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diff --git a/servo/components/style/shared_lock.rs b/servo/components/style/shared_lock.rs
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+++ b/servo/components/style/shared_lock.rs
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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 }
+    }
+
+    #[cfg(feature = "gecko")]
+    #[inline]
+    fn ptr(&self) -> *const SomethingZeroSizedButTyped {
+        self.cell
+            .as_ref()
+            .map(|cell| cell.as_ptr() as *const _)
+            .unwrap_or(ptr::null())
+    }
+
+    /// 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() }
+    }
+}
+
+impl<'a> SharedRwLockReadGuard<'a> {
+    #[inline]
+    #[cfg(feature = "gecko")]
+    fn ptr(&self) -> *const SomethingZeroSizedButTyped {
+        self.0
+            .as_ref()
+            .map(|r| &**r as *const _)
+            .unwrap_or(ptr::null())
+    }
+}
+
+/// 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, ptr: *const SomethingZeroSizedButTyped) -> bool {
+        ptr::eq(self.shared_lock.ptr(), ptr)
+    }
+
+    /// 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.ptr()),
+            "Locked::read_with called with a guard from an unrelated SharedRwLock: {:?} vs. {:?}",
+            self.shared_lock.ptr(),
+            guard.ptr(),
+        );
+        #[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(&*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,
+        }
+    }
+}