Web Replay: code generator should be able to encode/decode ViewState::Flags

• Component: WebCore Misc.
• Status: RESOLVED LATER

[Brian Burg]

ViewState::(enum) is an anonymous enum inside of a struct (ViewState). ViewState::Flags is a typedef for uint32_t, and the enum values are used to bitwise-construct and query a uint32_t containing several flags.

For replay we still want to serialize ViewState with string names, since ViewState::Flags could change in the future and we don't want reordering the flags to break serialization.
But, since the enum is un-named and ViewState::Flags is a uint32_t, the existing code will try to define new EncodingTraits specializations for uint32_t, which isn't allowed.

To sidestep this, I'll generate a wrapper class for each framework, so multiple specializations can exist for the same underlying type. Cases like ViewState will use a new flag in the inputs file to opt-in to this code generation behavior.


Sketch:

enum class JSEnum { FooBar, BarBaz };

template <JSEnum EnumName, typename StorageType>
class UncheckedJSEnum {
public:
    UncheckedJSEnum(StorageType value) : m_value(value) { }
    StorageType m_value;
};
    
template<> struct EncodingTraits<UncheckedJSEnum<JSEnum::FooBar,uint32_t>> {
  ...  
};

[Brian Burg]

Even simpler:

enum class WebEnums { FooBar, BarBaz };
struct ViewState {
    typedef uint32_t Flags;
};
template <typename EnumType, EnumType tag>
class Phantom { };
template<> struct EncodingTraits<Phantom<JSEnums,JSEnums::FooBar>> {
    typedef ViewState::Flags DecodedType;
    static EncodedValue encodeValue(const DecodedType&);
    static bool decodeValue(EncodedValue&, const DecodedType&);
};

[Brian Burg]

Created attachment 238625 [details]
WIP

[BJ Burg]

Closing web replay-related bugs until we resume working on the feature again.