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/* 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/. */
//! Implements parallel traversal over the DOM tree.
//!
//! This traversal is based on Rayon, and therefore its safety is largely
//! verified by the type system.
//!
//! The primary trickiness and fine print for the above relates to the
//! thread safety of the DOM nodes themselves. Accessing a DOM element
//! concurrently on multiple threads is actually mostly "safe", since all
//! the mutable state is protected by an AtomicRefCell, and so we'll
//! generally panic if something goes wrong. Still, we try to to enforce our
//! thread invariants at compile time whenever possible. As such, TNode and
//! TElement are not Send, so ordinary style system code cannot accidentally
//! share them with other threads. In the parallel traversal, we explicitly
//! invoke |unsafe { SendNode::new(n) }| to put nodes in containers that may
//! be sent to other threads. This occurs in only a handful of places and is
//! easy to grep for. At the time of this writing, there is no other unsafe
//! code in the parallel traversal.
#![deny(missing_docs)]
use context::TraversalStatistics;
use dom::{OpaqueNode, SendNode, TElement, TNode};
use rayon;
use scoped_tls::ScopedTLS;
use std::borrow::Borrow;
use time;
use traversal::{DomTraversal, PerLevelTraversalData, PreTraverseToken};
/// The chunk size used to split the parallel traversal nodes.
///
/// We send each `CHUNK_SIZE` nodes as a different work unit to the work queue.
pub const CHUNK_SIZE: usize = 64;
/// A parallel top down traversal, generic over `D`.
#[allow(unsafe_code)]
pub fn traverse_dom<E, D>(traversal: &D,
root: E,
known_root_dom_depth: Option<usize>,
token: PreTraverseToken,
queue: &rayon::ThreadPool)
where E: TElement,
D: DomTraversal<E>,
{
let dump_stats = TraversalStatistics::should_dump();
let start_time = if dump_stats { Some(time::precise_time_s()) } else { None };
debug_assert!(traversal.is_parallel());
// Handle Gecko's eager initial styling. We don't currently support it
// in conjunction with bottom-up traversal. If we did, we'd need to put
// it on the context to make it available to the bottom-up phase.
let (nodes, depth) = if token.traverse_unstyled_children_only() {
debug_assert!(!D::needs_postorder_traversal());
let mut children = vec![];
for kid in root.as_node().children() {
if kid.as_element().map_or(false, |el| el.get_data().is_none()) {
children.push(unsafe { SendNode::new(kid) });
}
}
(children, known_root_dom_depth.map(|x| x + 1))
} else {
(vec![unsafe { SendNode::new(root.as_node()) }], known_root_dom_depth)
};
let traversal_data = PerLevelTraversalData {
current_dom_depth: depth,
};
let tls = ScopedTLS::<D::ThreadLocalContext>::new(queue);
let root = root.as_node().opaque();
queue.install(|| {
rayon::scope(|scope| {
traverse_nodes(nodes, root, traversal_data, scope, traversal, &tls);
});
});
// Dump statistics to stdout if requested.
if dump_stats {
let slots = unsafe { tls.unsafe_get() };
let mut aggregate = slots.iter().fold(TraversalStatistics::default(), |acc, t| {
match *t.borrow() {
None => acc,
Some(ref cx) => &cx.borrow().statistics + &acc,
}
});
aggregate.finish(traversal, start_time.unwrap());
println!("{}", aggregate);
}
}
/// A parallel top-down DOM traversal.
#[inline(always)]
#[allow(unsafe_code)]
fn top_down_dom<'a, 'scope, E, D>(nodes: &'a [SendNode<E::ConcreteNode>],
root: OpaqueNode,
mut traversal_data: PerLevelTraversalData,
scope: &'a rayon::Scope<'scope>,
traversal: &'scope D,
tls: &'scope ScopedTLS<'scope, D::ThreadLocalContext>)
where E: TElement + 'scope,
D: DomTraversal<E>,
{
let mut discovered_child_nodes = vec![];
{
// Scope the borrow of the TLS so that the borrow is dropped before
// potentially traversing a child on this thread.
let mut tlc = tls.ensure(|| traversal.create_thread_local_context());
for n in nodes {
// Perform the appropriate traversal.
let node = **n;
let mut children_to_process = 0isize;
traversal.process_preorder(&mut traversal_data, &mut *tlc, node);
if let Some(el) = node.as_element() {
traversal.traverse_children(&mut *tlc, el, |_tlc, kid| {
children_to_process += 1;
discovered_child_nodes.push(unsafe { SendNode::new(kid) })
});
}
traversal.handle_postorder_traversal(&mut *tlc, root, node,
children_to_process);
}
}
if let Some(ref mut depth) = traversal_data.current_dom_depth {
*depth += 1;
}
traverse_nodes(discovered_child_nodes, root, traversal_data, scope, traversal, tls);
}
fn traverse_nodes<'a, 'scope, E, D>(nodes: Vec<SendNode<E::ConcreteNode>>, root: OpaqueNode,
traversal_data: PerLevelTraversalData,
scope: &'a rayon::Scope<'scope>,
traversal: &'scope D,
tls: &'scope ScopedTLS<'scope, D::ThreadLocalContext>)
where E: TElement + 'scope,
D: DomTraversal<E>,
{
if nodes.is_empty() {
return;
}
// Optimization: traverse directly and avoid a heap-allocating spawn() call if
// we're only pushing one work unit.
if nodes.len() <= CHUNK_SIZE {
let nodes = nodes.into_boxed_slice();
top_down_dom(&nodes, root, traversal_data, scope, traversal, tls);
return;
}
// General case.
for chunk in nodes.chunks(CHUNK_SIZE) {
let nodes = chunk.iter().cloned().collect::<Vec<_>>().into_boxed_slice();
let traversal_data = traversal_data.clone();
scope.spawn(move |scope| {
let nodes = nodes;
top_down_dom(&nodes, root, traversal_data, scope, traversal, tls)
})
}
}
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