Introduce trait WeakReferenceable

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

components/script/dom/bindings/codegen/Bindings.conf

@@ -21,4 +21,8 @@ DOMInterfaces = {
 #FIXME(jdm): This should be 'register': False, but then we don't generate enum types
 'TestBinding': {},
 
+'URL': {
+    'weakReferenceable': True,
+}
+
 }

components/script/dom/bindings/codegen/CodegenRust.py

@@ -1765,11 +1765,15 @@ class CGDOMJSClass(CGThing):
     def define(self):
         traceHook = 'Some(%s)' % TRACE_HOOK_NAME
         if self.descriptor.isGlobal():
+            assert not self.descriptor.weakReferenceable
             traceHook = "Some(js::jsapi::JS_GlobalObjectTraceHook)"
             flags = "JSCLASS_IS_GLOBAL | JSCLASS_DOM_GLOBAL"
             slots = "JSCLASS_GLOBAL_SLOT_COUNT + 1"
         else:
             flags = "0"
+            if self.descriptor.weakReferenceable:
+                slots = "2"
+            else:
                 slots = "1"
         return """\
 static Class: DOMJSClass = DOMJSClass {
@@ -2175,6 +2179,9 @@ let obj = RootedObject::new(cx, obj);\
                    "\n"
                    "JS_SetReservedSlot(obj.ptr, DOM_OBJECT_SLOT,\n"
                    "                   PrivateValue(raw as *const libc::c_void));")
+    if descriptor.weakReferenceable:
+        create += """
+JS_SetReservedSlot(obj.ptr, DOM_WEAK_SLOT, PrivateValue(ptr::null()));"""
     return create
 
 
@@ -4521,8 +4528,8 @@ class CGAbstractClassHook(CGAbstractExternMethod):
                                         args)
 
     def definition_body_prologue(self):
-        return CGGeneric("""\
-let this = private_from_object(obj) as *const %s;
+        return CGGeneric("""
+let this = native_from_object::<%s>(obj).unwrap();
 """ % self.descriptor.concreteType)
 
     def definition_body(self):
@@ -4541,6 +4548,24 @@ def finalizeHook(descriptor, hookName, context):
         release += """\
 finalize_global(obj);
 """
+    elif descriptor.weakReferenceable:
+        release += """\
+let weak_box_ptr = JS_GetReservedSlot(obj, DOM_WEAK_SLOT).to_private() as *mut WeakBox<%s>;
+if !weak_box_ptr.is_null() {
+    let count = {
+        let weak_box = &*weak_box_ptr;
+        assert!(weak_box.value.get().is_some());
+        assert!(weak_box.count.get() > 0);
+        weak_box.value.set(None);
+        let count = weak_box.count.get() - 1;
+        weak_box.count.set(count);
+        count
+    };
+    if count == 0 {
+        mem::drop(Box::from_raw(weak_box_ptr));
+    }
+}
+""" % descriptor.concreteType
     release += """\
 let _ = Box::from_raw(this as *mut %s);
 debug!("%s finalize: {:p}", this);\
@@ -4718,6 +4743,16 @@ class CGInterfaceTrait(CGThing):
         return self.cgRoot.define()
 
 
+class CGWeakReferenceableTrait(CGThing):
+    def __init__(self, descriptor):
+        CGThing.__init__(self)
+        assert descriptor.weakReferenceable
+        self.code = "impl WeakReferenceable for %s {}" % descriptor.interface.identifier.name
+
+    def define(self):
+        return self.code
+
+
 class CGDescriptor(CGThing):
     def __init__(self, descriptor):
         CGThing.__init__(self)
@@ -4826,6 +4861,8 @@ class CGDescriptor(CGThing):
         if not descriptor.interface.isCallback():
             cgThings.append(CGIDLInterface(descriptor))
             cgThings.append(CGInterfaceTrait(descriptor))
+            if descriptor.weakReferenceable:
+                cgThings.append(CGWeakReferenceableTrait(descriptor))
 
         cgThings = CGList(cgThings, "\n")
         # self.cgRoot = CGWrapper(CGNamespace(toBindingNamespace(descriptor.name),
@@ -5215,7 +5252,7 @@ class CGBindingRoot(CGThing):
             'dom::bindings::conversions::{ConversionBehavior, DOM_OBJECT_SLOT, IDLInterface}',
             'dom::bindings::conversions::{FromJSValConvertible, StringificationBehavior}',
             'dom::bindings::conversions::{ToJSValConvertible, jsid_to_str, native_from_handlevalue}',
-            'dom::bindings::conversions::{private_from_object, root_from_object}',
+            'dom::bindings::conversions::{native_from_object, private_from_object, root_from_object}',
             'dom::bindings::conversions::{root_from_handleobject, root_from_handlevalue}',
             'dom::bindings::codegen::{PrototypeList, RegisterBindings, UnionTypes}',
             'dom::bindings::codegen::Bindings::*',
@@ -5230,6 +5267,7 @@ class CGBindingRoot(CGThing):
             'dom::bindings::str::ByteString',
             'dom::bindings::str::USVString',
             'dom::bindings::trace::RootedVec',
+            'dom::bindings::weakref::{DOM_WEAK_SLOT, WeakBox, WeakReferenceable}',
             'mem::heap_size_of_raw_self_and_children',
             'libc',
             'util::str::DOMString',

components/script/dom/bindings/codegen/Configuration.py

@@ -168,6 +168,7 @@ class Descriptor(DescriptorProvider):
         self.register = desc.get('register', True)
         self.outerObjectHook = desc.get('outerObjectHook', 'None')
         self.proxy = False
+        self.weakReferenceable = desc.get('weakReferenceable', False)
 
         # If we're concrete, we need to crawl our ancestor interfaces and mark
         # them as having a concrete descendant.

components/script/dom/bindings/conversions.rs

@@ -729,7 +729,8 @@ pub unsafe fn private_from_proto_check<F>(mut obj: *mut JSObject, proto_check: F
     }
 }
 
-fn native_from_object<T>(obj: *mut JSObject) -> Result<*const T, ()>
+/// Get a `*const T` for a DOM object accessible from a `JSObject`.
+pub fn native_from_object<T>(obj: *mut JSObject) -> Result<*const T, ()>
     where T: Reflectable + IDLInterface
 {
     unsafe {

components/script/dom/bindings/mod.rs

@@ -147,6 +147,7 @@ pub mod str;
 pub mod structuredclone;
 pub mod trace;
 pub mod utils;
+pub mod weakref;
 pub mod xmlname;
 
 /// Generated JS-Rust bindings.

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());
+            }
+        }
+    }
+}

components/script/dom/testbinding.rs

@@ -15,7 +15,9 @@ use dom::bindings::js::Root;
 use dom::bindings::num::Finite;
 use dom::bindings::reflector::{Reflector, reflect_dom_object};
 use dom::bindings::str::{ByteString, USVString};
+use dom::bindings::weakref::MutableWeakRef;
 use dom::blob::Blob;
+use dom::url::URL;
 use js::jsapi::{HandleValue, JSContext, JSObject};
 use js::jsval::{JSVal, NullValue};
 use std::borrow::ToOwned;
@@ -26,12 +28,14 @@ use util::str::DOMString;
 #[dom_struct]
 pub struct TestBinding {
     reflector_: Reflector,
+    url: MutableWeakRef<URL>,
 }
 
 impl TestBinding {
     fn new_inherited() -> TestBinding {
         TestBinding {
             reflector_: Reflector::new(),
+            url: MutableWeakRef::new(None),
         }
     }
 
@@ -142,6 +146,12 @@ impl TestBindingMethods for TestBinding {
         Some(Blob::new(global_root_from_reflector(self).r(), None, ""))
     }
     fn SetInterfaceAttributeNullable(&self, _: Option<&Blob>) {}
+    fn GetInterfaceAttributeWeak(&self) -> Option<Root<URL>> {
+        self.url.root()
+    }
+    fn SetInterfaceAttributeWeak(&self, url: Option<&URL>) {
+        self.url.set(url);
+    }
     fn GetObjectAttributeNullable(&self, _: *mut JSContext) -> *mut JSObject { ptr::null_mut() }
     fn SetObjectAttributeNullable(&self, _: *mut JSContext, _: *mut JSObject) {}
     fn GetUnionAttributeNullable(&self) -> Option<HTMLElementOrLong> {

components/script/dom/webidls/TestBinding.webidl

@@ -113,6 +113,7 @@ interface TestBinding {
            attribute ByteString? byteStringAttributeNullable;
   readonly attribute TestEnum? enumAttributeNullable;
            attribute Blob? interfaceAttributeNullable;
+           attribute URL? interfaceAttributeWeak;
            attribute object? objectAttributeNullable;
            attribute (HTMLElement or long)? unionAttributeNullable;
            attribute (Event or DOMString)? union2AttributeNullable;

tests/wpt/mozilla/meta/MANIFEST.json

@@ -7,6 +7,10 @@
       {
         "path": "mozilla/prototypes.html",
         "url": "/_mozilla/mozilla/prototypes.html"
+      },
+      {
+        "path": "mozilla/weakref.html",
+        "url": "/_mozilla/mozilla/weakref.html"
       }
     ],
     "wdspec": []

tests/wpt/mozilla/tests/mozilla/weakref.html

@@ -0,0 +1,28 @@
+<html>
+<head>
+<script src="/resources/testharness.js"></script>
+<script src="/resources/testharnessreport.js"></script>
+</head>
+<script>
+test(function() {
+  // We don't use assert_equals() etc here because it somehow
+  // keeps hold of its passed arguments, and thus the weak
+  // reference is never freed before the end of the test.
+
+  var t = new TestBinding;
+  assert_true(t.interfaceAttributeWeak === null);
+
+  var url = new URL("http://blog.servo.org/");
+  t.interfaceAttributeWeak = url;
+  assert_true(t.interfaceAttributeWeak !== null);
+
+  gc();
+  assert_true(t.interfaceAttributeWeak !== null);
+
+  url = null;
+  gc();
+
+  assert_true(t.interfaceAttributeWeak === null);
+}, "Weak references work");
+</script>
+</html>