binding: Introduces ExceptionToPromiseScope.

third_party/WebKit/Source/bindings/core/v8/GeneratedCodeHelper.h

 // Copyright 2016 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.
 
 // This file is for functions that are used only by generated code.
 // CAUTION:
 // All functions defined in this file should be used by generated code only.
 // If you want to use them from hand-written code, please find appropriate
 // location and move them to that location.
 
 #ifndef GeneratedCodeHelper_h
 #define GeneratedCodeHelper_h
 
+#include "bindings/core/v8/ExceptionState.h"
+#include "bindings/core/v8/V8Binding.h"
 #include "core/CoreExport.h"
 #include "wtf/PassRefPtr.h"
 #include <v8.h>
 
 namespace blink {
 
+class ScriptState;
 class SerializedScriptValue;
 
 CORE_EXPORT void v8ConstructorAttributeGetter(v8::Local<v8::Name> propertyName, const v8::PropertyCallbackInfo<v8::Value>&);
 
 CORE_EXPORT v8::Local<v8::Value> v8Deserialize(v8::Isolate*, PassRefPtr<SerializedScriptValue>);
 
+// Throws an arity error of type TypeError.
+CORE_EXPORT void throwMinimumArityError(ExceptionState&, unsigned expected, unsigned actual);
+CORE_EXPORT void throwInvalidArityError(ExceptionState&, const char* validRange, unsigned actual);
+CORE_EXPORT void throwMinimumArityErrorForMethod(v8::Isolate*, const char* interface, const char* method, unsigned expected, unsigned actual);
+CORE_EXPORT void throwMinimumArityErrorForConstructor(v8::Isolate*, const char* interface, unsigned expected, unsigned actual);
+
+// ExceptionToPromiseScope converts a possible exception to a reject promise
+// and returns the promise instead of throwing the exception.
+//
+// Promise-returning DOM operations are required to always return a promise
+// and to never throw an exception.
+// See also http://heycam.github.io/webidl/#es-operations
+class CORE_EXPORT ExceptionToPromiseScope {

[haraken, 2016/09/02 16:24:13]

I still don't understand what GeneratedCodeHelper is for.
ExceptionToPromiseScope is currently used only auto-generated files but it can
potentially be used by general bindings (just like ExceptionState). It would
make more sense to put it to ExceptionState.h or something.

[Yuki, 2016/09/05 07:19:25]

I don't expect that this will be used in such a manner.  There should be no
reason to use this utility outside generated bindings.

It's strange, meaningless, inefficient, and even danger to nest
ExceptionToPromiseScope, and we'd like to have this utility at very beginning of
callback functions.  So, there should be no chance for other code to use this.

The reason why I put this utility here is because I don't want people to abuse
this utility without understanding what happens.

[haraken, 2016/09/05 08:24:22]

I'm confused...

Note that the logic ExceptionToPromiseScope provides is already used in multiple
places in non-generated bindings. For example,
PaymentRequest::onUpdatePaymentDetails implements the logic with
TrackExceptionState. Permissions::query implements the logic with
ExceptionState::reject.

I'm not sure how useful it is to introduce another helper class only for
generated bindings...

+    STACK_ALLOCATED();
+public:
+    ExceptionToPromiseScope(const v8::FunctionCallbackInfo<v8::Value>& info, ScriptState* scriptState, ExceptionState& exceptionState)

[haraken, 2016/09/02 16:24:13]

Would it make more sense to introduce another constructor of ExceptionState
(which takes care of promises) instead of introducing ExceptionToPromiseScope?

ExceptionToPromiseScope sounds like a family of ExceptionState to me.

[Yuki, 2016/09/05 07:19:25]

I don't think so.  ExceptionToPromiseScope depends on v8::FunctionCallbackInfo
and a ScriptState.  We don't want ExceptionState to depend on such things.
No.  ExceptionState is a general purpose utility.  The purpose of
ExceptionToPromiseScope is to implement Web IDL 4.6.7. > the fuction's behavior
1. > exception case > 1.

[haraken, 2016/09/05 08:24:22]

As I mentioned above, this functionality is implemented by TrackExceptionState
or ExceptionState::reject in non-generated bindings.

+        : m_info(info)
+        , m_scriptState(scriptState)
+        , m_exceptionState(exceptionState) { }
+    ~ExceptionToPromiseScope()
+    {
+        if (!m_exceptionState.hadException())
+            return;
+
+        v8SetReturnValue(m_info, m_exceptionState.reject(m_scriptState).v8Value());
+    }
+
+private:
+    const v8::FunctionCallbackInfo<v8::Value>& m_info;
+    ScriptState* m_scriptState;
+    ExceptionState& m_exceptionState;
+};
+
 } // namespace blink
 
 #endif // GeneratedCodeHelper_h