Introduce trait WeakReferenceable

This allows to take weak references of JS-managed DOM objects.

components/script/dom/bindings/weakref.rs

@@ -0,0 +1,175 @@
+/* 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/. */
+
+//! Weak-referenceable JS-managed DOM objects.
+//!
+//! IDL interfaces marked as `weakReferenceable` in `Bindings.conf`
+//! automatically implement the `WeakReferenceable` trait in codegen.
+//! The instance object is responsible for setting `None` in its own
+//! own `WeakBox` when it is collected, through the `DOM_WEAK_SLOT`
+//! slot. When all associated `WeakRef` values are dropped, the
+//! `WeakBox` itself is dropped too.
+
+use core::nonzero::NonZero;
+use dom::bindings::js::Root;
+use dom::bindings::reflector::Reflectable;
+use dom::bindings::trace::JSTraceable;
+use js::jsapi::{JSTracer, JS_GetReservedSlot, JS_SetReservedSlot};
+use js::jsval::PrivateValue;
+use libc::c_void;
+use std::cell::{Cell, UnsafeCell};
+use std::mem;
+use util::mem::HeapSizeOf;
+
+/// The index of the slot wherein a pointer to the weak holder cell is
+/// stored for weak-referenceable bindings. We use slot 1 for holding it,
+/// this is unsafe for globals, we disallow weak-referenceable globals
+/// directly in codegen.
+pub const DOM_WEAK_SLOT: u32 = 1;
+
+/// A weak reference to a JS-managed DOM object.
+#[allow_unrooted_interior]
+pub struct WeakRef<T: WeakReferenceable> {
+    ptr: NonZero<*mut WeakBox<T>>,
+}
+
+/// The inner box of weak references, public for the finalization in codegen.
+#[must_root]
+pub struct WeakBox<T: WeakReferenceable> {
+    /// The reference count. When it reaches zero, the `value` field should
+    /// have already been set to `None`. The pointee contributes one to the count.
+    pub count: Cell<usize>,
+    /// The pointer to the JS-managed object, set to None when it is collected.
+    pub value: Cell<Option<NonZero<*const T>>>,
+}
+
+/// Trait implemented by weak-referenceable interfaces.
+pub trait WeakReferenceable: Reflectable + Sized {
+    /// Downgrade a DOM object reference to a weak one.
+    fn downgrade(&self) -> WeakRef<Self> {
+        unsafe {
+            let object = self.reflector().get_jsobject().get();
+            let mut ptr =
+                JS_GetReservedSlot(object, DOM_WEAK_SLOT).to_private() as *mut WeakBox<Self>;
+            if ptr.is_null() {
+                debug!("Creating new WeakBox holder for {:p}.", self);
+                ptr = Box::into_raw(box WeakBox {
+                    count: Cell::new(1),
+                    value: Cell::new(Some(NonZero::new(self))),
+                });
+                JS_SetReservedSlot(object, DOM_WEAK_SLOT, PrivateValue(ptr as *const c_void));
+            }
+            let box_ = &*ptr;
+            assert!(box_.value.get().is_some());
+            let new_count = box_.count.get() + 1;
+            debug!("Incrementing WeakBox refcount for {:p} to {}.", self, new_count);
+            box_.count.set(new_count);
+            WeakRef { ptr: NonZero::new(ptr) }
+        }
+    }
+}
+
+impl<T: WeakReferenceable> WeakRef<T> {
+    /// Create a new weak reference from a `WeakReferenceable` interface instance.
+    /// This is just a convenience wrapper around `<T as WeakReferenceable>::downgrade`
+    /// to not have to import `WeakReferenceable`.
+    pub fn new(value: &T) -> Self {
+        value.downgrade()
+    }
+
+    /// Root a weak reference. Returns `None` if the object was already collected.
+    pub fn root(&self) -> Option<Root<T>> {
+        unsafe { &**self.ptr }.value.get().map(Root::new)
+    }
+
+    /// Return whether the weakly-referenced object is still alive.
+    pub fn is_alive(&self) -> bool {
+        unsafe { &**self.ptr }.value.get().is_some()
+    }
+}
+
+impl<T: WeakReferenceable> Clone for WeakRef<T> {
+    fn clone(&self) -> WeakRef<T> {
+        unsafe {
+            let box_ = &**self.ptr;
+            let new_count = box_.count.get() + 1;
+            box_.count.set(new_count);
+            WeakRef { ptr: self.ptr }
+        }
+    }
+}
+
+impl<T: WeakReferenceable> HeapSizeOf for WeakRef<T> {
+    fn heap_size_of_children(&self) -> usize {
+        0
+    }
+}
+
+no_jsmanaged_fields!(WeakRef<T: WeakReferenceable>);
+
+impl<T: WeakReferenceable> Drop for WeakRef<T> {
+    fn drop(&mut self) {
+        unsafe {
+            let (count, value) = {
+                let weak_box = &**self.ptr;
+                assert!(weak_box.count.get() > 0);
+                let count = weak_box.count.get() - 1;
+                weak_box.count.set(count);
+                (count, weak_box.value.get())
+            };
+            if count == 0 {
+                assert!(value.is_none());
+                mem::drop(Box::from_raw(*self.ptr));
+            }
+        }
+    }
+}
+
+/// A mutable weak reference to a JS-managed DOM object. On tracing,
+/// the contained weak reference is dropped if the pointee was already
+/// collected.
+pub struct MutableWeakRef<T: WeakReferenceable> {
+    cell: UnsafeCell<Option<WeakRef<T>>>,
+}
+
+impl<T: WeakReferenceable> MutableWeakRef<T> {
+    /// Create a new mutable weak reference.
+    pub fn new(value: Option<&T>) -> MutableWeakRef<T> {
+        MutableWeakRef {
+            cell: UnsafeCell::new(value.map(WeakRef::new)),
+        }
+    }
+
+    /// Set the pointee of a mutable weak reference.
+    pub fn set(&self, value: Option<&T>) {
+        unsafe { *self.cell.get() = value.map(WeakRef::new); }
+    }
+
+    /// Root a mutable weak reference. Returns `None` if the object
+    /// was already collected.
+    pub fn root(&self) -> Option<Root<T>> {
+        unsafe { &*self.cell.get() }.as_ref().and_then(WeakRef::root)
+    }
+}
+
+impl<T: WeakReferenceable> HeapSizeOf for MutableWeakRef<T> {
+    fn heap_size_of_children(&self) -> usize {
+        0
+    }
+}
+
+impl<T: WeakReferenceable> JSTraceable for MutableWeakRef<T> {
+    fn trace(&self, _: *mut JSTracer) {
+        let ptr = self.cell.get();
+        unsafe {
+            let should_drop = match *ptr {
+                Some(ref value) => !value.is_alive(),
+                None => false,
+            };
+            if should_drop {
+                mem::drop((*ptr).take().unwrap());
+            }
+        }
+    }
+}