src: refactor callback #defines into C++ templates

[addaleax]

Use template helpers instead of #defines to generate
the raw C callbacks that are passed to the HTTP parser library.

A nice effect of this is that it is more obvious what
parameters the Parser methods take.

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• commit message follows commit guidelines

Affected core subsystem(s)

src/node_http_parser.cc

[maclover7]

LGTM as long as CI passes (I'm assuming there's no weird cases here)

[joyeecheung]

It's possible to make the template more generic and avoid code duplication like this:

template <typename Parser, Parser> struct Proxy;
template <typename Parser, typename ...Args, int (Parser::*Member)(Args...)>
struct Proxy<int (Parser::*)(Args...), Member> {
  static int Raw(http_parser* p, Args ... args) {
    Parser* parser = ContainerOf(&Parser::parser_, p);
    return (parser->*Member)(std::forward<Args>(args)...);
  }
};

typedef int(Parser::*Call)();
typedef int(Parser::*DataCall)(const char* at, size_t length);

const struct http_parser_settings Parser::settings = {
  Proxy<Call, &Parser::on_message_begin>::Raw,
  Proxy<DataCall, &Parser::on_url>::Raw,
  ...
}

[addaleax]

Wow! I tried going into that direction but couldn’t get it to compile 😄 Would you prefer your approach? I feel like the current version is already a lot to digest, but your approach is kind of cool…

[joyeecheung]

I personally prefer DRYing code even it involves a bit more template magic (I mean, that's the point of using templates right XD)..but it's totally fine that you keep it this way.

[addaleax]

@joyeecheung Okay, done! Kinda still can’t believe it works. 😄

[joyeecheung]

CI: https://ci.nodejs.org/job/node-test-pull-request/12597/

[addaleax]

Landed in 6283077

[MylesBorins]

Should this be backported to v6.x-staging or v8.x-staging? If yes please follow the guide and raise a backport PR, if not let me know or add the dont-land-on label.