
None => return warn!("Layout channel unavailable"),
}
let cloned_stylesheet = sheet.clone();
let insertion_point2 = insertion_point.clone();
let _ = self.window.with_layout(move |layout| {
layout.process(Msg::AddStylesheet(
cloned_stylesheet,
insertion_point2.as_ref().map(|s| s.sheet.clone()),
));
});
DocumentOrShadowRoot::add_stylesheet(
owner,
@ -3851,15 +3848,15 @@ impl Document {
/// Remove a stylesheet owned by `owner` from the list of document sheets.
#[allow(crown::unrooted_must_root)] // Owner needs to be rooted already necessarily.
pub fn remove_stylesheet(&self, owner: &Element, s: &Arc<Stylesheet>) {
match self.window.layout_chan() {
Some(chan) => chan.send(Msg::RemoveStylesheet(s.clone())).unwrap(),
None => return warn!("Layout channel unavailable"),
}
pub fn remove_stylesheet(&self, owner: &Element, stylesheet: &Arc<Stylesheet>) {
let cloned_stylesheet = stylesheet.clone();
let _ = self
.window
.with_layout(|layout| layout.process(Msg::RemoveStylesheet(cloned_stylesheet)));
DocumentOrShadowRoot::remove_stylesheet(
owner,
s,
stylesheet,
StylesheetSetRef::Document(&mut *self.stylesheets.borrow_mut()),
)
}