layout_task.rs

/* This Source Code Form is subject to the terms of the Mozilla Public
 * License, v. 2.0. If a copy of the MPL was not distributed with this
 * file, You can obtain one at http://mozilla.org/MPL/2.0/. */

//! The layout task. Performs layout on the DOM, builds display lists and sends them to be
//! rendered.

use css::node_style::StyledNode;
use construct::FlowConstructionResult;
use context::SharedLayoutContext;
use flow::{Flow, ImmutableFlowUtils, MutableFlowUtils, MutableOwnedFlowUtils};
use flow;
use flow_ref::FlowRef;
use incremental::{Reflow, Repaint};
use layout_debug;
use parallel::UnsafeFlow;
use parallel;
use sequential;
use util::{LayoutDataAccess, LayoutDataWrapper, OpaqueNodeMethods, ToGfxColor};
use wrapper::{LayoutNode, TLayoutNode, ThreadSafeLayoutNode};

use collections::dlist::DList;
use encoding::EncodingRef;
use encoding::all::UTF_8;
use geom::point::Point2D;
use geom::rect::Rect;
use geom::size::Size2D;
use gfx::display_list::{ContentStackingLevel, DisplayItem, DisplayItemIterator, DisplayList};
use gfx::display_list::{OpaqueNode};
use gfx::render_task::{RenderInitMsg, RenderChan, RenderLayer};
use gfx::{render_task, color};
use layout_traits;
use layout_traits::{LayoutControlMsg, LayoutTaskFactory};
use script::dom::bindings::js::JS;
use script::dom::node::{ElementNodeTypeId, LayoutDataRef, Node};
use script::dom::element::{HTMLBodyElementTypeId, HTMLHtmlElementTypeId};
use script::layout_interface::{AddStylesheetMsg, LoadStylesheetMsg, ScriptLayoutChan};
use script::layout_interface::{TrustedNodeAddress, ContentBoxesResponse, ExitNowMsg};
use script::layout_interface::{ContentBoxResponse, HitTestResponse, MouseOverResponse};
use script::layout_interface::{LayoutChan, Msg, PrepareToExitMsg};
use script::layout_interface::{GetRPCMsg, LayoutRPC, ReapLayoutDataMsg, Reflow};
use script::layout_interface::{ReflowForDisplay, ReflowMsg};
use script_traits::{SendEventMsg, ReflowEvent, ReflowCompleteMsg, OpaqueScriptLayoutChannel};
use script_traits::{ScriptControlChan, UntrustedNodeAddress};
use servo_msg::compositor_msg::Scrollable;
use servo_msg::constellation_msg::{ConstellationChan, PipelineId, Failure, FailureMsg};
use servo_net::image_cache_task::{ImageCacheTask, ImageResponseMsg};
use gfx::font_cache_task::{FontCacheTask};
use servo_net::local_image_cache::{ImageResponder, LocalImageCache};
use servo_net::resource_task::{ResourceTask, load_bytes_iter};
use servo_util::geometry::Au;
use servo_util::geometry;
use servo_util::logical_geometry::LogicalPoint;
use servo_util::opts;
use servo_util::smallvec::{SmallVec, SmallVec1, VecLike};
use servo_util::task::spawn_named_with_send_on_failure;
use servo_util::task_state;
use servo_util::time::{TimeProfilerChan, profile};
use servo_util::time;
use servo_util::workqueue::WorkQueue;
use std::cell::Cell;
use std::comm::{channel, Sender, Receiver, Select};
use std::mem;
use std::ptr;
use style::{AuthorOrigin, Stylesheet, Stylist, TNode, iter_font_face_rules};
use style::{Device, Screen};
use sync::{Arc, Mutex, MutexGuard};
use url::Url;

/// Mutable data belonging to the LayoutTask.
///
/// This needs to be protected by a mutex so we can do fast RPCs.
pub struct LayoutTaskData {
    /// The local image cache.
    pub local_image_cache: Arc<Mutex<LocalImageCache<UntrustedNodeAddress>>>,

    /// The size of the viewport.
    pub screen_size: Size2D<Au>,

    /// A cached display list.
    pub display_list: Option<Arc<DisplayList>>,

    pub stylist: Box<Stylist>,

    /// The workers that we use for parallel operation.
    pub parallel_traversal: Option<WorkQueue<*const SharedLayoutContext, UnsafeFlow>>,

    /// The dirty rect. Used during display list construction.
    pub dirty: Rect<Au>,

    /// Starts at zero, and increased by one every time a layout completes.
    /// This can be used to easily check for invalid stale data.
    pub generation: uint,

    /// True if a style sheet was added since the last reflow. Currently, this causes all nodes to
    /// be dirtied at the next reflow.
    pub stylesheet_dirty: bool,
}

/// Information needed by the layout task.
pub struct LayoutTask {
    /// The ID of the pipeline that we belong to.
    pub id: PipelineId,

    /// The port on which we receive messages from the script task.
    pub port: Receiver<Msg>,

    /// The port on which we receive messages from the constellation
    pub pipeline_port: Receiver<LayoutControlMsg>,

    //// The channel to send messages to ourself.
    pub chan: LayoutChan,

    /// The channel on which messages can be sent to the constellation.
    pub constellation_chan: ConstellationChan,

    /// The channel on which messages can be sent to the script task.
    pub script_chan: ScriptControlChan,

    /// The channel on which messages can be sent to the painting task.
    pub render_chan: RenderChan,

    /// The channel on which messages can be sent to the time profiler.
    pub time_profiler_chan: TimeProfilerChan,

    /// The channel on which messages can be sent to the resource task.
    pub resource_task: ResourceTask,

    /// The channel on which messages can be sent to the image cache.
    pub image_cache_task: ImageCacheTask,

    /// Public interface to the font cache task.
    pub font_cache_task: FontCacheTask,

    /// Is this the first reflow in this LayoutTask?
    pub first_reflow: Cell<bool>,

    /// A mutex to allow for fast, read-only RPC of layout's internal data
    /// structures, while still letting the LayoutTask modify them.
    ///
    /// All the other elements of this struct are read-only.
    pub rw_data: Arc<Mutex<LayoutTaskData>>,

    /// The media queries device state.
    /// TODO: Handle updating this when window size changes etc.
    pub device: Device,
}

struct LayoutImageResponder {
    id: PipelineId,
    script_chan: ScriptControlChan,
}

impl ImageResponder<UntrustedNodeAddress> for LayoutImageResponder {
    fn respond(&self) -> proc(ImageResponseMsg, UntrustedNodeAddress):Send {
        let id = self.id.clone();
        let script_chan = self.script_chan.clone();
        let f: proc(ImageResponseMsg, UntrustedNodeAddress):Send =
            proc(_, node_address) {
                let ScriptControlChan(chan) = script_chan;
                debug!("Dirtying {:x}", node_address as uint);
                let mut nodes = SmallVec1::new();
                nodes.vec_push(node_address);
                drop(chan.send_opt(SendEventMsg(id.clone(), ReflowEvent(nodes))))
            };
        f
    }
}

impl LayoutTaskFactory for LayoutTask {
    /// Spawns a new layout task.
    fn create(_phantom: Option<&mut LayoutTask>,
                  id: PipelineId,
                  chan: OpaqueScriptLayoutChannel,
                  pipeline_port: Receiver<LayoutControlMsg>,
                  constellation_chan: ConstellationChan,
                  failure_msg: Failure,
                  script_chan: ScriptControlChan,
                  render_chan: RenderChan,
                  resource_task: ResourceTask,
                  img_cache_task: ImageCacheTask,
                  font_cache_task: FontCacheTask,
                  time_profiler_chan: TimeProfilerChan,
                  shutdown_chan: Sender<()>) {
        let ConstellationChan(con_chan) = constellation_chan.clone();
        spawn_named_with_send_on_failure("LayoutTask", task_state::Layout, proc() {
            { // Ensures layout task is destroyed before we send shutdown message
                let sender = chan.sender();
                let layout =
                    LayoutTask::new(
                        id,
                        chan.receiver(),
                        LayoutChan(sender),
                        pipeline_port,
                        constellation_chan,
                        script_chan,
                        render_chan,
                        resource_task,
                        img_cache_task,
                        font_cache_task,
                        time_profiler_chan);
                layout.start();
            }
            shutdown_chan.send(());
        }, FailureMsg(failure_msg), con_chan, false);
    }
}

/// The `LayoutTask` `rw_data` lock must remain locked until the first reflow,
/// as RPC calls don't make sense until then. Use this in combination with
/// `LayoutTask::lock_rw_data` and `LayoutTask::return_rw_data`.
enum RWGuard<'a> {
    /// If the lock was previously held, from when the task started.
    Held(MutexGuard<'a, LayoutTaskData>),
    /// If the lock was just used, and has been returned since there has been
    /// a reflow already.
    Used(MutexGuard<'a, LayoutTaskData>),
}

impl<'a> Deref<LayoutTaskData> for RWGuard<'a> {
    fn deref(&self) -> &LayoutTaskData {
        match *self {
            Held(ref x) => x.deref(),
            Used(ref x) => x.deref(),
        }
    }
}

impl<'a> DerefMut<LayoutTaskData> for RWGuard<'a> {
    fn deref_mut(&mut self) -> &mut LayoutTaskData {
        match *self {
            Held(ref mut x) => x.deref_mut(),
            Used(ref mut x) => x.deref_mut(),
        }
    }
}

impl LayoutTask {
    /// Creates a new `LayoutTask` structure.
    fn new(id: PipelineId,
           port: Receiver<Msg>,
           chan: LayoutChan,
           pipeline_port: Receiver<LayoutControlMsg>,
           constellation_chan: ConstellationChan,
           script_chan: ScriptControlChan,
           render_chan: RenderChan,
           resource_task: ResourceTask,
           image_cache_task: ImageCacheTask,
           font_cache_task: FontCacheTask,
           time_profiler_chan: TimeProfilerChan)
           -> LayoutTask {
        let local_image_cache = Arc::new(Mutex::new(LocalImageCache::new(image_cache_task.clone())));
        let screen_size = Size2D(Au(0), Au(0));
        let device = Device::new(Screen, opts::get().initial_window_size.as_f32());
        let parallel_traversal = if opts::get().layout_threads != 1 {
            Some(WorkQueue::new("LayoutWorker", task_state::Layout,
                                opts::get().layout_threads, ptr::null()))
        } else {
            None
        };

        LayoutTask {
            id: id,
            port: port,
            pipeline_port: pipeline_port,
            chan: chan,
            constellation_chan: constellation_chan,
            script_chan: script_chan,
            render_chan: render_chan,
            time_profiler_chan: time_profiler_chan,
            resource_task: resource_task,
            image_cache_task: image_cache_task.clone(),
            font_cache_task: font_cache_task,
            first_reflow: Cell::new(true),
            device: device,
            rw_data: Arc::new(Mutex::new(
                LayoutTaskData {
                    local_image_cache: local_image_cache,
                    screen_size: screen_size,
                    display_list: None,
                    stylist: box Stylist::new(&device),
                    parallel_traversal: parallel_traversal,
                    dirty: Rect::zero(),
                    generation: 0,
                    stylesheet_dirty: false,
              })),
        }
    }

    /// Starts listening on the port.
    fn start(self) {
        let mut possibly_locked_rw_data = Some(self.rw_data.lock());
        while self.handle_request(&mut possibly_locked_rw_data) {
            // Loop indefinitely.
        }
    }

    // Create a layout context for use in building display lists, hit testing, &c.
    fn build_shared_layout_context(
      &self,
      rw_data: &LayoutTaskData,
      reflow_root: &LayoutNode,
      url: &Url)
          -> SharedLayoutContext {
        SharedLayoutContext {
            image_cache: rw_data.local_image_cache.clone(),
            screen_size: rw_data.screen_size.clone(),
            constellation_chan: self.constellation_chan.clone(),
            layout_chan: self.chan.clone(),
            font_cache_task: self.font_cache_task.clone(),
            stylist: &*rw_data.stylist,
            url: (*url).clone(),
            reflow_root: OpaqueNodeMethods::from_layout_node(reflow_root),
            dirty: Rect::zero(),
            generation: rw_data.generation,
        }
    }

    /// Receives and dispatches messages from the script and constellation tasks
    fn handle_request<'a>(&'a self, possibly_locked_rw_data: &mut Option<MutexGuard<'a, LayoutTaskData>>) -> bool {
        enum PortToRead {
            Pipeline,
            Script,
        }

        let port_to_read = {
            let sel = Select::new();
            let mut port1 = sel.handle(&self.port);
            let mut port2 = sel.handle(&self.pipeline_port);
            unsafe {
                port1.add();
                port2.add();
            }
            let ret = sel.wait();
            if ret == port1.id() {
                Script
            } else if ret == port2.id() {
                Pipeline
            } else {
                fail!("invalid select result");
            }
        };

        match port_to_read {
            Pipeline => match self.pipeline_port.recv() {
                layout_traits::ExitNowMsg => self.handle_script_request(ExitNowMsg, possibly_locked_rw_data),
            },
            Script => {
                let msg = self.port.recv();
                self.handle_script_request(msg, possibly_locked_rw_data)
            }
        }
    }

    /// If no reflow has happened yet, this will just return the lock in
    /// `possibly_locked_rw_data`. Otherwise, it will acquire the `rw_data` lock.
    ///
    /// If you do not wish RPCs to remain blocked, just drop the `RWGuard`
    /// returned from this function. If you _do_ wish for them to remain blocked,
    /// use `return_rw_data`.
    fn lock_rw_data<'a>(&'a self, possibly_locked_rw_data: &mut Option<MutexGuard<'a, LayoutTaskData>>) -> RWGuard<'a> {
        match possibly_locked_rw_data.take() {
            None    => Used(self.rw_data.lock()),
            Some(x) => Held(x),
        }
    }

    /// If no reflow has ever been trigger, this will keep the lock, locked
    /// (and saved in `possibly_locked_rw_data`). If it has been, the lock will
    /// be unlocked.
    fn return_rw_data<'a>(possibly_locked_rw_data: &mut Option<MutexGuard<'a, LayoutTaskData>>, rw_data: RWGuard<'a>) {
        match rw_data {
            Used(x) => drop(x),
            Held(x) => *possibly_locked_rw_data = Some(x),
        }
    }

    /// Receives and dispatches messages from the script task.
    fn handle_script_request<'a>(&'a self, request: Msg, possibly_locked_rw_data: &mut Option<MutexGuard<'a, LayoutTaskData>>) -> bool {
        match request {
            AddStylesheetMsg(sheet) => self.handle_add_stylesheet(sheet, possibly_locked_rw_data),
            LoadStylesheetMsg(url) => self.handle_load_stylesheet(url, possibly_locked_rw_data),
            GetRPCMsg(response_chan) => {
                response_chan.send(
                    box LayoutRPCImpl(
                        self.rw_data.clone()) as Box<LayoutRPC + Send>);
            },
            ReflowMsg(data) => {
                profile(time::LayoutPerformCategory, Some((&data.url, data.iframe, self.first_reflow.get())),
                self.time_profiler_chan.clone(), || {
                    self.handle_reflow(&*data, possibly_locked_rw_data);
                });
            },
            ReapLayoutDataMsg(dead_layout_data) => {
                unsafe {
                    LayoutTask::handle_reap_layout_data(dead_layout_data)
                }
            },
            PrepareToExitMsg(response_chan) => {
                debug!("layout: PrepareToExitMsg received");
                self.prepare_to_exit(response_chan, possibly_locked_rw_data);
                return false
            },
            ExitNowMsg => {
                debug!("layout: ExitNowMsg received");
                self.exit_now(possibly_locked_rw_data);
                return false
            }
        }

        true
    }

    /// Enters a quiescent state in which no new messages except for `ReapLayoutDataMsg` will be
    /// processed until an `ExitNowMsg` is received. A pong is immediately sent on the given
    /// response channel.
    fn prepare_to_exit<'a>(&'a self, response_chan: Sender<()>, possibly_locked_rw_data: &mut Option<MutexGuard<'a, LayoutTaskData>>) {
        response_chan.send(());
        loop {
            match self.port.recv() {
                ReapLayoutDataMsg(dead_layout_data) => {
                    unsafe {
                        LayoutTask::handle_reap_layout_data(dead_layout_data)
                    }
                }
                ExitNowMsg => {
                    debug!("layout task is exiting...");
                    self.exit_now(possibly_locked_rw_data);
                    break
                }
                _ => {
                    fail!("layout: message that wasn't `ExitNowMsg` received after \
                           `PrepareToExitMsg`")
                }
            }
        }
    }

    /// Shuts down the layout task now. If there are any DOM nodes left, layout will now (safely)
    /// crash.
    fn exit_now<'a>(&'a self, possibly_locked_rw_data: &mut Option<MutexGuard<'a, LayoutTaskData>>) {
        let (response_chan, response_port) = channel();

        {
            let mut rw_data = self.lock_rw_data(possibly_locked_rw_data);
            match rw_data.deref_mut().parallel_traversal {
                None => {}
                Some(ref mut traversal) => traversal.shutdown(),
            }
            LayoutTask::return_rw_data(possibly_locked_rw_data, rw_data);
        }

        self.render_chan.send(render_task::ExitMsg(Some(response_chan)));
        response_port.recv()
    }

    fn handle_load_stylesheet<'a>(&'a self, url: Url, possibly_locked_rw_data: &mut Option<MutexGuard<'a, LayoutTaskData>>) {
        // TODO: Get the actual value. http://dev.w3.org/csswg/css-syntax/#environment-encoding
        let environment_encoding = UTF_8 as EncodingRef;

        let (metadata, iter) = load_bytes_iter(&self.resource_task, url);
        let protocol_encoding_label = metadata.charset.as_ref().map(|s| s.as_slice());
        let final_url = metadata.final_url;

        let sheet = Stylesheet::from_bytes_iter(iter, final_url, protocol_encoding_label, Some(environment_encoding));
        self.handle_add_stylesheet(sheet, possibly_locked_rw_data);
    }

    fn handle_add_stylesheet<'a>(&'a self, sheet: Stylesheet, possibly_locked_rw_data: &mut Option<MutexGuard<'a, LayoutTaskData>>) {
        // Find all font-face rules and notify the font cache of them.
        // GWTODO: Need to handle unloading web fonts (when we handle unloading stylesheets!)
        iter_font_face_rules(&sheet, &self.device, |family, src| {
            self.font_cache_task.add_web_font(family.to_string(), (*src).clone());
        });
        let mut rw_data = self.lock_rw_data(possibly_locked_rw_data);
        rw_data.stylist.add_stylesheet(sheet, AuthorOrigin, &self.device);
        rw_data.stylesheet_dirty = true;
        LayoutTask::return_rw_data(possibly_locked_rw_data, rw_data);
    }

    /// Retrieves the flow tree root from the root node.
    fn try_get_layout_root(&self, node: LayoutNode) -> Option<FlowRef> {
        let mut layout_data_ref = node.mutate_layout_data();
        let layout_data =
            match layout_data_ref.as_mut() {
                None              => return None,
                Some(layout_data) => layout_data,
            };

        let result = layout_data.data.flow_construction_result.swap_out();

        let mut flow = match result {
            FlowConstructionResult(mut flow, abs_descendants) => {
                // Note: Assuming that the root has display 'static' (as per
                // CSS Section 9.3.1). Otherwise, if it were absolutely
                // positioned, it would return a reference to itself in
                // `abs_descendants` and would lead to a circular reference.
                // Set Root as CB for any remaining absolute descendants.
                flow.set_absolute_descendants(abs_descendants);
                flow
            }
            _ => return None,
        };

        flow.mark_as_root();

        Some(flow)
    }

    fn get_layout_root(&self, node: LayoutNode) -> FlowRef {
        self.try_get_layout_root(node).expect("no layout root")
    }

    /// Performs layout constraint solving.
    ///
    /// This corresponds to `Reflow()` in Gecko and `layout()` in WebKit/Blink and should be
    /// benchmarked against those two. It is marked `#[inline(never)]` to aid profiling.
    #[inline(never)]
    fn solve_constraints<'a>(&self,
                         layout_root: &mut FlowRef,
                         shared_layout_context: &SharedLayoutContext) {
        let _scope = layout_debug_scope!("solve_constraints");
        sequential::traverse_flow_tree_preorder(layout_root, shared_layout_context);
    }

    /// Performs layout constraint solving in parallel.
    ///
    /// This corresponds to `Reflow()` in Gecko and `layout()` in WebKit/Blink and should be
    /// benchmarked against those two. It is marked `#[inline(never)]` to aid profiling.
    #[inline(never)]
    fn solve_constraints_parallel(&self,
                                  data: &Reflow,
                                  rw_data: &mut LayoutTaskData,
                                  layout_root: &mut FlowRef,
                                  shared_layout_context: &SharedLayoutContext) {
        let _scope = layout_debug_scope!("solve_constraints_parallel");

        match rw_data.parallel_traversal {
            None => fail!("solve_contraints_parallel() called with no parallel traversal ready"),
            Some(ref mut traversal) => {
                // NOTE: this currently computes borders, so any pruning should separate that
                // operation out.
                parallel::traverse_flow_tree_preorder(layout_root,
                                                      &data.url,
                                                      data.iframe,
                                                      self.first_reflow.get(),
                                                      self.time_profiler_chan.clone(),
                                                      shared_layout_context,
                                                      traversal);
            }
        }
    }

    /// Verifies that every node was either marked as a leaf or as a nonleaf in the flow tree.
    /// This is only on in debug builds.
    #[inline(never)]
    #[cfg(debug)]
    fn verify_flow_tree(&self, layout_root: &mut FlowRef) {
        let mut traversal = traversal::FlowTreeVerification;
        layout_root.traverse_preorder(&mut traversal);
    }

    #[cfg(not(debug))]
    fn verify_flow_tree(&self, _: &mut FlowRef) {
    }

    /// The high-level routine that performs layout tasks.
    fn handle_reflow<'a>(&'a self, data: &Reflow, possibly_locked_rw_data: &mut Option<MutexGuard<'a, LayoutTaskData>>) {
        // FIXME: Isolate this transmutation into a "bridge" module.
        // FIXME(rust#16366): The following line had to be moved because of a
        // rustc bug. It should be in the next unsafe block.
        let mut node: JS<Node> = unsafe { JS::from_trusted_node_address(data.document_root) };
        let node: &mut LayoutNode = unsafe {
            mem::transmute(&mut node)
        };

        debug!("layout: received layout request for: {:s}", data.url.serialize());
        debug!("layout: parsed Node tree");
        debug!("{:?}", node.dump());

        let mut rw_data = self.lock_rw_data(possibly_locked_rw_data);

        {
            // Reset the image cache.
            let mut local_image_cache = rw_data.local_image_cache.lock();
            local_image_cache.next_round(self.make_on_image_available_cb());
        }

        // TODO: Calculate the "actual viewport":
        // http://www.w3.org/TR/css-device-adapt/#actual-viewport
        let viewport_size = data.window_size.initial_viewport;

        let old_screen_size = rw_data.screen_size;
        let current_screen_size = Size2D(Au::from_frac32_px(viewport_size.width.get()),
                                         Au::from_frac32_px(viewport_size.height.get()));
        rw_data.screen_size = current_screen_size;

        // Create a layout context for use throughout the following passes.
        let mut shared_layout_ctx =
            self.build_shared_layout_context(
                rw_data.deref(),
                node,
                &data.url);

        // Handle conditions where the entire flow tree is invalid.
        let mut needs_dirtying = rw_data.stylesheet_dirty;

        let mut needs_reflow = current_screen_size != old_screen_size;

        // If the entire flow tree is invalid, then it will be reflowed anyhow.
        needs_reflow &= !needs_dirtying;

        unsafe {
            if needs_dirtying {
                LayoutTask::dirty_all_nodes(node);
            }
        }

        if needs_reflow {
            self.try_get_layout_root(*node).map(
                |mut flow| LayoutTask::reflow_all_nodes(flow.deref_mut()));
        }

        rw_data.stylesheet_dirty = false;

        let mut layout_root = profile(time::LayoutStyleRecalcCategory,
                                      Some((&data.url,
                                      data.iframe,
                                      self.first_reflow.get())),
                                      self.time_profiler_chan.clone(),
                                      || {
            // Perform CSS selector matching and flow construction.
            let rw_data = rw_data.deref_mut();
            match rw_data.parallel_traversal {
                None => {
                    sequential::traverse_dom_preorder(*node, &shared_layout_ctx);
                }
                Some(ref mut traversal) => {
                    parallel::traverse_dom_preorder(*node, &shared_layout_ctx, traversal)
                }
            }

            self.get_layout_root((*node).clone())
        });

        profile(time::LayoutRestyleDamagePropagation,
                Some((&data.url, data.iframe, self.first_reflow.get())),
                self.time_profiler_chan.clone(),
                || {
            layout_root.propagate_restyle_damage();
        });

        // Verification of the flow tree, which ensures that all nodes were either marked as leaves
        // or as non-leaves. This becomes a no-op in release builds. (It is inconsequential to
        // memory safety but is a useful debugging tool.)
        self.verify_flow_tree(&mut layout_root);

        if opts::get().trace_layout {
            layout_debug::begin_trace(layout_root.clone());
        }

        // Perform the primary layout passes over the flow tree to compute the locations of all
        // the boxes.
        profile(time::LayoutMainCategory, Some((&data.url, data.iframe, self.first_reflow.get())),
                self.time_profiler_chan.clone(), || {
            let rw_data = rw_data.deref_mut();
            match rw_data.parallel_traversal {
                None => {
                    // Sequential mode.
                    self.solve_constraints(&mut layout_root, &shared_layout_ctx)
                }
                Some(_) => {
                    // Parallel mode.
                    self.solve_constraints_parallel(data, rw_data, &mut layout_root, &mut shared_layout_ctx)
                }
            }
        });

        // Build the display list if necessary, and send it to the renderer.
        if data.goal == ReflowForDisplay {
            let writing_mode = flow::base(layout_root.deref()).writing_mode;
            profile(time::LayoutDispListBuildCategory, Some((&data.url, data.iframe, self.first_reflow.get())), self.time_profiler_chan.clone(), || {
                shared_layout_ctx.dirty = flow::base(layout_root.deref()).position.to_physical(
                    writing_mode, rw_data.screen_size);
                flow::mut_base(layout_root.deref_mut()).abs_position =
                    LogicalPoint::zero(writing_mode).to_physical(writing_mode, rw_data.screen_size);

                let rw_data = rw_data.deref_mut();
                match rw_data.parallel_traversal {
                    None => {
                        sequential::build_display_list_for_subtree(
                            &mut layout_root,
                            &shared_layout_ctx);
                    }
                    Some(ref mut traversal) => {
                        parallel::build_display_list_for_subtree(
                            &mut layout_root,
                            &data.url,
                            data.iframe,
                            self.first_reflow.get(),
                            self.time_profiler_chan.clone(),
                            &shared_layout_ctx,
                            traversal);
                    }
                }

                debug!("Done building display list. Display List = {}",
                       flow::base(layout_root.deref()).display_list);

                let root_display_list =
                    mem::replace(&mut flow::mut_base(layout_root.deref_mut()).display_list,
                                 DisplayList::new());
                root_display_list.debug();
                let display_list = Arc::new(root_display_list.flatten(ContentStackingLevel));

                // FIXME(pcwalton): This is really ugly and can't handle overflow: scroll. Refactor
                // it with extreme prejudice.
                let mut color = color::rgba(1.0, 1.0, 1.0, 1.0);
                for child in node.traverse_preorder() {
                    if child.type_id() == Some(ElementNodeTypeId(HTMLHtmlElementTypeId)) ||
                            child.type_id() == Some(ElementNodeTypeId(HTMLBodyElementTypeId)) {
                        let element_bg_color = {
                            let thread_safe_child = ThreadSafeLayoutNode::new(&child);
                            thread_safe_child.style()
                                             .resolve_color(thread_safe_child.style()
                                                                             .get_background()
                                                                             .background_color)
                                             .to_gfx_color()
                        };
                        match element_bg_color {
                            color::rgba(0., 0., 0., 0.) => {}
                            _ => {
                                color = element_bg_color;
                                break;
                           }
                        }
                    }
                }

                let root_size = {
                    let root_flow = flow::base(layout_root.deref());
                    root_flow.position.size.to_physical(root_flow.writing_mode)
                };
                let root_size = Size2D(root_size.width.to_nearest_px() as uint,
                                       root_size.height.to_nearest_px() as uint);
                let render_layer = RenderLayer {
                    id: layout_root.layer_id(0),
                    display_list: display_list.clone(),
                    position: Rect(Point2D(0u, 0u), root_size),
                    background_color: color,
                    scroll_policy: Scrollable,
                };

                rw_data.display_list = Some(display_list.clone());

                // TODO(pcwalton): Eventually, when we have incremental reflow, this will have to
                // be smarter in order to handle retained layer contents properly from reflow to
                // reflow.
                let mut layers = SmallVec1::new();
                layers.push(render_layer);
                for layer in mem::replace(&mut flow::mut_base(layout_root.deref_mut()).layers,
                                          DList::new()).into_iter() {
                    layers.push(layer)
                }

                debug!("Layout done!");

                self.render_chan.send(RenderInitMsg(layers));
            });
        }

        self.first_reflow.set(false);

        if opts::get().trace_layout {
            layout_debug::end_trace();
        }

        if opts::get().dump_flow_tree {
            layout_root.dump();
        }

        rw_data.generation += 1;

        // Tell script that we're done.
        //
        // FIXME(pcwalton): This should probably be *one* channel, but we can't fix this without
        // either select or a filtered recv() that only looks for messages of a given type.
        data.script_join_chan.send(());
        let ScriptControlChan(ref chan) = data.script_chan;
        chan.send(ReflowCompleteMsg(self.id, data.id));
    }

    unsafe fn dirty_all_nodes(node: &mut LayoutNode) {
        for node in node.traverse_preorder() {
            // TODO(cgaebel): mark nodes which are sensitive to media queries as
            // "changed":
            // > node.set_changed(true);
            node.set_dirty(true);
            node.set_dirty_siblings(true);
            node.set_dirty_descendants(true);
        }
    }

    fn reflow_all_nodes(flow: &mut Flow) {
        flow::mut_base(flow).restyle_damage.insert(Reflow | Repaint);

        for child in flow::child_iter(flow) {
            LayoutTask::reflow_all_nodes(child);
        }
    }

    // When images can't be loaded in time to display they trigger
    // this callback in some task somewhere. This will send a message
    // to the script task, and ultimately cause the image to be
    // re-requested. We probably don't need to go all the way back to
    // the script task for this.
    fn make_on_image_available_cb(&self) -> Box<ImageResponder<UntrustedNodeAddress>+Send> {
        // This has a crazy signature because the image cache needs to
        // make multiple copies of the callback, and the dom event
        // channel is not a copyable type, so this is actually a
        // little factory to produce callbacks
        box LayoutImageResponder {
            id: self.id.clone(),
            script_chan: self.script_chan.clone(),
        } as Box<ImageResponder<UntrustedNodeAddress>+Send>
    }

    /// Handles a message to destroy layout data. Layout data must be destroyed on *this* task
    /// because it contains local managed pointers.
    unsafe fn handle_reap_layout_data(layout_data: LayoutDataRef) {
        let mut layout_data_ref = layout_data.borrow_mut();
        let _: Option<LayoutDataWrapper> = mem::transmute(
            mem::replace(&mut *layout_data_ref, None));
    }
}

struct LayoutRPCImpl(Arc<Mutex<LayoutTaskData>>);

impl LayoutRPC for LayoutRPCImpl {
    // The neat thing here is that in order to answer the following two queries we only
    // need to compare nodes for equality. Thus we can safely work only with `OpaqueNode`.
    fn content_box(&self, node: TrustedNodeAddress) -> ContentBoxResponse {
        let node: OpaqueNode = OpaqueNodeMethods::from_script_node(node);
        fn union_boxes_for_node(accumulator: &mut Option<Rect<Au>>,
                                mut iter: DisplayItemIterator,
                                node: OpaqueNode) {
            for item in iter {
                if item.base().node == node {
                    match *accumulator {
                        None => *accumulator = Some(item.base().bounds),
                        Some(ref mut acc) => *acc = acc.union(&item.base().bounds),
                    }
                }
            }
        }

        let mut rect = None;
        {
            let &LayoutRPCImpl(ref rw_data) = self;
            let rw_data = rw_data.lock();
            match rw_data.display_list {
                None => fail!("no display list!"),
                Some(ref display_list) => {
                    union_boxes_for_node(&mut rect, display_list.iter(), node)
                }
            }
        }
        ContentBoxResponse(rect.unwrap_or(Rect::zero()))
    }

    /// Requests the dimensions of all the content boxes, as in the `getClientRects()` call.
    fn content_boxes(&self, node: TrustedNodeAddress) -> ContentBoxesResponse {
        let node: OpaqueNode = OpaqueNodeMethods::from_script_node(node);

        fn add_boxes_for_node(accumulator: &mut Vec<Rect<Au>>,
                              mut iter: DisplayItemIterator,
                              node: OpaqueNode) {
            for item in iter {
                if item.base().node == node {
                    accumulator.push(item.base().bounds)
                }
            }
        }

        let mut boxes = vec!();
        {
            let &LayoutRPCImpl(ref rw_data) = self;
            let rw_data = rw_data.lock();
            match rw_data.display_list {
                None => fail!("no display list!"),
                Some(ref display_list) => {
                    add_boxes_for_node(&mut boxes, display_list.iter(), node)
                }
            }
        }
        ContentBoxesResponse(boxes)
    }

    /// Requests the node containing the point of interest
    fn hit_test(&self, _: TrustedNodeAddress, point: Point2D<f32>) -> Result<HitTestResponse, ()> {
        fn hit_test<'a,I>(point: Point2D<Au>, mut iterator: I)
                          -> Option<HitTestResponse>
                          where I: Iterator<&'a DisplayItem> {
            for item in iterator {
                // TODO(tikue): This check should really be performed by a method of `DisplayItem`.
                if geometry::rect_contains_point(item.base().clip_rect, point) &&
                        geometry::rect_contains_point(item.bounds(), point) {
                    return Some(HitTestResponse(item.base().node.to_untrusted_node_address()))
                }
            }
            None
        }
        let point = Point2D(Au::from_frac_px(point.x as f64), Au::from_frac_px(point.y as f64));
        let resp = {
            let &LayoutRPCImpl(ref rw_data) = self;
            let rw_data = rw_data.lock();
            match rw_data.display_list {
                None => fail!("no display list!"),
                Some(ref display_list) => hit_test(point, display_list.list.iter().rev()),
            }
        };

        if resp.is_some() {
            return Ok(resp.unwrap());
        }
        Err(())
    }

    fn mouse_over(&self, _: TrustedNodeAddress, point: Point2D<f32>)
                  -> Result<MouseOverResponse, ()> {
        fn mouse_over_test<'a,I>(point: Point2D<Au>,
                                 mut iterator: I,
                                 result: &mut Vec<UntrustedNodeAddress>)
                                 where I: Iterator<&'a DisplayItem> {
            for item in iterator {
                // TODO(tikue): This check should really be performed by a method of `DisplayItem`.
                if geometry::rect_contains_point(item.bounds(), point) {
                    result.push(item.base().node.to_untrusted_node_address())
                }
            }
        }

        let mut mouse_over_list: Vec<UntrustedNodeAddress> = vec!();
        let point = Point2D(Au::from_frac_px(point.x as f64), Au::from_frac_px(point.y as f64));
        {
            let &LayoutRPCImpl(ref rw_data) = self;
            let rw_data = rw_data.lock();
            match rw_data.display_list {
                None => fail!("no display list!"),
                Some(ref display_list) => {
                    mouse_over_test(point, display_list.list.iter().rev(), &mut mouse_over_list);
                }
            };
        }

        if mouse_over_list.is_empty() {
            Err(())
        } else {
            Ok(MouseOverResponse(mouse_over_list))
        }
    }
}