Function enable_cancellation

src/windows/libwinio.cpp:430–453

template <typename T, typename O>
static void enable_cancellation(
    const int_fd& fd,
    const Future<T>& future,
    const std::shared_ptr<O>& overlapped)
{
  // We capture a `std::weak_ptr` in the `.onDiscard` callback, so that
  // we can cancel the IO operation with the overlapped object if it still
  // exists. The captured `shared_ptr` in the `.onAny` callback ensures that
  // we keep the overlapped object alive until the IOCP callbacks are done.
  auto overlapped_weak = std::weak_ptr<O>(overlapped);

  future.onDiscard([fd, overlapped_weak]() {
    std::shared_ptr<O> cancel = overlapped_weak.lock();
    if (static_cast<bool>(cancel)) {
      // We were able to get a reference to the overlapped object, so let's
      // try to cancel the IO operation. Note that there is technically a
      // race here. We could be between the IO completion event and deleting
      // the overlapped object. In that case, this function will just no-op.
      ::CancelIoEx(fd, &cancel->base.overlapped);
    }
  });

  future.onAny([overlapped]() {});
}


static Future<size_t> read_internal(