bindings: Use V8 MaybeLocal APIs in V8DeviceMotionEventCustom

Source/bindings/core/v8/V8BindingMacros.h

 /*
  * Copyright (C) 2010 Google Inc. All rights reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions are
  * met:
  *
  *     * Redistributions of source code must retain the above copyright
  * notice, this list of conditions and the following disclaimer.
  *     * Redistributions in binary form must reproduce the above
@@ skipping 59 matching lines @@
 #define TOSTRING_VOID(type, var, value) \
     type var(value);                    \
     if (UNLIKELY(!var.prepare()))       \
         return;
 
 #define TOSTRING_DEFAULT(type, var, value, retVal) \
     type var(value);                               \
     if (UNLIKELY(!var.prepare()))                  \
         return retVal;
 
+// Use this macro to call V8 APIs which return MaybeLocal<T>.
+#define CALL_V8_WITH_CHECK_MAYBELOCAL(type, outValue, callExpression, failureExpression) \

[Yuki, 2015/03/04 09:04:22]

From the point of view of users of this macro, I'd prefer outVariable or
varName, etc. to outValue.  It's variable rather than value.

[bashi, 2015/03/04 10:12:45]

Done.

+    v8::Local<type> outValue; \
+    if (!(callExpression).ToLocal(&outValue) || outValue.IsEmpty()) \

[Jens Widell, 2015/03/04 09:01:44]

Should ToLocal() failing and the resulting local being empty necessarily be
treated the same?

I'd interpret ToLocal() failing as "V8 is being shut down, abort, abort!"
whereas the resulting local being empty is a "normal failure".

In the first case, we should presumably try to do as little as possible; just
unwind the stack. In the second case, we might want to extract an exception from
a TryCatch and store it in an ExceptionState, or create and return a rejected
promise, or something like that. Things that might themselves fail if ToLocal()
failed, perhaps?

[bashi, 2015/03/04 10:12:45]

If I read ToLocal() correctly, ToLocal() just fails if the |val_| is null, so
there is no implication of abnormal termination when ToLocal() fails. We can
check v8::IsExecutionTerminating(), but in this case, I couldn't find a strong
reason to use ExceptionState (we would need to add an ExceptionState instance in
V8DeviceMotionEventCustom.cpp, then need to call throwIfNeeded()). We could add
macros which use ExceptionState when needed.

Instead, I added TryCatch to make sure we rethrow it.

+        failureExpression

[Yuki, 2015/03/04 09:04:22]

do { failureExpression; } while (0)
would be better if you'd like to support cases like
  MACRO(..., foo(); bar(); return);

[bashi, 2015/03/04 10:12:45]

I changed to take returnValue as we should return as soon as possible.

+
 } // namespace blink
 
 #endif // V8BindingMacros_h