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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 */
//! Stack-scoped thread-local storage for rayon thread pools.
use crate::global_style_data::STYLO_MAX_THREADS;
use rayon;
use std::cell::{Ref, RefCell, RefMut};
use std::ops::DerefMut;
/// A scoped TLS set, that is alive during the `'scope` lifetime.
/// We use this on Servo to construct thread-local contexts, but clear them once
/// we're done with restyling.
/// Note that the cleanup is done on the thread that owns the scoped TLS, thus
/// the Send bound.
pub struct ScopedTLS<'scope, T: Send> {
pool: Option<&'scope rayon::ThreadPool>,
slots: [RefCell<Option<T>>; STYLO_MAX_THREADS],
/// The scoped TLS is `Sync` because no more than one worker thread can access a
/// given slot.
unsafe impl<'scope, T: Send> Sync for ScopedTLS<'scope, T> {}
impl<'scope, T: Send> ScopedTLS<'scope, T> {
/// Create a new scoped TLS that will last as long as this rayon threadpool
/// reference.
pub fn new(pool: Option<&'scope rayon::ThreadPool>) -> Self {
debug_assert!(pool.map_or(true, |p| p.current_num_threads() <= STYLO_MAX_THREADS));
ScopedTLS {
slots: Default::default(),
/// Returns the index corresponding to the calling thread in the thread pool.
pub fn current_thread_index(&self) -> usize {
self.pool.map_or(0, |p| p.current_thread_index().unwrap())
/// Return an immutable reference to the `Option<T>` that this thread owns.
pub fn borrow(&self) -> Ref<Option<T>> {
let idx = self.current_thread_index();
/// Return a mutable reference to the `Option<T>` that this thread owns.
pub fn borrow_mut(&self) -> RefMut<Option<T>> {
let idx = self.current_thread_index();
/// Ensure that the current data this thread owns is initialized, or
/// initialize it using `f`. We want ensure() to be fast and inline, and we
/// want to inline the memmove that initializes the Option<T>. But we don't
/// want to inline space for the entire large T struct in our stack frame.
/// That's why we hand `f` a mutable borrow to write to instead of just
/// having it return a T.
pub fn ensure<F: FnOnce(&mut Option<T>)>(&self, f: F) -> RefMut<T> {
let mut opt = self.borrow_mut();
if opt.is_none() {
RefMut::map(opt, |x| x.as_mut().unwrap())
/// Returns the slots. Safe because if we have a mut reference the tls can't be referenced by
/// any other thread.
pub fn slots(&mut self) -> &mut [RefCell<Option<T>>] {
&mut self.slots