withTimeout.nim

  proc waitableCallback(param: pointer,
                        timerOrWaitFired: WINBOOL): void {.stdcall.} =
    var p = getGlobalDispatcher()
    var o = cast[PCustomOverlapped](param)
    discard postQueuedCompletionStatus(p.ioPort, timerOrWaitFired, 
                                       cast[CompletionKey](o.data.fd), 
                                       cast[pointer](o))

  proc withTimeout*[T](fut: Future[T], timeout: int): Future[void] =
    var
      hWaitHandle: Handle
      hEvent: Handle
      winTimeout: ULONG

    var retFuture = newFuture[void]("withTimeout")
    hEvent = getGlobalDispatcher().hWaitEvent

    if not fut.finished:
      if timeout == -1:
        winTimeout = cast[ULONG](winlean.INFINITE)
      else:
        winTimeout = cast[ULONG](timeout)

      let regFlags = cast[ULONG](WT_EXECUTEINWAITTHREAD or WT_EXECUTEONLYONCE)

      var ol = PCustomOverlapped()
      GC_ref(ol)
      ol.data = CompletionData(cb:
        proc(fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
          discard unregisterWait(hWaitHandle)
          getGlobalDispatcher().handles.excl(cast[AsyncFD](hWaitHandle))
          retFuture.complete()
        )
      let ret = registerWaitForSingleObject(addr hWaitHandle, hEvent,
                                            waitableCallback, cast[pointer](ol),
                                            winTimeout, regFlags)
      if ret == 0:
        raiseOSError(osLastError())
      getGlobalDispatcher().handles.incl(cast[AsyncFD](hWaitHandle))

      fut.callback = proc() =
        if not retFuture.finished:
          discard unregisterWait(hWaitHandle)
          getGlobalDispatcher().handles.excl(cast[AsyncFD](hWaitHandle))
          GC_unref(ol)
          retFuture.complete()
    else:
      retFuture.complete()
    return retFuture