src/req_wrap-inl.h

#ifndef SRC_REQ_WRAP_INL_H_
#define SRC_REQ_WRAP_INL_H_

#if defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS

#include "req_wrap.h"
#include "async_wrap-inl.h"
#include "uv.h"

namespace node {

ReqWrapBase::ReqWrapBase(Environment* env) {
  CHECK(env->has_run_bootstrapping_code());
  env->req_wrap_queue()->PushBack(this);
}

template <typename T>
ReqWrap<T>::ReqWrap(Environment* env,
                    v8::Local<v8::Object> object,
                    AsyncWrap::ProviderType provider)
    : AsyncWrap(env, object, provider),
      ReqWrapBase(env) {
  Reset();
}

template <typename T>
ReqWrap<T>::~ReqWrap() {
  CHECK_EQ(false, persistent().IsEmpty());
}

template <typename T>
void ReqWrap<T>::Dispatched() {
  req_.data = this;
}

template <typename T>
void ReqWrap<T>::Reset() {
  original_callback_ = nullptr;
  req_.data = nullptr;
}

template <typename T>
ReqWrap<T>* ReqWrap<T>::from_req(T* req) {
  return ContainerOf(&ReqWrap<T>::req_, req);
}

template <typename T>
void ReqWrap<T>::Cancel() {
  if (req_.data == this)  // Only cancel if already dispatched.
    uv_cancel(reinterpret_cast<uv_req_t*>(&req_));
}

template <typename T>
AsyncWrap* ReqWrap<T>::GetAsyncWrap() {
  return this;
}

// Below is dark template magic designed to invoke libuv functions that
// initialize uv_req_t instances in a unified fashion, to allow easier
// tracking of active/inactive requests.

// Invoke a generic libuv function that initializes uv_req_t instances.
// This is, unfortunately, necessary since they come in three different
// variants that can not all be invoked in the same way:
// - int uv_foo(uv_loop_t* loop, uv_req_t* request, ...);
// - int uv_foo(uv_req_t* request, ...);
// - void uv_foo(uv_req_t* request, ...);
template <typename ReqT, typename T>
struct CallLibuvFunction;

// Detect `int uv_foo(uv_loop_t* loop, uv_req_t* request, ...);`.
template <typename ReqT, typename... Args>
struct CallLibuvFunction<ReqT, int(*)(uv_loop_t*, ReqT*, Args...)> {
  using T = int(*)(uv_loop_t*, ReqT*, Args...);
  template <typename... PassedArgs>
  static int Call(T fn, uv_loop_t* loop, ReqT* req, PassedArgs... args) {
    return fn(loop, req, args...);
  }
};

// Detect `int uv_foo(uv_req_t* request, ...);`.
template <typename ReqT, typename... Args>
struct CallLibuvFunction<ReqT, int(*)(ReqT*, Args...)> {
  using T = int(*)(ReqT*, Args...);
  template <typename... PassedArgs>
  static int Call(T fn, uv_loop_t* loop, ReqT* req, PassedArgs... args) {
    return fn(req, args...);
  }
};

// Detect `void uv_foo(uv_req_t* request, ...);`.
template <typename ReqT, typename... Args>
struct CallLibuvFunction<ReqT, void(*)(ReqT*, Args...)> {
  using T = void(*)(ReqT*, Args...);
  template <typename... PassedArgs>
  static int Call(T fn, uv_loop_t* loop, ReqT* req, PassedArgs... args) {
    fn(req, args...);
    return 0;
  }
};

// This is slightly darker magic: This template is 'applied' to each parameter
// passed to the libuv function. If the parameter type (aka `T`) is a
// function type, it is assumed that this it is the request callback, and a
// wrapper that calls the original callback is created.
// If not, the parameter is passed through verbatim.
template <typename ReqT, typename T>
struct MakeLibuvRequestCallback {
  static T For(ReqWrap<ReqT>* req_wrap, T v) {
    static_assert(!is_callable<T>::value,
                  "MakeLibuvRequestCallback missed a callback");
    return v;
  }
};

// Match the `void callback(uv_req_t*, ...);` signature that all libuv
// callbacks use.
template <typename ReqT, typename... Args>
struct MakeLibuvRequestCallback<ReqT, void(*)(ReqT*, Args...)> {
  using F = void(*)(ReqT* req, Args... args);

  static void Wrapper(ReqT* req, Args... args) {
    ReqWrap<ReqT>* req_wrap = ReqWrap<ReqT>::from_req(req);
    req_wrap->env()->DecreaseWaitingRequestCounter();
    F original_callback = reinterpret_cast<F>(req_wrap->original_callback_);
    original_callback(req, args...);
  }

  static F For(ReqWrap<ReqT>* req_wrap, F v) {
    CHECK_NULL(req_wrap->original_callback_);
    req_wrap->original_callback_ =
        reinterpret_cast<typename ReqWrap<ReqT>::callback_t>(v);
    return Wrapper;
  }
};

template <typename T>
template <typename LibuvFunction, typename... Args>
int ReqWrap<T>::Dispatch(LibuvFunction fn, Args... args) {
  Dispatched();

  // This expands as:
  //
  // int err = fn(env()->event_loop(), req(), arg1, arg2, Wrapper, arg3, ...)
  //              ^                                       ^        ^
  //              |                                       |        |
  //              \-- Omitted if `fn` has no              |        |
  //                  first `uv_loop_t*` argument         |        |
  //                                                      |        |
  //        A function callback whose first argument      |        |
  //        matches the libuv request type is replaced ---/        |
  //        by the `Wrapper` method defined above                  |
  //                                                               |
  //               Other (non-function) arguments are passed  -----/
  //               through verbatim
  int err = CallLibuvFunction<T, LibuvFunction>::Call(
      fn,
      env()->event_loop(),
      req(),
      MakeLibuvRequestCallback<T, Args>::For(this, args)...);
  if (err >= 0)
    env()->IncreaseWaitingRequestCounter();
  return err;
}

}  // namespace node

#endif  // defined(NODE_WANT_INTERNALS) && NODE_WANT_INTERNALS

#endif  // SRC_REQ_WRAP_INL_H_