API functions returning Promises should not throw exceptions.

Source/bindings/scripts/v8_types.py

 # Copyright (C) 2013 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
 # copyright notice, this list of conditions and the following disclaimer
@@ skipping 396 matching lines @@
     if idl_type.base_type == 'DOMString' and not idl_type.array_or_sequence_type:
         format_string = 'V8TRYCATCH_FOR_V8STRINGRESOURCE_VOID({cpp_type}, {variable_name}, {cpp_value})'
     elif idl_type.is_integer_type:
         format_string = 'V8TRYCATCH_EXCEPTION_VOID({cpp_type}, {variable_name}, {cpp_value}, exceptionState)'
     else:
         format_string = 'V8TRYCATCH_VOID({cpp_type}, {variable_name}, {cpp_value})'
 
     cpp_value = v8_value_to_cpp_value(idl_type, extended_attributes, v8_value, index)
     return format_string.format(cpp_type=this_cpp_type, cpp_value=cpp_value, variable_name=variable_name)
 
+
+def v8_value_to_local_cpp_value_async(idl_type, extended_attributes, v8_value, variable_name, index=None):

[Nils Barth (inactive), 2014/04/10 07:48:03]

You're just adding 'info' here, right?
If so, could you just add it to the above v8_value_to_local_cpp_value
with a flag?
I think it would be clearer to refactor the previous to use an 'arguments'
variable instead; I'll do that separately.

[Nils Barth (inactive), 2014/04/10 07:59:47]

Oh, and you're tacking on _PROMISE.
Anyway, I've posted a refactoring CL:
Refactor v8_types.v8_value_to_local_cpp_value for clarity and to make Promises
easier to implement
https://codereview.chromium.org/231833005/
...so I think you can just add:
async=False
async: If true, the generated code returns a rejected Promise when it sees an
error rather than throwing an exception.

if async:
    args.append('info')
...
if async:
    v8trycatch_void += '_PROMISE'

[yhirano, 2014/04/11 10:19:04]

Thanks, done

+    """Returns an expression that converts a V8 value to a C++ value and stores it as a local value.
+    The generated code returns a rejected Promise when it sees an error rather than throwing an exception.
+    """
+    this_cpp_type = idl_type.cpp_type_args(extended_attributes=extended_attributes, used_as_argument=True)
+
+    idl_type = idl_type.preprocessed_type
+
+    if idl_type.base_type == 'DOMString' and not idl_type.array_or_sequence_type:
+        format_string = 'V8TRYCATCH_FOR_V8STRINGRESOURCE_VOID_PROMISE({cpp_type}, {variable_name}, {cpp_value}, info)'
+    elif idl_type.is_integer_type:
+        format_string = 'V8TRYCATCH_EXCEPTION_VOID_PROMISE({cpp_type}, {variable_name}, {cpp_value}, info, exceptionState)'
+    else:
+        format_string = 'V8TRYCATCH_VOID_PROMISE({cpp_type}, {variable_name}, {cpp_value}, info)'
+
+    cpp_value = v8_value_to_cpp_value(idl_type, extended_attributes, v8_value, index)
+    return format_string.format(cpp_type=this_cpp_type, cpp_value=cpp_value, variable_name=variable_name)
+
+
 IdlType.v8_value_to_local_cpp_value = v8_value_to_local_cpp_value
+IdlType.v8_value_to_local_cpp_value_async = v8_value_to_local_cpp_value_async
 IdlUnionType.v8_value_to_local_cpp_value = v8_value_to_local_cpp_value
 
 
 ################################################################################
 # C++ -> V8
 ################################################################################
 
 def preprocess_idl_type(idl_type):
     if idl_type.is_enum:
         # Enumerations are internally DOMStrings
@@ skipping 174 matching lines @@
 def cpp_value_to_v8_value(idl_type, cpp_value, isolate='info.GetIsolate()', creation_context='', extended_attributes=None):
     """Returns an expression that converts a C++ value to a V8 value."""
     # the isolate parameter is needed for callback interfaces
     idl_type, cpp_value = preprocess_idl_type_and_value(idl_type, cpp_value, extended_attributes)
     this_v8_conversion_type = idl_type.v8_conversion_type(extended_attributes)
     format_string = CPP_VALUE_TO_V8_VALUE[this_v8_conversion_type]
     statement = format_string.format(cpp_value=cpp_value, isolate=isolate, creation_context=creation_context)
     return statement
 
 IdlType.cpp_value_to_v8_value = cpp_value_to_v8_value