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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 https://mozilla.org/MPL/2.0/. */
//! Different objects protected by the same lock
use crate::str::{CssString, CssStringWriter};
use crate::stylesheets::Origin;
#[cfg(feature = "gecko")]
use atomic_refcell::{AtomicRef, AtomicRefCell, AtomicRefMut};
#[cfg(feature = "servo")]
use parking_lot::RwLock;
use servo_arc::Arc;
use std::cell::UnsafeCell;
use std::fmt;
#[cfg(feature = "servo")]
use std::mem;
#[cfg(feature = "gecko")]
use std::ptr;
use to_shmem::{SharedMemoryBuilder, ToShmem};
/// A shared read/write lock that can protect multiple objects.
///
/// In Gecko builds, we don't need the blocking behavior, just the safety. As
/// such we implement this with an AtomicRefCell instead in Gecko builds,
/// which is ~2x as fast, and panics (rather than deadlocking) when things go
/// wrong (which is much easier to debug on CI).
///
/// Servo needs the blocking behavior for its unsynchronized animation setup,
/// but that may not be web-compatible and may need to be changed (at which
/// point Servo could use AtomicRefCell too).
///
/// Gecko also needs the ability to have "read only" SharedRwLocks, which are
/// used for objects stored in (read only) shared memory. Attempting to acquire
/// write access to objects protected by a read only SharedRwLock will panic.
#[derive(Clone)]
#[cfg_attr(feature = "servo", derive(MallocSizeOf))]
pub struct SharedRwLock {
#[cfg(feature = "servo")]
#[cfg_attr(feature = "servo", ignore_malloc_size_of = "Arc")]
arc: Arc<RwLock<()>>,
#[cfg(feature = "gecko")]
cell: Option<Arc<AtomicRefCell<SomethingZeroSizedButTyped>>>,
}
#[cfg(feature = "gecko")]
struct SomethingZeroSizedButTyped;
impl fmt::Debug for SharedRwLock {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
f.write_str("SharedRwLock")
}
}
impl SharedRwLock {
/// Create a new shared lock (servo).
#[cfg(feature = "servo")]
pub fn new() -> Self {
SharedRwLock {
arc: Arc::new(RwLock::new(())),
}
}
/// Create a new shared lock (gecko).
#[cfg(feature = "gecko")]
pub fn new() -> Self {
SharedRwLock {
cell: Some(Arc::new(AtomicRefCell::new(SomethingZeroSizedButTyped))),
}
}
/// Create a new global shared lock (servo).
#[cfg(feature = "servo")]
pub fn new_leaked() -> Self {
SharedRwLock {
arc: Arc::new_leaked(RwLock::new(())),
}
}
/// Create a new global shared lock (gecko).
#[cfg(feature = "gecko")]
pub fn new_leaked() -> Self {
SharedRwLock {
cell: Some(Arc::new_leaked(AtomicRefCell::new(
SomethingZeroSizedButTyped,
))),
}
}
/// Create a new read-only shared lock (gecko).
#[cfg(feature = "gecko")]
pub fn read_only() -> Self {
SharedRwLock { cell: None }
}
/// Wrap the given data to make its access protected by this lock.
pub fn wrap<T>(&self, data: T) -> Locked<T> {
Locked {
shared_lock: self.clone(),
data: UnsafeCell::new(data),
}
}
/// Obtain the lock for reading (servo).
#[cfg(feature = "servo")]
pub fn read(&self) -> SharedRwLockReadGuard {
mem::forget(self.arc.read());
SharedRwLockReadGuard(self)
}
/// Obtain the lock for reading (gecko).
#[cfg(feature = "gecko")]
pub fn read(&self) -> SharedRwLockReadGuard {
SharedRwLockReadGuard(self.cell.as_ref().map(|cell| cell.borrow()))
}
/// Obtain the lock for writing (servo).
#[cfg(feature = "servo")]
pub fn write(&self) -> SharedRwLockWriteGuard {
mem::forget(self.arc.write());
SharedRwLockWriteGuard(self)
}
/// Obtain the lock for writing (gecko).
#[cfg(feature = "gecko")]
pub fn write(&self) -> SharedRwLockWriteGuard {
SharedRwLockWriteGuard(self.cell.as_ref().unwrap().borrow_mut())
}
}
/// Proof that a shared lock was obtained for reading (servo).
#[cfg(feature = "servo")]
pub struct SharedRwLockReadGuard<'a>(&'a SharedRwLock);
/// Proof that a shared lock was obtained for reading (gecko).
#[cfg(feature = "gecko")]
pub struct SharedRwLockReadGuard<'a>(Option<AtomicRef<'a, SomethingZeroSizedButTyped>>);
#[cfg(feature = "servo")]
impl<'a> Drop for SharedRwLockReadGuard<'a> {
fn drop(&mut self) {
// Unsafe: self.lock is private to this module, only ever set after `read()`,
// and never copied or cloned (see `compile_time_assert` below).
unsafe { self.0.arc.force_unlock_read() }
}
}
/// Proof that a shared lock was obtained for writing (servo).
#[cfg(feature = "servo")]
pub struct SharedRwLockWriteGuard<'a>(&'a SharedRwLock);
/// Proof that a shared lock was obtained for writing (gecko).
#[cfg(feature = "gecko")]
pub struct SharedRwLockWriteGuard<'a>(AtomicRefMut<'a, SomethingZeroSizedButTyped>);
#[cfg(feature = "servo")]
impl<'a> Drop for SharedRwLockWriteGuard<'a> {
fn drop(&mut self) {
// Unsafe: self.lock is private to this module, only ever set after `write()`,
// and never copied or cloned (see `compile_time_assert` below).
unsafe { self.0.arc.force_unlock_write() }
}
}
/// Data protect by a shared lock.
pub struct Locked<T> {
shared_lock: SharedRwLock,
data: UnsafeCell<T>,
}
// Unsafe: the data inside `UnsafeCell` is only accessed in `read_with` and `write_with`,
// where guards ensure synchronization.
unsafe impl<T: Send> Send for Locked<T> {}
unsafe impl<T: Send + Sync> Sync for Locked<T> {}
impl<T: fmt::Debug> fmt::Debug for Locked<T> {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
let guard = self.shared_lock.read();
self.read_with(&guard).fmt(f)
}
}
impl<T> Locked<T> {
#[cfg(feature = "gecko")]
#[inline]
fn is_read_only_lock(&self) -> bool {
self.shared_lock.cell.is_none()
}
#[cfg(feature = "servo")]
fn same_lock_as(&self, lock: &SharedRwLock) -> bool {
Arc::ptr_eq(&self.shared_lock.arc, &lock.arc)
}
#[cfg(feature = "gecko")]
fn same_lock_as(&self, derefed_guard: Option<&SomethingZeroSizedButTyped>) -> bool {
ptr::eq(
self.shared_lock
.cell
.as_ref()
.map(|cell| cell.as_ptr())
.unwrap_or(ptr::null_mut()),
derefed_guard
.map(|guard| guard as *const _ as *mut _)
.unwrap_or(ptr::null_mut()),
)
}
/// Access the data for reading.
pub fn read_with<'a>(&'a self, guard: &'a SharedRwLockReadGuard) -> &'a T {
#[cfg(feature = "gecko")]
assert!(
self.is_read_only_lock() || self.same_lock_as(guard.0.as_ref().map(|r| &**r)),
"Locked::read_with called with a guard from an unrelated SharedRwLock"
);
#[cfg(not(feature = "gecko"))]
assert!(self.same_lock_as(&guard.0));
let ptr = self.data.get();
// Unsafe:
//
// * The guard guarantees that the lock is taken for reading,
// and we’ve checked that it’s the correct lock.
// * The returned reference borrows *both* the data and the guard,
// so that it can outlive neither.
unsafe { &*ptr }
}
/// Access the data for reading without verifying the lock. Use with caution.
#[cfg(feature = "gecko")]
pub unsafe fn read_unchecked<'a>(&'a self) -> &'a T {
let ptr = self.data.get();
&*ptr
}
/// Access the data for writing.
pub fn write_with<'a>(&'a self, guard: &'a mut SharedRwLockWriteGuard) -> &'a mut T {
#[cfg(feature = "gecko")]
assert!(
!self.is_read_only_lock() && self.same_lock_as(Some(&guard.0)),
"Locked::write_with called with a guard from a read only or unrelated SharedRwLock"
);
#[cfg(not(feature = "gecko"))]
assert!(self.same_lock_as(&guard.0));
let ptr = self.data.get();
// Unsafe:
//
// * The guard guarantees that the lock is taken for writing,
// and we’ve checked that it’s the correct lock.
// * The returned reference borrows *both* the data and the guard,
// so that it can outlive neither.
// * We require a mutable borrow of the guard,
// so that one write guard can only be used once at a time.
unsafe { &mut *ptr }
}
}
#[cfg(feature = "gecko")]
impl<T: ToShmem> ToShmem for Locked<T> {
fn to_shmem(&self, builder: &mut SharedMemoryBuilder) -> to_shmem::Result<Self> {
use std::mem::ManuallyDrop;
let guard = self.shared_lock.read();
Ok(ManuallyDrop::new(Locked {
shared_lock: SharedRwLock::read_only(),
data: UnsafeCell::new(ManuallyDrop::into_inner(
self.read_with(&guard).to_shmem(builder)?,
)),
}))
}
}
#[cfg(feature = "servo")]
impl<T: ToShmem> ToShmem for Locked<T> {
fn to_shmem(&self, _builder: &mut SharedMemoryBuilder) -> to_shmem::Result<Self> {
panic!("ToShmem not supported in Servo currently")
}
}
#[allow(dead_code)]
mod compile_time_assert {
use super::{SharedRwLockReadGuard, SharedRwLockWriteGuard};
trait Marker1 {}
impl<T: Clone> Marker1 for T {}
impl<'a> Marker1 for SharedRwLockReadGuard<'a> {} // Assert SharedRwLockReadGuard: !Clone
impl<'a> Marker1 for SharedRwLockWriteGuard<'a> {} // Assert SharedRwLockWriteGuard: !Clone
trait Marker2 {}
impl<T: Copy> Marker2 for T {}
impl<'a> Marker2 for SharedRwLockReadGuard<'a> {} // Assert SharedRwLockReadGuard: !Copy
impl<'a> Marker2 for SharedRwLockWriteGuard<'a> {} // Assert SharedRwLockWriteGuard: !Copy
}
/// Like ToCss, but with a lock guard given by the caller, and with the writer specified
/// concretely rather than with a parameter.
pub trait ToCssWithGuard {
/// Serialize `self` in CSS syntax, writing to `dest`, using the given lock guard.
fn to_css(&self, guard: &SharedRwLockReadGuard, dest: &mut CssStringWriter) -> fmt::Result;
/// Serialize `self` in CSS syntax using the given lock guard and return a string.
///
/// (This is a convenience wrapper for `to_css` and probably should not be overridden.)
#[inline]
fn to_css_string(&self, guard: &SharedRwLockReadGuard) -> CssString {
let mut s = CssString::new();
self.to_css(guard, &mut s).unwrap();
s
}
}
/// Parameters needed for deep clones.
#[cfg(feature = "gecko")]
pub struct DeepCloneParams {
/// The new sheet we're cloning rules into.
pub reference_sheet: *const crate::gecko_bindings::structs::StyleSheet,
}
/// Parameters needed for deep clones.
#[cfg(feature = "servo")]
pub struct DeepCloneParams;
/// A trait to do a deep clone of a given CSS type. Gets a lock and a read
/// guard, in order to be able to read and clone nested structures.
pub trait DeepCloneWithLock: Sized {
/// Deep clones this object.
fn deep_clone_with_lock(
&self,
lock: &SharedRwLock,
guard: &SharedRwLockReadGuard,
params: &DeepCloneParams,
) -> Self;
}
/// Guards for a document
#[derive(Clone)]
pub struct StylesheetGuards<'a> {
/// For author-origin stylesheets.
pub author: &'a SharedRwLockReadGuard<'a>,
/// For user-agent-origin and user-origin stylesheets
pub ua_or_user: &'a SharedRwLockReadGuard<'a>,
}
impl<'a> StylesheetGuards<'a> {
/// Get the guard for a given stylesheet origin.
pub fn for_origin(&self, origin: Origin) -> &SharedRwLockReadGuard<'a> {
match origin {
Origin::Author => &self.author,
_ => &self.ua_or_user,
}
}
/// Same guard for all origins
pub fn same(guard: &'a SharedRwLockReadGuard<'a>) -> Self {
StylesheetGuards {
author: guard,
ua_or_user: guard,
}
}
}