How to migrate from ev_io_* to uv_poll_* for IO polling

For native modules that wanted to watch for IO events on file descriptors, the original functions to do this was ev_io_*(), exposed by libev. You would have code that looked something like this:

/** THIS IS THE OLD API, DON'T COPY THIS, SEE BELOW!! **/

struct my_struct {
  Persistent<Function> callback;
  int fd;
};

/* the "on fd IO event" callback; called on the main thread */
void on_fd_event (EV_P_ ev_io *io_watcher, int events) {
  HandleScope scope;

  my_struct *m = (my_struct *) io_watcher->data;

  // stop watcher
  ev_io_stop(EV_DEFAULT_UC_ io_watcher);
  delete io_watcher;

  /* Some code here that works with `fd` (and closes it if needed) and produces `Handle<Value> result` */

  Handle<Value> argv[1];
  argv[0] = Integer::New(result);

  TryCatch try_catch;
  m->callback->Call(Context::GetCurrent()->Global(), 1, argv);

  // cleanup
  m->callback.Dispose();
  delete m;

  if (try_catch.HasCaught())
    FatalException(try_catch);
}

/* the JS entry point */
Handle<Value> start_fd_watch (const Arguments& args) {
  HandleScope scope;

  /* Some code here that opens `fd` and do something */

  my_struct *m = new my_struct;
  m->fd = fd;
  m->callback = Persistent<Function>::New(Local<Function>::Cast(args[0]));

  // create IO watcher and start watching
  ev_io* io_watcher = new ev_io;
  io_watcher->data = m;
  ev_init(io_watcher, on_fd_event);
  ev_io_set(io_watcher, fd, EV_READ /* or other flags */);
  ev_io_start(EV_DEFAULT_UC_ io_watcher);

  return Undefined();
}

Now, there is a new preferred method to poll IO events: uv_poll_*(), exposed by libuv. The API is similar but slightly different:

/** THIS IS THE CURRENT API. COPY THIS!!! **/

struct my_struct {
  Persistent<Function> callback;
  int fd;
};

/* the "on fd IO event" callback; called on the main thread */
void on_fd_event (uv_poll_t* handle, int status, int events) {
  HandleScope scope;

  my_struct *m = (my_struct *) handle->data;

  // stop watcher
  uv_poll_stop(handle);
  delete handle;

  /* Some code here that works with `fd` (and closes it if needed) and produces `Handle<Value> result` */

  Handle<Value> argv[1];
  argv[0] = Integer::New(result);

  TryCatch try_catch;
  m->callback->Call(Context::GetCurrent()->Global(), 1, argv);

  // cleanup
  m->callback.Dispose();
  delete m;

  if (try_catch.HasCaught())
    FatalException(try_catch);
}

/* the JS entry point */
Handle<Value> start_fd_watch (const Arguments& args) {
  HandleScope scope;

  /* Some code here that opens `fd` and do something */

  my_struct *m = new my_struct;
  m->fd = fd;
  m->callback = Persistent<Function>::New(Local<Function>::Cast(args[0]));

  // create poll handle and start polling
  uv_poll_t* handle = new uv_poll_t;
  handle->data = m;
  uv_poll_init(uv_default_loop(), handle, fd);
  uv_poll_start(handle, EV_READ /* or other flags */, on_fd_event);

  return Undefined();
}

Rundown:

• Call uv_queue_work() instead of eio_custom() to initiate async work on the thread pool.
• The "after work" callback functions now accept a uv_work_t * pointer as their arguments, instead of a eio_req * pointer.
• You must manually create the uv_work_t instance, and pass it to uv_queue_work(). It's not done for you like with libeio.
• You can attach a custom void * to the data field of the uv_work_t struct.
• Don't forget to free() or delete the uv_work_t struct at the end of the "after work" callback function.