Add a helper for implementing Promise-valued properties.

Source/bindings/core/v8/ScriptPromiseProperty.h

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+#ifndef ScriptPromiseProperty_h
+#define ScriptPromiseProperty_h
+
+#include "bindings/core/v8/ScriptPromise.h"
+#include "bindings/core/v8/ScriptPromisePropertyBase.h"
+#include "bindings/core/v8/V8Binding.h"
+#include "wtf/Noncopyable.h"
+#include "wtf/PassRefPtr.h"
+
+namespace WebCore {
+
+class ExecutionContext;
+
+// ScriptPromiseProperty is a helper for implementing a DOM method or
+// attribute whose value is a Promise, and the same Promise must be

[haraken, 2014/07/02 09:04:31]

whose value is a Promise => that returns a Promise

+// returned each time.
+//
+// ScriptPromiseProperty does not keep Promises or worlds alive to
+// deliver Promise resolution/rejection to them; the Promise
+// resolution/rejections are delivered if the wrapper for the object
+// with the property is alive. This is achieved by keeping a weak

[haraken, 2014/07/02 09:04:31]

the wrapper for the object with the property => the holder's wrapper

+// reference from ScriptPromiseProperty to the holder's wrapper, and
+// references in hidden values between the wrapper, promise and
+// resolver (coincidentally the Resolver and Promise may be the same
+// object, but that is an implementation detail of v8.)
+//
+//                                             -----> Resolver
+//                                            /
+// ScriptPromiseProperty - - -> Holder Wrapper <------.
+//                                            \       |
+//                                             -----> Promise
+//
+// To avoid exposing the action of the garbage collector to script,
+// you should keep the wrapper alive as long as a promise may be
+// settled.
+//
+// ScriptPromiseProperty only supports the main world.
+// FIXME: Implement support for isolated worlds.
+template<typename HolderType, typename ResolvedType, typename RejectedType>
+class ScriptPromiseProperty : public ScriptPromisePropertyBase {
+    WTF_MAKE_NONCOPYABLE(ScriptPromiseProperty);
+public:
+    // Creates a ScriptPromiseProperty that will create Promises in
+    // the specified ExecutionContext for a property of 'holder'
+    // (typically ScriptPromiseProperty should be a member of the
+    // property holder). 'name' must be a unique name for the hidden
+    // value backing the property.
+    template<typename PassHolderType>
+    static PassRefPtrWillBeRawPtr<ScriptPromiseProperty<HolderType, ResolvedType, RejectedType> > create(ExecutionContext*, PassHolderType, const char* name);
+
+    virtual ~ScriptPromiseProperty() { }
+
+    template<typename PassResolvedType>
+    void resolve(PassResolvedType);
+
+    template<typename PassRejectedType>
+    void reject(PassRejectedType);
+
+private:
+    template<typename PassHolderType>
+    ScriptPromiseProperty(ExecutionContext*, PassHolderType, const char* name);
+
+    virtual v8::Handle<v8::Object> holder(v8::Handle<v8::Object> creationContext, v8::Isolate*) OVERRIDE;
+    virtual v8::Handle<v8::Value> resolvedValue(v8::Handle<v8::Object> creationContext, v8::Isolate*) OVERRIDE;
+    virtual v8::Handle<v8::Value> rejectedValue(v8::Handle<v8::Object> creationContext, v8::Isolate*) OVERRIDE;
+
+    HolderType m_holder;
+    ResolvedType m_resolved;
+    RejectedType m_rejected;
+};
+
+template<typename HolderType, typename ResolvedType, typename RejectedType>
+template<typename PassHolderType>
+PassRefPtrWillBeRawPtr<ScriptPromiseProperty<HolderType, ResolvedType, RejectedType> > ScriptPromiseProperty<HolderType, ResolvedType, RejectedType>::create(ExecutionContext* executionContext, PassHolderType holder, const char* name)
+{
+    RefPtr<ScriptPromiseProperty<HolderType, ResolvedType, RejectedType> > property = adoptRef(new ScriptPromiseProperty<HolderType, ResolvedType, RejectedType>(executionContext, holder, name));
+    return property.release();

[haraken, 2014/07/02 09:04:31]

return adoptRef(new ScriptPromiseProperty(...));

+}
+
+template<typename HolderType, typename ResolvedType, typename RejectedType>
+template<typename PassHolderType>
+ScriptPromiseProperty<HolderType, ResolvedType, RejectedType>::ScriptPromiseProperty(ExecutionContext* executionContext, PassHolderType holder, const char* name)
+    : ScriptPromisePropertyBase(executionContext, name)
+    , m_holder(holder)
+{
+}
+
+template<typename HolderType, typename ResolvedType, typename RejectedType>
+template<typename PassResolvedType>
+void ScriptPromiseProperty<HolderType, ResolvedType, RejectedType>::resolve(PassResolvedType value)
+{
+    if (!(executionContext() && state() == Pending))

[haraken, 2014/07/02 09:04:31]

Is it valid that resolve() is called while state() is not Pending? I wonder if
this could be ASSERT(state() == Pending).

[dominicc (has gone to gerrit), 2014/07/03 02:13:39]

I originally did it this way, but yhirano says this should not assert because JS
promises don't.

But I think we should revisit this. C++ != JavaScript; trying to settle the
Promise twice indicates a bug. I've put this back to asserting.

+        return;
+    m_resolved = value;
+    settle(Resolved);

[haraken, 2014/07/02 09:04:31]

settle => resolveOrReject (to align with the function name in
ScriptPromiseResolver)

+}
+
+template<typename HolderType, typename ResolvedType, typename RejectedType>
+template<typename PassRejectedType>
+void ScriptPromiseProperty<HolderType, ResolvedType, RejectedType>::reject(PassRejectedType value)
+{
+    if (!(executionContext() && state() == Pending))
+        return;
+    m_rejected = value;
+    settle(Rejected);
+}
+
+template<typename HolderType, typename ResolvedType, typename RejectedType>
+v8::Handle<v8::Object> ScriptPromiseProperty<HolderType, ResolvedType, RejectedType>::holder(v8::Handle<v8::Object> creationContext, v8::Isolate* isolate)
+{
+    v8::Handle<v8::Value> value = V8ValueTraits<HolderType>::toV8Value(m_holder, creationContext, isolate);

[haraken, 2014/07/02 09:04:31]

Is it guaranteed that we're in a correct context when you call this toV8Value?
If it's not guaranteed, we need to enter a ScriptState::Scope before calling
toV8Value.

I'm fine with the rule that the caller side of these methods should be in a
correct context. Either way, we want to be clear about the rule; I'm a bit
concerned that the current Promise implementation is unclear about the rule and
it might lead to cross-world leakage.

[dominicc (has gone to gerrit), 2014/07/03 02:13:39]

Yes, the ScriptPromisePropertyBase::promise and ::resolveOrReject handle this. I
added a comment block to the declarations of these in ScriptPromisePropertyBase.
This is why we need ScriptPromiseProperty. If you use ScriptPromiseResolver and
try to cache the Promise it is easy to make a mistake and return a Promise set
up in the first caller's context (because everyone is copy-pasting
CallWith=ScriptState.) Instead the Promise should be in the context associated
with the holder.

You can see ScriptPromisePropertyBase do this in ::promise and
::resolveOrReject. The first thing ::promise does is enter the context for
(holder's execution context, caller's world). ::resolveOrReject is different
because when we support multiple worlds we have to loop through those worlds. So
it enters the context for (cached wrapper's creation context).

+    return value.As<v8::Object>();
+}
+
+template<typename HolderType, typename ResolvedType, typename RejectedType>
+v8::Handle<v8::Value> ScriptPromiseProperty<HolderType, ResolvedType, RejectedType>::resolvedValue(v8::Handle<v8::Object> creationContext, v8::Isolate* isolate)
+{
+    ASSERT(state() == Resolved);
+    return V8ValueTraits<ResolvedType>::toV8Value(m_resolved, creationContext, isolate);
+}
+
+template<typename HolderType, typename ResolvedType, typename RejectedType>
+v8::Handle<v8::Value> ScriptPromiseProperty<HolderType, ResolvedType, RejectedType>::rejectedValue(v8::Handle<v8::Object> creationContext, v8::Isolate* isolate)
+{
+    ASSERT(state() == Rejected);
+    return V8ValueTraits<RejectedType>::toV8Value(m_rejected, creationContext, isolate);
+}
+
+} // namespace WebCore
+
+#endif // ScriptPromiseProperty_h