#
#
#            Nim's Runtime Library
#        (c) Copyright 2015 Dominik Picheta
#
#    See the file "copying.txt", included in this
#    distribution, for details about the copyright.
#

include "system/inclrtl"

import os, oids, tables, strutils, times, heapqueue, lists

import nativesockets, net, deques

export Port, SocketFlag

#{.injectStmt: newGcInvariant().}

## AsyncDispatch
## *************
##
## This module implements asynchronous IO. This includes a dispatcher,
## a ``Future`` type implementation, and an ``async`` macro which allows
## asynchronous code to be written in a synchronous style with the ``await``
## keyword.
##
## The dispatcher acts as a kind of event loop. You must call ``poll`` on it
## (or a function which does so for you such as ``waitFor`` or ``runForever``)
## in order to poll for any outstanding events. The underlying implementation
## is based on epoll on Linux, IO Completion Ports on Windows and select on
## other operating systems.
##
## The ``poll`` function will not, on its own, return any events. Instead
## an appropriate ``Future`` object will be completed. A ``Future`` is a
## type which holds a value which is not yet available, but which *may* be
## available in the future. You can check whether a future is finished
## by using the ``finished`` function. When a future is finished it means that
## either the value that it holds is now available or it holds an error instead.
## The latter situation occurs when the operation to complete a future fails
## with an exception. You can distinguish between the two situations with the
## ``failed`` function.
##
## Future objects can also store a callback procedure which will be called
## automatically once the future completes.
##
## Futures therefore can be thought of as an implementation of the proactor
## pattern. In this
## pattern you make a request for an action, and once that action is fulfilled
## a future is completed with the result of that action. Requests can be
## made by calling the appropriate functions. For example: calling the ``recv``
## function will create a request for some data to be read from a socket. The
## future which the ``recv`` function returns will then complete once the
## requested amount of data is read **or** an exception occurs.
##
## Code to read some data from a socket may look something like this:
##
##   .. code-block::nim
##      var future = socket.recv(100)
##      future.callback =
##        proc () =
##          echo(future.read)
##
## All asynchronous functions returning a ``Future`` will not block. They
## will not however return immediately. An asynchronous function will have
## code which will be executed before an asynchronous request is made, in most
## cases this code sets up the request.
##
## In the above example, the ``recv`` function will return a brand new
## ``Future`` instance once the request for data to be read from the socket
## is made. This ``Future`` instance will complete once the requested amount
## of data is read, in this case it is 100 bytes. The second line sets a
## callback on this future which will be called once the future completes.
## All the callback does is write the data stored in the future to ``stdout``.
## The ``read`` function is used for this and it checks whether the future
## completes with an error for you (if it did it will simply raise the
## error), if there is no error however it returns the value of the future.
##
## Asynchronous procedures
## -----------------------
##
## Asynchronous procedures remove the pain of working with callbacks. They do
## this by allowing you to write asynchronous code the same way as you would
## write synchronous code.
##
## An asynchronous procedure is marked using the ``{.async.}`` pragma.
## When marking a procedure with the ``{.async.}`` pragma it must have a
## ``Future[T]`` return type or no return type at all. If you do not specify
## a return type then ``Future[void]`` is assumed.
##
## Inside asynchronous procedures ``await`` can be used to call any
## procedures which return a
## ``Future``; this includes asynchronous procedures. When a procedure is
## "awaited", the asynchronous procedure it is awaited in will
## suspend its execution
## until the awaited procedure's Future completes. At which point the
## asynchronous procedure will resume its execution. During the period
## when an asynchronous procedure is suspended other asynchronous procedures
## will be run by the dispatcher.
##
## The ``await`` call may be used in many contexts. It can be used on the right
## hand side of a variable declaration: ``var data = await socket.recv(100)``,
## in which case the variable will be set to the value of the future
## automatically. It can be used to await a ``Future`` object, and it can
## be used to await a procedure returning a ``Future[void]``:
## ``await socket.send("foobar")``.
##
## Discarding futures
## ------------------
##
## Futures should **never** be discarded. This is because they may contain
## errors. If you do not care for the result of a Future then you should
## use the ``asyncCheck`` procedure instead of the ``discard`` keyword.
##
## Examples
## --------
##
## For examples take a look at the documentation for the modules implementing
## asynchronous IO. A good place to start is the
## `asyncnet module <asyncnet.html>`_.
##
## Limitations/Bugs
## ----------------
##
## * The effect system (``raises: []``) does not work with async procedures.
## * Can't await in a ``except`` body
## * Forward declarations for async procs are broken,
##   link includes workaround: https://github.com/nim-lang/Nim/issues/3182.
## * FutureVar[T] needs to be completed manually.

# TODO: Check if yielded future is nil and throw a more meaningful exception

include "../includes/asyncfutures"

type
  PDispatcherBase = ref object of RootRef
    timers: HeapQueue[tuple[finishAt: float, fut: Future[void]]]
    callbacks: Deque[proc ()]

proc processTimers(p: PDispatcherBase) {.inline.} =
  #Process just part if timers at a step
  var count = p.timers.len
  let t = epochTime()
  while count > 0 and t >= p.timers[0].finishAt:
    p.timers.pop().fut.complete()
    dec count

proc processPendingCallbacks(p: PDispatcherBase) =
  while p.callbacks.len > 0:
    var cb = p.callbacks.popFirst()
    cb()

proc adjustedTimeout(p: PDispatcherBase, timeout: int): int {.inline.} =
  # If dispatcher has active timers this proc returns the timeout
  # of the nearest timer. Returns `timeout` otherwise.
  result = timeout
  if p.timers.len > 0:
    let timerTimeout = p.timers[0].finishAt
    let curTime = epochTime()
    if timeout == -1 or (curTime + (timeout / 1000)) > timerTimeout:
      result = int((timerTimeout - curTime) * 1000)
      if result < 0: result = 0

when defined(windows) or defined(nimdoc):
  import winlean, sets, hashes
  type
    CompletionKey = ULONG_PTR

    CompletionData* = object
      fd*: AsyncFD # TODO: Rename this.
      cb*: proc (fd: AsyncFD, bytesTransferred: Dword,
                errcode: OSErrorCode) {.closure,gcsafe.}
      cell*: ForeignCell # we need this `cell` to protect our `cb` environment,
                         # when using RegisterWaitForSingleObject, because
                         # waiting is done in different thread.

    PDispatcher* = ref object of PDispatcherBase
      ioPort: Handle
      handles: HashSet[AsyncFD]

    CustomOverlapped = object of OVERLAPPED
      data*: CompletionData

    PCustomOverlapped* = ref CustomOverlapped

    AsyncFD* = distinct int

    PostCallbackData = object
      ioPort: Handle
      handleFd: AsyncFD
      waitFd: Handle
      ovl: PCustomOverlapped
    PostCallbackDataPtr = ptr PostCallbackData

    AsyncEventImpl = object
      hEvent: Handle
      hWaiter: Handle
      pcd: PostCallbackDataPtr
    AsyncEvent* = ptr AsyncEventImpl

    Callback = proc (fd: AsyncFD): bool {.closure,gcsafe.}
  {.deprecated: [TCompletionKey: CompletionKey, TAsyncFD: AsyncFD,
                TCustomOverlapped: CustomOverlapped, TCompletionData: CompletionData].}

  proc hash(x: AsyncFD): Hash {.borrow.}
  proc `==`*(x: AsyncFD, y: AsyncFD): bool {.borrow.}

  proc newDispatcher*(): PDispatcher =
    ## Creates a new Dispatcher instance.
    new result
    result.ioPort = createIoCompletionPort(INVALID_HANDLE_VALUE, 0, 0, 1)
    result.handles = initSet[AsyncFD]()
    result.timers.newHeapQueue()
    result.callbacks = initDeque[proc ()](64)

  var gDisp{.threadvar.}: PDispatcher ## Global dispatcher
  proc getGlobalDispatcher*(): PDispatcher =
    ## Retrieves the global thread-local dispatcher.
    if gDisp.isNil: gDisp = newDispatcher()
    result = gDisp

  proc register*(fd: AsyncFD) =
    ## Registers ``fd`` with the dispatcher.
    let p = getGlobalDispatcher()
    if createIoCompletionPort(fd.Handle, p.ioPort,
                              cast[CompletionKey](fd), 1) == 0:
      raiseOSError(osLastError())
    p.handles.incl(fd)

  proc verifyPresence(fd: AsyncFD) =
    ## Ensures that file descriptor has been registered with the dispatcher.
    let p = getGlobalDispatcher()
    if fd notin p.handles:
      raise newException(ValueError,
        "Operation performed on a socket which has not been registered with" &
        " the dispatcher yet.")

  proc hasPendingOperations*(): bool =
    ## Returns `true` if the global dispatcher has pending operations.
    let p = getGlobalDispatcher()
    p.handles.len != 0 or p.timers.len != 0 or p.callbacks.len != 0

  proc poll*(timeout = 500) =
    ## Waits for completion events and processes them. Raises ``ValueError``
    ## if there are no pending operations.
    let p = getGlobalDispatcher()
    if p.handles.len == 0 and p.timers.len == 0 and p.callbacks.len == 0:
      raise newException(ValueError,
        "No handles or timers registered in dispatcher.")

    let at = p.adjustedTimeout(timeout)
    var llTimeout =
      if at == -1: winlean.INFINITE
      else: at.int32

    if p.handles.len != 0:
      var lpNumberOfBytesTransferred: Dword
      var lpCompletionKey: ULONG_PTR
      var customOverlapped: PCustomOverlapped
      let res = getQueuedCompletionStatus(p.ioPort,
          addr lpNumberOfBytesTransferred, addr lpCompletionKey,
          cast[ptr POVERLAPPED](addr customOverlapped), llTimeout).bool

      # http://stackoverflow.com/a/12277264/492186
      # TODO: http://www.serverframework.com/handling-multiple-pending-socket-read-and-write-operations.html
      if res:
        # This is useful for ensuring the reliability of the overlapped struct.
        assert customOverlapped.data.fd == lpCompletionKey.AsyncFD

        customOverlapped.data.cb(customOverlapped.data.fd,
            lpNumberOfBytesTransferred, OSErrorCode(-1))

        # If cell.data != nil, then system.protect(rawEnv(cb)) was called,
        # so we need to dispose our `cb` environment, because it is not needed
        # anymore.
        if customOverlapped.data.cell.data != nil:
          system.dispose(customOverlapped.data.cell)

        GC_unref(customOverlapped)
      else:
        let errCode = osLastError()
        if customOverlapped != nil:
          assert customOverlapped.data.fd == lpCompletionKey.AsyncFD
          customOverlapped.data.cb(customOverlapped.data.fd,
              lpNumberOfBytesTransferred, errCode)
          if customOverlapped.data.cell.data != nil:
            system.dispose(customOverlapped.data.cell)
          GC_unref(customOverlapped)
        else:
          if errCode.int32 == WAIT_TIMEOUT:
            # Timed out
            discard
          else: raiseOSError(errCode)

    # Timer processing.
    processTimers(p)
    # Callback queue processing
    processPendingCallbacks(p)

  var acceptEx*: WSAPROC_ACCEPTEX
  var connectEx*: WSAPROC_CONNECTEX
  var getAcceptExSockAddrs*: WSAPROC_GETACCEPTEXSOCKADDRS

  proc initPointer(s: SocketHandle, fun: var pointer, guid: var GUID): bool =
    # Ref: https://github.com/powdahound/twisted/blob/master/twisted/internet/iocpreactor/iocpsupport/winsock_pointers.c
    var bytesRet: Dword
    fun = nil
    result = WSAIoctl(s, SIO_GET_EXTENSION_FUNCTION_POINTER, addr guid,
                      sizeof(GUID).Dword, addr fun, sizeof(pointer).Dword,
                      addr bytesRet, nil, nil) == 0

  proc initAll() =
    let dummySock = newNativeSocket()
    if dummySock == INVALID_SOCKET:
      raiseOSError(osLastError())
    var fun: pointer = nil
    if not initPointer(dummySock, fun, WSAID_CONNECTEX):
      raiseOSError(osLastError())
    connectEx = cast[WSAPROC_CONNECTEX](fun)
    if not initPointer(dummySock, fun, WSAID_ACCEPTEX):
      raiseOSError(osLastError())
    acceptEx = cast[WSAPROC_ACCEPTEX](fun)
    if not initPointer(dummySock, fun, WSAID_GETACCEPTEXSOCKADDRS):
      raiseOSError(osLastError())
    getAcceptExSockAddrs = cast[WSAPROC_GETACCEPTEXSOCKADDRS](fun)
    close(dummySock)

  proc connect*(socket: AsyncFD, address: string, port: Port,
    domain = nativesockets.AF_INET): Future[void] =
    ## Connects ``socket`` to server at ``address:port``.
    ##
    ## Returns a ``Future`` which will complete when the connection succeeds
    ## or an error occurs.
    verifyPresence(socket)
    var retFuture = newFuture[void]("connect")
    # Apparently ``ConnectEx`` expects the socket to be initially bound:
    var saddr: Sockaddr_in
    saddr.sin_family = int16(toInt(domain))
    saddr.sin_port = 0
    saddr.sin_addr.s_addr = INADDR_ANY
    if bindAddr(socket.SocketHandle, cast[ptr SockAddr](addr(saddr)),
                  sizeof(saddr).SockLen) < 0'i32:
      raiseOSError(osLastError())

    var aiList = getAddrInfo(address, port, domain)
    var success = false
    var lastError: OSErrorCode
    var it = aiList
    while it != nil:
      # "the OVERLAPPED structure must remain valid until the I/O completes"
      # http://blogs.msdn.com/b/oldnewthing/archive/2011/02/02/10123392.aspx
      var ol = PCustomOverlapped()
      GC_ref(ol)
      ol.data = CompletionData(fd: socket, cb:
        proc (fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
          if not retFuture.finished:
            if errcode == OSErrorCode(-1):
              retFuture.complete()
            else:
              retFuture.fail(newException(OSError, osErrorMsg(errcode)))
      )

      var ret = connectEx(socket.SocketHandle, it.ai_addr,
                          sizeof(Sockaddr_in).cint, nil, 0, nil,
                          cast[POVERLAPPED](ol))
      if ret:
        # Request to connect completed immediately.
        success = true
        retFuture.complete()
        # We don't deallocate ``ol`` here because even though this completed
        # immediately poll will still be notified about its completion and it will
        # free ``ol``.
        break
      else:
        lastError = osLastError()
        if lastError.int32 == ERROR_IO_PENDING:
          # In this case ``ol`` will be deallocated in ``poll``.
          success = true
          break
        else:
          GC_unref(ol)
          success = false
      it = it.ai_next

    freeAddrInfo(aiList)
    if not success:
      retFuture.fail(newException(OSError, osErrorMsg(lastError)))
    return retFuture

  proc recv*(socket: AsyncFD, size: int,
             flags = {SocketFlag.SafeDisconn}): Future[string] =
    ## Reads **up to** ``size`` bytes from ``socket``. Returned future will
    ## complete once all the data requested is read, a part of the data has been
    ## read, or the socket has disconnected in which case the future will
    ## complete with a value of ``""``.
    ##
    ## **Warning**: The ``Peek`` socket flag is not supported on Windows.


    # Things to note:
    #   * When WSARecv completes immediately then ``bytesReceived`` is very
    #     unreliable.
    #   * Still need to implement message-oriented socket disconnection,
    #     '\0' in the message currently signifies a socket disconnect. Who
    #     knows what will happen when someone sends that to our socket.
    verifyPresence(socket)
    assert SocketFlag.Peek notin flags, "Peek not supported on Windows."

    var retFuture = newFuture[string]("recv")
    var dataBuf: TWSABuf
    dataBuf.buf = cast[cstring](alloc0(size))
    dataBuf.len = size.ULONG

    var bytesReceived: Dword
    var flagsio = flags.toOSFlags().Dword
    var ol = PCustomOverlapped()
    GC_ref(ol)
    ol.data = CompletionData(fd: socket, cb:
      proc (fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
        if not retFuture.finished:
          if errcode == OSErrorCode(-1):
            if bytesCount == 0 and dataBuf.buf[0] == '\0':
              retFuture.complete("")
            else:
              var data = newString(bytesCount)
              assert bytesCount <= size
              copyMem(addr data[0], addr dataBuf.buf[0], bytesCount)
              retFuture.complete($data)
          else:
            if flags.isDisconnectionError(errcode):
              retFuture.complete("")
            else:
              retFuture.fail(newException(OSError, osErrorMsg(errcode)))
        if dataBuf.buf != nil:
          dealloc dataBuf.buf
          dataBuf.buf = nil
    )

    let ret = WSARecv(socket.SocketHandle, addr dataBuf, 1, addr bytesReceived,
                      addr flagsio, cast[POVERLAPPED](ol), nil)
    if ret == -1:
      let err = osLastError()
      if err.int32 != ERROR_IO_PENDING:
        if dataBuf.buf != nil:
          dealloc dataBuf.buf
          dataBuf.buf = nil
        GC_unref(ol)
        if flags.isDisconnectionError(err):
          retFuture.complete("")
        else:
          retFuture.fail(newException(OSError, osErrorMsg(err)))
    elif ret == 0:
      # Request completed immediately.
      if bytesReceived != 0:
        var data = newString(bytesReceived)
        assert bytesReceived <= size
        copyMem(addr data[0], addr dataBuf.buf[0], bytesReceived)
        retFuture.complete($data)
      else:
        if hasOverlappedIoCompleted(cast[POVERLAPPED](ol)):
          retFuture.complete("")
    return retFuture

  proc recvInto*(socket: AsyncFD, buf: pointer, size: int,
                flags = {SocketFlag.SafeDisconn}): Future[int] =
    ## Reads **up to** ``size`` bytes from ``socket`` into ``buf``, which must
    ## at least be of that size. Returned future will complete once all the
    ## data requested is read, a part of the data has been read, or the socket
    ## has disconnected in which case the future will complete with a value of
    ## ``0``.
    ##
    ## **Warning**: The ``Peek`` socket flag is not supported on Windows.


    # Things to note:
    #   * When WSARecv completes immediately then ``bytesReceived`` is very
    #     unreliable.
    #   * Still need to implement message-oriented socket disconnection,
    #     '\0' in the message currently signifies a socket disconnect. Who
    #     knows what will happen when someone sends that to our socket.
    verifyPresence(socket)
    assert SocketFlag.Peek notin flags, "Peek not supported on Windows."

    var retFuture = newFuture[int]("recvInto")

    #buf[] = '\0'
    var dataBuf: TWSABuf
    dataBuf.buf = cast[cstring](buf)
    dataBuf.len = size.ULONG

    var bytesReceived: Dword
    var flagsio = flags.toOSFlags().Dword
    var ol = PCustomOverlapped()
    GC_ref(ol)
    ol.data = CompletionData(fd: socket, cb:
      proc (fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
        if not retFuture.finished:
          if errcode == OSErrorCode(-1):
            retFuture.complete(bytesCount)
          else:
            if flags.isDisconnectionError(errcode):
              retFuture.complete(0)
            else:
              retFuture.fail(newException(OSError, osErrorMsg(errcode)))
        if dataBuf.buf != nil:
          dataBuf.buf = nil
    )

    let ret = WSARecv(socket.SocketHandle, addr dataBuf, 1, addr bytesReceived,
                      addr flagsio, cast[POVERLAPPED](ol), nil)
    if ret == -1:
      let err = osLastError()
      if err.int32 != ERROR_IO_PENDING:
        if dataBuf.buf != nil:
          dataBuf.buf = nil
        GC_unref(ol)
        if flags.isDisconnectionError(err):
          retFuture.complete(0)
        else:
          retFuture.fail(newException(OSError, osErrorMsg(err)))
    elif ret == 0:
      # Request completed immediately.
      if bytesReceived != 0:
        assert bytesReceived <= size
        retFuture.complete(bytesReceived)
      else:
        if hasOverlappedIoCompleted(cast[POVERLAPPED](ol)):
          retFuture.complete(bytesReceived)
    return retFuture

  proc send*(socket: AsyncFD, buf: pointer, size: int,
             flags = {SocketFlag.SafeDisconn}): Future[void] =
    ## Sends ``size`` bytes from ``buf`` to ``socket``. The returned future
    ## will complete once all data has been sent.
    ## **WARNING**: Use it with caution. If ``buf`` refers to GC'ed object,
    ## you must use GC_ref/GC_unref calls to avoid early freeing of the buffer.
    verifyPresence(socket)
    var retFuture = newFuture[void]("send")

    var dataBuf: TWSABuf
    dataBuf.buf = cast[cstring](buf)
    dataBuf.len = size.ULONG

    var bytesReceived, lowFlags: Dword
    var ol = PCustomOverlapped()
    GC_ref(ol)
    ol.data = CompletionData(fd: socket, cb:
      proc (fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
        if not retFuture.finished:
          if errcode == OSErrorCode(-1):
            retFuture.complete()
          else:
            if flags.isDisconnectionError(errcode):
              retFuture.complete()
            else:
              retFuture.fail(newException(OSError, osErrorMsg(errcode)))
    )

    let ret = WSASend(socket.SocketHandle, addr dataBuf, 1, addr bytesReceived,
                      lowFlags, cast[POVERLAPPED](ol), nil)
    if ret == -1:
      let err = osLastError()
      if err.int32 != ERROR_IO_PENDING:
        GC_unref(ol)
        if flags.isDisconnectionError(err):
          retFuture.complete()
        else:
          retFuture.fail(newException(OSError, osErrorMsg(err)))
    else:
      retFuture.complete()
      # We don't deallocate ``ol`` here because even though this completed
      # immediately poll will still be notified about its completion and it will
      # free ``ol``.
    return retFuture

  proc send*(socket: AsyncFD, data: string,
             flags = {SocketFlag.SafeDisconn}): Future[void] =
    ## Sends ``data`` to ``socket``. The returned future will complete once all
    ## data has been sent.
    verifyPresence(socket)
    var retFuture = newFuture[void]("send")

    var dataBuf: TWSABuf
    dataBuf.buf = data
    GC_ref(data) # we need to protect data until send operation is completed
                 # or failed.
    dataBuf.len = data.len.ULONG

    var bytesReceived, lowFlags: Dword
    var ol = PCustomOverlapped()
    GC_ref(ol)
    ol.data = CompletionData(fd: socket, cb:
      proc (fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
        GC_unref(data) # if operation completed `data` must be released.
        if not retFuture.finished:
          if errcode == OSErrorCode(-1):
            retFuture.complete()
          else:
            if flags.isDisconnectionError(errcode):
              retFuture.complete()
            else:
              retFuture.fail(newException(OSError, osErrorMsg(errcode)))
    )

    let ret = WSASend(socket.SocketHandle, addr dataBuf, 1, addr bytesReceived,
                      lowFlags, cast[POVERLAPPED](ol), nil)
    if ret == -1:
      let err = osLastError()
      if err.int32 != ERROR_IO_PENDING:
        GC_unref(ol)
        GC_unref(data) # if operation failed `data` must be released, because
                       # completion routine will not be called.
        if flags.isDisconnectionError(err):
          retFuture.complete()
        else:
          retFuture.fail(newException(OSError, osErrorMsg(err)))
    else:
      retFuture.complete()
      # We don't deallocate ``ol`` here because even though this completed
      # immediately poll will still be notified about its completion and it will
      # free ``ol``.
    return retFuture

  proc sendTo*(socket: AsyncFD, data: pointer, size: int, saddr: ptr SockAddr,
               saddrLen: Socklen,
               flags = {SocketFlag.SafeDisconn}): Future[void] =
    ## Sends ``data`` to specified destination ``saddr``, using
    ## socket ``socket``. The returned future will complete once all data
    ## has been sent.
    verifyPresence(socket)
    var retFuture = newFuture[void]("sendTo")
    var dataBuf: TWSABuf
    dataBuf.buf = cast[cstring](data)
    dataBuf.len = size.ULONG
    var bytesSent = 0.Dword
    var lowFlags = 0.Dword

    # we will preserve address in our stack
    var staddr: array[128, char] # SOCKADDR_STORAGE size is 128 bytes
    var stalen: cint = cint(saddrLen)
    zeroMem(addr(staddr[0]), 128)
    copyMem(addr(staddr[0]), saddr, saddrLen)

    var ol = PCustomOverlapped()
    GC_ref(ol)
    ol.data = CompletionData(fd: socket, cb:
      proc (fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
        if not retFuture.finished:
          if errcode == OSErrorCode(-1):
            retFuture.complete()
          else:
            retFuture.fail(newException(OSError, osErrorMsg(errcode)))
    )

    let ret = WSASendTo(socket.SocketHandle, addr dataBuf, 1, addr bytesSent,
                        lowFlags, cast[ptr SockAddr](addr(staddr[0])),
                        stalen, cast[POVERLAPPED](ol), nil)
    if ret == -1:
      let err = osLastError()
      if err.int32 != ERROR_IO_PENDING:
        GC_unref(ol)
        retFuture.fail(newException(OSError, osErrorMsg(err)))
    else:
      retFuture.complete()
      # We don't deallocate ``ol`` here because even though this completed
      # immediately poll will still be notified about its completion and it will
      # free ``ol``.
    return retFuture

  proc recvFromInto*(socket: AsyncFD, data: pointer, size: int,
                     saddr: ptr SockAddr, saddrLen: ptr SockLen,
                     flags = {SocketFlag.SafeDisconn}): Future[int] =
    ## Receives a datagram data from ``socket`` into ``buf``, which must
    ## be at least of size ``size``, address of datagram's sender will be
    ## stored into ``saddr`` and ``saddrLen``. Returned future will complete
    ## once one datagram has been received, and will return size of packet
    ## received.
    verifyPresence(socket)
    var retFuture = newFuture[int]("recvFromInto")

    var dataBuf = TWSABuf(buf: cast[cstring](data), len: size.ULONG)

    var bytesReceived = 0.Dword
    var lowFlags = 0.Dword

    var ol = PCustomOverlapped()
    GC_ref(ol)
    ol.data = CompletionData(fd: socket, cb:
      proc (fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
        if not retFuture.finished:
          if errcode == OSErrorCode(-1):
            assert bytesCount <= size
            retFuture.complete(bytesCount)
          else:
            # datagram sockets don't have disconnection,
            # so we can just raise an exception
            retFuture.fail(newException(OSError, osErrorMsg(errcode)))
    )

    let res = WSARecvFrom(socket.SocketHandle, addr dataBuf, 1,
                          addr bytesReceived, addr lowFlags,
                          saddr, cast[ptr cint](saddrLen),
                          cast[POVERLAPPED](ol), nil)
    if res == -1:
      let err = osLastError()
      if err.int32 != ERROR_IO_PENDING:
        GC_unref(ol)
        retFuture.fail(newException(OSError, osErrorMsg(err)))
    else:
      # Request completed immediately.
      if bytesReceived != 0:
        assert bytesReceived <= size
        retFuture.complete(bytesReceived)
      else:
        if hasOverlappedIoCompleted(cast[POVERLAPPED](ol)):
          retFuture.complete(bytesReceived)
    return retFuture

  proc acceptAddr*(socket: AsyncFD, flags = {SocketFlag.SafeDisconn}):
      Future[tuple[address: string, client: AsyncFD]] =
    ## Accepts a new connection. Returns a future containing the client socket
    ## corresponding to that connection and the remote address of the client.
    ## The future will complete when the connection is successfully accepted.
    ##
    ## The resulting client socket is automatically registered to the
    ## dispatcher.
    ##
    ## The ``accept`` call may result in an error if the connecting socket
    ## disconnects during the duration of the ``accept``. If the ``SafeDisconn``
    ## flag is specified then this error will not be raised and instead
    ## accept will be called again.
    verifyPresence(socket)
    var retFuture = newFuture[tuple[address: string, client: AsyncFD]]("acceptAddr")

    var clientSock = newNativeSocket()
    if clientSock == osInvalidSocket: raiseOSError(osLastError())

    const lpOutputLen = 1024
    var lpOutputBuf = newString(lpOutputLen)
    var dwBytesReceived: Dword
    let dwReceiveDataLength = 0.Dword # We don't want any data to be read.
    let dwLocalAddressLength = Dword(sizeof(Sockaddr_in) + 16)
    let dwRemoteAddressLength = Dword(sizeof(Sockaddr_in) + 16)

    template failAccept(errcode) =
      if flags.isDisconnectionError(errcode):
        var newAcceptFut = acceptAddr(socket, flags)
        newAcceptFut.callback =
          proc () =
            if newAcceptFut.failed:
              retFuture.fail(newAcceptFut.readError)
            else:
              retFuture.complete(newAcceptFut.read)
      else:
        retFuture.fail(newException(OSError, osErrorMsg(errcode)))

    template completeAccept() {.dirty.} =
      var listenSock = socket
      let setoptRet = setsockopt(clientSock, SOL_SOCKET,
          SO_UPDATE_ACCEPT_CONTEXT, addr listenSock,
          sizeof(listenSock).SockLen)
      if setoptRet != 0:
        let errcode = osLastError()
        discard clientSock.closeSocket()
        failAccept(errcode)
      else:
        var localSockaddr, remoteSockaddr: ptr SockAddr
        var localLen, remoteLen: int32
        getAcceptExSockaddrs(addr lpOutputBuf[0], dwReceiveDataLength,
                             dwLocalAddressLength, dwRemoteAddressLength,
                             addr localSockaddr, addr localLen,
                             addr remoteSockaddr, addr remoteLen)
        register(clientSock.AsyncFD)
        # TODO: IPv6. Check ``sa_family``. http://stackoverflow.com/a/9212542/492186
        retFuture.complete(
          (address: $inet_ntoa(cast[ptr Sockaddr_in](remoteSockAddr).sin_addr),
          client: clientSock.AsyncFD)
        )

    var ol = PCustomOverlapped()
    GC_ref(ol)
    ol.data = CompletionData(fd: socket, cb:
      proc (fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
        if not retFuture.finished:
          if errcode == OSErrorCode(-1):
            completeAccept()
          else:
            failAccept(errcode)
    )

    # http://msdn.microsoft.com/en-us/library/windows/desktop/ms737524%28v=vs.85%29.aspx
    let ret = acceptEx(socket.SocketHandle, clientSock, addr lpOutputBuf[0],
                       dwReceiveDataLength,
                       dwLocalAddressLength,
                       dwRemoteAddressLength,
                       addr dwBytesReceived, cast[POVERLAPPED](ol))

    if not ret:
      let err = osLastError()
      if err.int32 != ERROR_IO_PENDING:
        failAccept(err)
        GC_unref(ol)
    else:
      completeAccept()
      # We don't deallocate ``ol`` here because even though this completed
      # immediately poll will still be notified about its completion and it will
      # free ``ol``.

    return retFuture

  proc newAsyncNativeSocket*(domain, sockType, protocol: cint): AsyncFD =
    ## Creates a new socket and registers it with the dispatcher implicitly.
    result = newNativeSocket(domain, sockType, protocol).AsyncFD
    result.SocketHandle.setBlocking(false)
    register(result)

  proc newAsyncNativeSocket*(domain: Domain = nativesockets.AF_INET,
                             sockType: SockType = SOCK_STREAM,
                             protocol: Protocol = IPPROTO_TCP): AsyncFD =
    ## Creates a new socket and registers it with the dispatcher implicitly.
    result = newNativeSocket(domain, sockType, protocol).AsyncFD
    result.SocketHandle.setBlocking(false)
    register(result)

  proc closeSocket*(socket: AsyncFD) =
    ## Closes a socket and ensures that it is unregistered.
    socket.SocketHandle.close()
    getGlobalDispatcher().handles.excl(socket)

  proc unregister*(fd: AsyncFD) =
    ## Unregisters ``fd``.
    getGlobalDispatcher().handles.excl(fd)

  {.push stackTrace:off.}
  proc waitableCallback(param: pointer,
                        timerOrWaitFired: WINBOOL): void {.stdcall.} =
    var p = cast[PostCallbackDataPtr](param)
    discard postQueuedCompletionStatus(p.ioPort, timerOrWaitFired.Dword,
                                       ULONG_PTR(p.handleFd),
                                       cast[pointer](p.ovl))
  {.pop.}

  proc registerWaitableEvent(fd: AsyncFD, cb: Callback; mask: Dword) =
    let p = getGlobalDispatcher()
    var flags = (WT_EXECUTEINWAITTHREAD or WT_EXECUTEONLYONCE).Dword
    var hEvent = wsaCreateEvent()
    if hEvent == 0:
      raiseOSError(osLastError())
    var pcd = cast[PostCallbackDataPtr](allocShared0(sizeof(PostCallbackData)))
    pcd.ioPort = p.ioPort
    pcd.handleFd = fd
    var ol = PCustomOverlapped()
    GC_ref(ol)

    ol.data = CompletionData(fd: fd, cb:
      proc(fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
        # we excluding our `fd` because cb(fd) can register own handler
        # for this `fd`
        p.handles.excl(fd)
        # unregisterWait() is called before callback, because appropriate
        # winsockets function can re-enable event.
        # https://msdn.microsoft.com/en-us/library/windows/desktop/ms741576(v=vs.85).aspx
        if unregisterWait(pcd.waitFd) == 0:
          let err = osLastError()
          if err.int32 != ERROR_IO_PENDING:
            deallocShared(cast[pointer](pcd))
            discard wsaCloseEvent(hEvent)
            raiseOSError(err)
        if cb(fd):
          # callback returned `true`, so we free all allocated resources
          deallocShared(cast[pointer](pcd))
          if not wsaCloseEvent(hEvent):
            raiseOSError(osLastError())
          # pcd.ovl will be unrefed in poll().
        else:
          # callback returned `false` we need to continue
          if p.handles.contains(fd):
            # new callback was already registered with `fd`, so we free all
            # allocated resources. This happens because in callback `cb`
            # addRead/addWrite was called with same `fd`.
            deallocShared(cast[pointer](pcd))
            if not wsaCloseEvent(hEvent):
              raiseOSError(osLastError())
          else:
            # we need to include `fd` again
            p.handles.incl(fd)
            # and register WaitForSingleObject again
            if not registerWaitForSingleObject(addr(pcd.waitFd), hEvent,
                                    cast[WAITORTIMERCALLBACK](waitableCallback),
                                       cast[pointer](pcd), INFINITE, flags):
              # pcd.ovl will be unrefed in poll()
              let err = osLastError()
              deallocShared(cast[pointer](pcd))
              discard wsaCloseEvent(hEvent)
              raiseOSError(err)
            else:
              # we incref `pcd.ovl` and `protect` callback one more time,
              # because it will be unrefed and disposed in `poll()` after
              # callback finishes.
              GC_ref(pcd.ovl)
              pcd.ovl.data.cell = system.protect(rawEnv(pcd.ovl.data.cb))
    )
    # We need to protect our callback environment value, so GC will not free it
    # accidentally.
    ol.data.cell = system.protect(rawEnv(ol.data.cb))

    # This is main part of `hacky way` is using WSAEventSelect, so `hEvent`
    # will be signaled when appropriate `mask` events will be triggered.
    if wsaEventSelect(fd.SocketHandle, hEvent, mask) != 0:
      let err = osLastError()
      GC_unref(ol)
      deallocShared(cast[pointer](pcd))
      discard wsaCloseEvent(hEvent)
      raiseOSError(err)

    pcd.ovl = ol
    if not registerWaitForSingleObject(addr(pcd.waitFd), hEvent,
                                    cast[WAITORTIMERCALLBACK](waitableCallback),
                                       cast[pointer](pcd), INFINITE, flags):
      let err = osLastError()
      GC_unref(ol)
      deallocShared(cast[pointer](pcd))
      discard wsaCloseEvent(hEvent)
      raiseOSError(err)
    p.handles.incl(fd)

  proc addRead*(fd: AsyncFD, cb: Callback) =
    ## Start watching the file descriptor for read availability and then call
    ## the callback ``cb``.
    ##
    ## This is not ``pure`` mechanism for Windows Completion Ports (IOCP),
    ## so if you can avoid it, please do it. Use `addRead` only if really
    ## need it (main usecase is adaptation of `unix like` libraries to be
    ## asynchronous on Windows).
    ## If you use this function, you dont need to use asyncdispatch.recv()
    ## or asyncdispatch.accept(), because they are using IOCP, please use
    ## nativesockets.recv() and nativesockets.accept() instead.
    ##
    ## Be sure your callback ``cb`` returns ``true``, if you want to remove
    ## watch of `read` notifications, and ``false``, if you want to continue
    ## receiving notifies.
    registerWaitableEvent(fd, cb, FD_READ or FD_ACCEPT or FD_OOB or FD_CLOSE)

  proc addWrite*(fd: AsyncFD, cb: Callback) =
    ## Start watching the file descriptor for write availability and then call
    ## the callback ``cb``.
    ##
    ## This is not ``pure`` mechanism for Windows Completion Ports (IOCP),
    ## so if you can avoid it, please do it. Use `addWrite` only if really
    ## need it (main usecase is adaptation of `unix like` libraries to be
    ## asynchronous on Windows).
    ## If you use this function, you dont need to use asyncdispatch.send()
    ## or asyncdispatch.connect(), because they are using IOCP, please use
    ## nativesockets.send() and nativesockets.connect() instead.
    ##
    ## Be sure your callback ``cb`` returns ``true``, if you want to remove
    ## watch of `write` notifications, and ``false``, if you want to continue
    ## receiving notifies.
    registerWaitableEvent(fd, cb, FD_WRITE or FD_CONNECT or FD_CLOSE)

  template registerWaitableHandle(p, hEvent, flags, pcd, timeout,
                                  handleCallback) =
    let handleFD = AsyncFD(hEvent)
    pcd.ioPort = p.ioPort
    pcd.handleFd = handleFD
    var ol = PCustomOverlapped()
    GC_ref(ol)
    ol.data.fd = handleFD
    ol.data.cb = handleCallback
    # We need to protect our callback environment value, so GC will not free it
    # accidentally.
    ol.data.cell = system.protect(rawEnv(ol.data.cb))

    pcd.ovl = ol
    if not registerWaitForSingleObject(addr(pcd.waitFd), hEvent,
                                    cast[WAITORTIMERCALLBACK](waitableCallback),
                                    cast[pointer](pcd), timeout.Dword, flags):
      let err = osLastError()
      GC_unref(ol)
      deallocShared(cast[pointer](pcd))
      discard closeHandle(hEvent)
      raiseOSError(err)
    p.handles.incl(handleFD)

  template closeWaitable(handle: untyped) =
    let waitFd = pcd.waitFd
    deallocShared(cast[pointer](pcd))
    p.handles.excl(fd)
    if unregisterWait(waitFd) == 0:
        let err = osLastError()
        if err.int32 != ERROR_IO_PENDING:
          discard closeHandle(handle)
          raiseOSError(err)
    if closeHandle(handle) == 0:
      raiseOSError(osLastError())

  proc addTimer*(timeout: int, oneshot: bool, cb: Callback) =
    ## Registers callback ``cb`` to be called when timer expired.
    ## ``timeout`` - timeout value in milliseconds.
    ## ``oneshot`` - `true`, to generate only one timeout event, `false`, to
    ## generate timeout events periodically.

    doAssert(timeout > 0)
    let p = getGlobalDispatcher()

    var hEvent = createEvent(nil, 1, 0, nil)
    if hEvent == INVALID_HANDLE_VALUE:
      raiseOSError(osLastError())

    var pcd = cast[PostCallbackDataPtr](allocShared0(sizeof(PostCallbackData)))
    var flags = WT_EXECUTEINWAITTHREAD.Dword
    if oneshot: flags = flags or WT_EXECUTEONLYONCE

    proc timercb(fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
      let res = cb(fd)
      if res or oneshot:
        closeWaitable(hEvent)
      else:
        # if callback returned `false`, then it wants to be called again, so
        # we need to ref and protect `pcd.ovl` again, because it will be
        # unrefed and disposed in `poll()`.
        GC_ref(pcd.ovl)
        pcd.ovl.data.cell = system.protect(rawEnv(pcd.ovl.data.cb))

    registerWaitableHandle(p, hEvent, flags, pcd, timeout, timercb)

  proc addProcess*(pid: int, cb: Callback) =
    ## Registers callback ``cb`` to be called when process with pid ``pid``
    ## exited.
    let p = getGlobalDispatcher()
    let procFlags = SYNCHRONIZE
    var hProcess = openProcess(procFlags, 0, pid.Dword)
    if hProcess == INVALID_HANDLE_VALUE:
      raiseOSError(osLastError())

    var pcd = cast[PostCallbackDataPtr](allocShared0(sizeof(PostCallbackData)))
    var flags = WT_EXECUTEINWAITTHREAD.Dword

    proc proccb(fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
      closeWaitable(hProcess)
      discard cb(fd)

    registerWaitableHandle(p, hProcess, flags, pcd, INFINITE, proccb)

  proc newAsyncEvent*(): AsyncEvent =
    ## Creates new ``AsyncEvent`` object.
    ## New ``AsyncEvent`` object is not automatically registered with
    ## dispatcher like ``AsyncSocket``.
    var sa = SECURITY_ATTRIBUTES(
      nLength: sizeof(SECURITY_ATTRIBUTES).cint,
      bInheritHandle: 1
    )
    var event = createEvent(addr(sa), 0'i32, 0'i32, nil)
    if event == INVALID_HANDLE_VALUE:
      raiseOSError(osLastError())
    result = cast[AsyncEvent](allocShared0(sizeof(AsyncEventImpl)))
    result.hEvent = event

  proc setEvent*(ev: AsyncEvent) =
    ## Set event ``ev`` to signaled state.
    if setEvent(ev.hEvent) == 0:
      raiseOSError(osLastError())

  proc unregister*(ev: AsyncEvent) =
    ## Unregisters event ``ev``.
    doAssert(ev.hWaiter != 0, "Event is not registered in the queue!")
    let p = getGlobalDispatcher()
    p.handles.excl(AsyncFD(ev.hEvent))
    if unregisterWait(ev.hWaiter) == 0:
      let err = osLastError()
      if err.int32 != ERROR_IO_PENDING:
        raiseOSError(err)
    ev.hWaiter = 0

  proc close*(ev: AsyncEvent) =
    ## Closes event ``ev``.
    let res = closeHandle(ev.hEvent)
    deallocShared(cast[pointer](ev))
    if res == 0:
      raiseOSError(osLastError())

  proc addEvent*(ev: AsyncEvent, cb: Callback) =
    ## Registers callback ``cb`` to be called when ``ev`` will be signaled
    doAssert(ev.hWaiter == 0, "Event is already registered in the queue!")

    let p = getGlobalDispatcher()
    let hEvent = ev.hEvent

    var pcd = cast[PostCallbackDataPtr](allocShared0(sizeof(PostCallbackData)))
    var flags = WT_EXECUTEINWAITTHREAD.Dword

    proc eventcb(fd: AsyncFD, bytesCount: Dword, errcode: OSErrorCode) =
      if ev.hWaiter != 0:
        if cb(fd):
          # we need this check to avoid exception, if `unregister(event)` was
          # called in callback.
          deallocShared(cast[pointer](pcd))
          if ev.hWaiter != 0:
            unregister(ev)
        else:
          # if callback returned `false`, then it wants to be called again, so
          # we need to ref and protect `pcd.ovl` again, because it will be
          # unrefed and disposed in `poll()`.
          GC_ref(pcd.ovl)
          pcd.ovl.data.cell = system.protect(rawEnv(pcd.ovl.data.cb))
      else:
        # if ev.hWaiter == 0, then event was unregistered before `poll()` call.
        deallocShared(cast[pointer](pcd))

    registerWaitableHandle(p, hEvent, flags, pcd, INFINITE, eventcb)
    ev.hWaiter = pcd.waitFd

  initAll()
else:
  import ioselectors
  from posix import EINTR, EAGAIN, EINPROGRESS, EWOULDBLOCK, MSG_PEEK,
                    MSG_NOSIGNAL
  const
    InitCallbackListSize = 4         # initial size of callbacks sequence,
                                     # associated with file/socket descriptor.
    InitDelayedCallbackListSize = 64 # initial size of delayed callbacks
                                     # queue.
  type
    AsyncFD* = distinct cint
    Callback = proc (fd: AsyncFD): bool {.closure,gcsafe.}

    AsyncData = object
      readList: seq[Callback]
      writeList: seq[Callback]

    AsyncEvent* = distinct SelectEvent

    PDispatcher* = ref object of PDispatcherBase
      selector: Selector[AsyncData]
  {.deprecated: [TAsyncFD: AsyncFD, TCallback: Callback].}

  proc `==`*(x, y: AsyncFD): bool {.borrow.}
  proc `==`*(x, y: AsyncEvent): bool {.borrow.}

  template newAsyncData(): AsyncData =
    AsyncData(
      readList: newSeqOfCap[Callback](InitCallbackListSize),
      writeList: newSeqOfCap[Callback](InitCallbackListSize)
    )

  proc newDispatcher*(): PDispatcher =
    new result
    result.selector = newSelector[AsyncData]()
    result.timers.newHeapQueue()
    result.callbacks = initDeque[proc ()](InitDelayedCallbackListSize)

  var gDisp{.threadvar.}: PDispatcher ## Global dispatcher
  proc getGlobalDispatcher*(): PDispatcher =
    if gDisp.isNil: gDisp = newDispatcher()
    result = gDisp

  proc register*(fd: AsyncFD) =
    let p = getGlobalDispatcher()
    var data = newAsyncData()
    p.selector.registerHandle(fd.SocketHandle, {}, data)

  proc newAsyncNativeSocket*(domain: cint, sockType: cint,
                             protocol: cint): AsyncFD =
    result = newNativeSocket(domain, sockType, protocol).AsyncFD
    result.SocketHandle.setBlocking(false)
    when defined(macosx):
      result.SocketHandle.setSockOptInt(SOL_SOCKET, SO_NOSIGPIPE, 1)
    register(result)

  proc newAsyncNativeSocket*(domain: Domain = AF_INET,
                             sockType: SockType = SOCK_STREAM,
                             protocol: Protocol = IPPROTO_TCP): AsyncFD =
    result = newNativeSocket(domain, sockType, protocol).AsyncFD
    result.SocketHandle.setBlocking(false)
    when defined(macosx):
      result.SocketHandle.setSockOptInt(SOL_SOCKET, SO_NOSIGPIPE, 1)
    register(result)

  proc closeSocket*(sock: AsyncFD) =
    let disp = getGlobalDispatcher()
    disp.selector.unregister(sock.SocketHandle)
    sock.SocketHandle.close()

  proc unregister*(fd: AsyncFD) =
    getGlobalDispatcher().selector.unregister(fd.SocketHandle)

  proc unregister*(ev: AsyncEvent) =
    getGlobalDispatcher().selector.unregister(SelectEvent(ev))

  proc addRead*(fd: AsyncFD, cb: Callback) =
    let p = getGlobalDispatcher()
    var newEvents = {Event.Read}
    withData(p.selector, fd.SocketHandle, adata) do:
      adata.readList.add(cb)
      newEvents.incl(Event.Read)
      if len(adata.writeList) != 0: newEvents.incl(Event.Write)
    do:
      raise newException(ValueError, "File descriptor not registered.")
    p.selector.updateHandle(fd.SocketHandle, newEvents)

  proc addWrite*(fd: AsyncFD, cb: Callback) =
    let p = getGlobalDispatcher()
    var newEvents = {Event.Write}
    withData(p.selector, fd.SocketHandle, adata) do:
      adata.writeList.add(cb)
      newEvents.incl(Event.Write)
      if len(adata.readList) != 0: newEvents.incl(Event.Read)
    do:
      raise newException(ValueError, "File descriptor not registered.")
    p.selector.updateHandle(fd.SocketHandle, newEvents)

  proc hasPendingOperations*(): bool =
    let p = getGlobalDispatcher()
    not p.selector.isEmpty() or p.timers.len != 0 or p.callbacks.len != 0

  template processBasicCallbacks(ident, rwlist: untyped) =
    # Process pending descriptor's and AsyncEvent callbacks.
    # Invoke every callback stored in `rwlist`, until first one
    # returned `false`, which means callback wants to stay
    # alive. In such case all remaining callbacks will be added
    # to `rwlist` again, in the order they have been inserted.
    #
    # `rwlist` associated with file descriptor MUST BE emptied before
    # dispatching callback (See https://github.com/nim-lang/Nim/issues/5128),
    # or it can be possible to fall into endless cycle.
    var curList: seq[Callback]

    withData(p.selector, ident, adata) do:
      shallowCopy(curList, adata.rwlist)
      adata.rwlist = newSeqOfCap[Callback](InitCallbackListSize)

    let newLength = max(len(curList), InitCallbackListSize)
    var newList = newSeqOfCap[Callback](newLength)

    for cb in curList:
      if len(newList) > 0:
        newList.add(cb)
      else:
        if not cb(fd.AsyncFD):
          newList.add(cb)

    withData(p.selector, ident, adata) do:
      # descriptor still present in queue.
      adata.rwlist = newList & adata.rwlist
      rLength = len(adata.readList)
      wLength = len(adata.writeList)
    do:
      # descriptor was unregistered in callback via `unregister()`.
      rLength = -1
      wLength = -1

  template processCustomCallbacks(ident: untyped) =
    # Process pending custom event callbacks. Custom events are
    # {Event.Timer, Event.Signal, Event.Process, Event.Vnode}.
    # There can be only one callback registered with one descriptor,
    # so there no need to iterate over list.
    var curList: seq[Callback]

    withData(p.selector, ident, adata) do:
      shallowCopy(curList, adata.readList)
      adata.readList = newSeqOfCap[Callback](InitCallbackListSize)

    let newLength = len(curList)
    var newList = newSeqOfCap[Callback](newLength)

    var cb = curList[0]
    if not cb(fd.AsyncFD):
      newList.add(cb)

    withData(p.selector, ident, adata) do:
      # descriptor still present in queue.
      adata.readList = newList & adata.readList
      if len(adata.readList) == 0:
        # if no callbacks registered with descriptor, unregister it.
        p.selector.unregister(fd)
    do:
      # descriptor was unregistered in callback via `unregister()`.
      discard

  proc poll*(timeout = 500) =
    var keys: array[64, ReadyKey]

    let p = getGlobalDispatcher()
    when ioselSupportedPlatform:
      let customSet = {Event.Timer, Event.Signal, Event.Process,
                       Event.Vnode}

    if p.selector.isEmpty() and p.timers.len == 0 and p.callbacks.len == 0:
      raise newException(ValueError,
        "No handles or timers registered in dispatcher.")

    if not p.selector.isEmpty():
      var count = p.selector.selectInto(p.adjustedTimeout(timeout), keys)
      var i = 0
      while i < count:
        var custom = false
        let fd = keys[i].fd
        let events = keys[i].events
        var rLength = 0 # len(data.readList) after callback
        var wLength = 0 # len(data.writeList) after callback

        if Event.Read in events or events == {Event.Error}:
          processBasicCallbacks(fd, readList)

        if Event.Write in events or events == {Event.Error}:
          processBasicCallbacks(fd, writeList)

        if Event.User in events or events == {Event.Error}:
          processBasicCallbacks(fd, readList)
          custom = true
          if rLength == 0:
            p.selector.unregister(fd)

        when ioselSupportedPlatform:
          if (customSet * events) != {}:
            custom = true
            processCustomCallbacks(fd)

        # because state `data` can be modified in callback we need to update
        # descriptor events with currently registered callbacks.
        if not custom:
          var newEvents: set[Event] = {}
          if rLength != -1 and wLength != -1:
            if rLength > 0: incl(newEvents, Event.Read)
            if wLength > 0: incl(newEvents, Event.Write)
            p.selector.updateHandle(SocketHandle(fd), newEvents)
        inc(i)

    # Timer processing.
    processTimers(p)
    # Callback queue processing
    processPendingCallbacks(p)

  proc connect*(socket: AsyncFD, address: string, port: Port,
    domain = AF_INET): Future[void] =
    var retFuture = newFuture[void]("connect")

    proc cb(fd: AsyncFD): bool =
      var ret = SocketHandle(fd).getSockOptInt(cint(SOL_SOCKET), cint(SO_ERROR))
      if ret == 0:
          # We have connected.
          retFuture.complete()
          return true
      elif ret == EINTR:
          # interrupted, keep waiting
          return false
      else:
          retFuture.fail(newException(OSError, osErrorMsg(OSErrorCode(ret))))
          return true

    assert getSockDomain(socket.SocketHandle) == domain
    var aiList = getAddrInfo(address, port, domain)
    var success = false
    var lastError: OSErrorCode
    var it = aiList
    while it != nil:
      var ret = connect(socket.SocketHandle, it.ai_addr, it.ai_addrlen.Socklen)
      if ret == 0:
        # Request to connect completed immediately.
        success = true
        retFuture.complete()
        break
      else:
        lastError = osLastError()
        if lastError.int32 == EINTR or lastError.int32 == EINPROGRESS:
          success = true
          addWrite(socket, cb)
          break
        else:
          success = false
      it = it.ai_next

    freeAddrInfo(aiList)
    if not success:
      retFuture.fail(newException(OSError, osErrorMsg(lastError)))
    return retFuture

  proc recv*(socket: AsyncFD, size: int,
             flags = {SocketFlag.SafeDisconn}): Future[string] =
    var retFuture = newFuture[string]("recv")

    var readBuffer = newString(size)

    proc cb(sock: AsyncFD): bool =
      result = true
      let res = recv(sock.SocketHandle, addr readBuffer[0], size.cint,
                     flags.toOSFlags())
      if res < 0:
        let lastError = osLastError()
        if lastError.int32 notin {EINTR, EWOULDBLOCK, EAGAIN}:
          if flags.isDisconnectionError(lastError):
            retFuture.complete("")
          else:
            retFuture.fail(newException(OSError, osErrorMsg(lastError)))
        else:
          result = false # We still want this callback to be called.
      elif res == 0:
        # Disconnected
        retFuture.complete("")
      else:
        readBuffer.setLen(res)
        retFuture.complete(readBuffer)
    # TODO: The following causes a massive slowdown.
    #if not cb(socket):
    addRead(socket, cb)
    return retFuture

  proc recvInto*(socket: AsyncFD, buf: pointer, size: int,
                 flags = {SocketFlag.SafeDisconn}): Future[int] =
    var retFuture = newFuture[int]("recvInto")

    proc cb(sock: AsyncFD): bool =
      result = true
      let res = recv(sock.SocketHandle, buf, size.cint,
                     flags.toOSFlags())
      if res < 0:
        let lastError = osLastError()
        if lastError.int32 notin {EINTR, EWOULDBLOCK, EAGAIN}:
          if flags.isDisconnectionError(lastError):
            retFuture.complete(0)
          else:
            retFuture.fail(newException(OSError, osErrorMsg(lastError)))
        else:
          result = false # We still want this callback to be called.
      else:
        retFuture.complete(res)
    # TODO: The following causes a massive slowdown.
    #if not cb(socket):
    addRead(socket, cb)
    return retFuture

  proc send*(socket: AsyncFD, buf: pointer, size: int,
             flags = {SocketFlag.SafeDisconn}): Future[void] =
    var retFuture = newFuture[void]("send")

    var written = 0

    proc cb(sock: AsyncFD): bool =
      result = true
      let netSize = size-written
      var d = cast[cstring](buf)
      let res = send(sock.SocketHandle, addr d[written], netSize.cint,
                     MSG_NOSIGNAL)
      if res < 0:
        let lastError = osLastError()
        if lastError.int32 notin {EINTR, EWOULDBLOCK, EAGAIN}:
          if flags.isDisconnectionError(lastError):
            retFuture.complete()
          else:
            retFuture.fail(newException(OSError, osErrorMsg(lastError)))
        else:
          result = false # We still want this callback to be called.
      else:
        written.inc(res)
        if res != netSize:
          result = false # We still have data to send.
        else:
          retFuture.complete()
    # TODO: The following causes crashes.
    #if not cb(socket):
    addWrite(socket, cb)
    return retFuture

  proc send*(socket: AsyncFD, data: string,
             flags = {SocketFlag.SafeDisconn}): Future[void] =
    var retFuture = newFuture[void]("send")

    var written = 0

    proc cb(sock: AsyncFD): bool =
      result = true
      let netSize = data.len-written
      var d = data.cstring
      let res = send(sock.SocketHandle, addr d[written], netSize.cint,
                     MSG_NOSIGNAL)
      if res < 0:
        let lastError = osLastError()
        if lastError.int32 notin {EINTR, EWOULDBLOCK, EAGAIN}:
          if flags.isDisconnectionError(lastError):
            retFuture.complete()
          else:
            retFuture.fail(newException(OSError, osErrorMsg(lastError)))
        else:
          result = false # We still want this callback to be called.
      else:
        written.inc(res)
        if res != netSize:
          result = false # We still have data to send.
        else:
          retFuture.complete()
    # TODO: The following causes crashes.
    #if not cb(socket):
    addWrite(socket, cb)
    return retFuture

  proc sendTo*(socket: AsyncFD, data: pointer, size: int, saddr: ptr SockAddr,
               saddrLen: SockLen,
               flags = {SocketFlag.SafeDisconn}): Future[void] =
    ## Sends ``data`` of size ``size`` in bytes to specified destination
    ## (``saddr`` of size ``saddrLen`` in bytes, using socket ``socket``.
    ## The returned future will complete once all data has been sent.
    var retFuture = newFuture[void]("sendTo")

    # we will preserve address in our stack
    var staddr: array[128, char] # SOCKADDR_STORAGE size is 128 bytes
    var stalen = saddrLen
    zeroMem(addr(staddr[0]), 128)
    copyMem(addr(staddr[0]), saddr, saddrLen)

    proc cb(sock: AsyncFD): bool =
      result = true
      let res = sendto(sock.SocketHandle, data, size, MSG_NOSIGNAL,
                       cast[ptr SockAddr](addr(staddr[0])), stalen)
      if res < 0:
        let lastError = osLastError()
        if lastError.int32 notin {EINTR, EWOULDBLOCK, EAGAIN}:
          retFuture.fail(newException(OSError, osErrorMsg(lastError)))
        else:
          result = false # We still want this callback to be called.
      else:
        retFuture.complete()

    addWrite(socket, cb)
    return retFuture

  proc recvFromInto*(socket: AsyncFD, data: pointer, size: int,
                     saddr: ptr SockAddr, saddrLen: ptr SockLen,
                     flags = {SocketFlag.SafeDisconn}): Future[int] =
    ## Receives a datagram data from ``socket`` into ``data``, which must
    ## be at least of size ``size`` in bytes, address of datagram's sender
    ## will be stored into ``saddr`` and ``saddrLen``. Returned future will
    ## complete once one datagram has been received, and will return size
    ## of packet received.
    var retFuture = newFuture[int]("recvFromInto")
    proc cb(sock: AsyncFD): bool =
      result = true
      let res = recvfrom(sock.SocketHandle, data, size.cint, flags.toOSFlags(),
                         saddr, saddrLen)
      if res < 0:
        let lastError = osLastError()
        if lastError.int32 notin {EINTR, EWOULDBLOCK, EAGAIN}:
          retFuture.fail(newException(OSError, osErrorMsg(lastError)))
        else:
          result = false
      else:
        retFuture.complete(res)
    addRead(socket, cb)
    return retFuture

  proc acceptAddr*(socket: AsyncFD, flags = {SocketFlag.SafeDisconn}):
      Future[tuple[address: string, client: AsyncFD]] =
    var retFuture = newFuture[tuple[address: string,
        client: AsyncFD]]("acceptAddr")
    proc cb(sock: AsyncFD): bool =
      result = true
      var sockAddress: Sockaddr_storage
      var addrLen = sizeof(sockAddress).Socklen
      var client = accept(sock.SocketHandle,
                          cast[ptr SockAddr](addr(sockAddress)), addr(addrLen))
      if client == osInvalidSocket:
        let lastError = osLastError()
        assert lastError.int32 notin {EWOULDBLOCK, EAGAIN}
        if lastError.int32 == EINTR:
          return false
        else:
          if flags.isDisconnectionError(lastError):
            return false
          else:
            retFuture.fail(newException(OSError, osErrorMsg(lastError)))
      else:
        register(client.AsyncFD)
        retFuture.complete((getAddrString(cast[ptr SockAddr](addr sockAddress)),
                            client.AsyncFD))
    addRead(socket, cb)
    return retFuture

  when ioselSupportedPlatform:

    proc addTimer*(timeout: int, oneshot: bool, cb: Callback) =
      ## Start watching for timeout expiration, and then call the
      ## callback ``cb``.
      ## ``timeout`` - time in milliseconds,
      ## ``oneshot`` - if ``true`` only one event will be dispatched,
      ## if ``false`` continuous events every ``timeout`` milliseconds.
      let p = getGlobalDispatcher()
      var data = newAsyncData()
      data.readList.add(cb)
      p.selector.registerTimer(timeout, oneshot, data)

    proc addSignal*(signal: int, cb: Callback) =
      ## Start watching signal ``signal``, and when signal appears, call the
      ## callback ``cb``.
      let p = getGlobalDispatcher()
      var data = newAsyncData()
      data.readList.add(cb)
      p.selector.registerSignal(signal, data)

    proc addProcess*(pid: int, cb: Callback) =
      ## Start watching for process exit with pid ``pid``, and then call
      ## the callback ``cb``.
      let p = getGlobalDispatcher()
      var data = newAsyncData()
      data.readList.add(cb)
      p.selector.registerProcess(pid, data)

  proc newAsyncEvent*(): AsyncEvent =
    ## Creates new ``AsyncEvent``.
    result = AsyncEvent(newSelectEvent())

  proc setEvent*(ev: AsyncEvent) =
    ## Sets new ``AsyncEvent`` to signaled state.
    setEvent(SelectEvent(ev))

  proc close*(ev: AsyncEvent) =
    ## Closes ``AsyncEvent``
    close(SelectEvent(ev))

  proc addEvent*(ev: AsyncEvent, cb: Callback) =
    ## Start watching for event ``ev``, and call callback ``cb``, when
    ## ev will be set to signaled state.
    let p = getGlobalDispatcher()
    var data = newAsyncData()
    data.readList.add(cb)
    p.selector.registerEvent(SelectEvent(ev), data)

proc sleepAsync*(ms: int): Future[void] =
  ## Suspends the execution of the current async procedure for the next
  ## ``ms`` milliseconds.
  var retFuture = newFuture[void]("sleepAsync")
  let p = getGlobalDispatcher()
  p.timers.push((epochTime() + (ms / 1000), retFuture))
  return retFuture

proc withTimeout*[T](fut: Future[T], timeout: int): Future[bool] =
  ## Returns a future which will complete once ``fut`` completes or after
  ## ``timeout`` milliseconds has elapsed.
  ##
  ## If ``fut`` completes first the returned future will hold true,
  ## otherwise, if ``timeout`` milliseconds has elapsed first, the returned
  ## future will hold false.

  var retFuture = newFuture[bool]("asyncdispatch.`withTimeout`")
  var timeoutFuture = sleepAsync(timeout)
  fut.callback =
    proc () =
      if not retFuture.finished: retFuture.complete(true)
  timeoutFuture.callback =
    proc () =
      if not retFuture.finished: retFuture.complete(false)
  return retFuture

proc accept*(socket: AsyncFD,
    flags = {SocketFlag.SafeDisconn}): Future[AsyncFD] =
  ## Accepts a new connection. Returns a future containing the client socket
  ## corresponding to that connection.
  ## The future will complete when the connection is successfully accepted.
  var retFut = newFuture[AsyncFD]("accept")
  var fut = acceptAddr(socket, flags)
  fut.callback =
    proc (future: Future[tuple[address: string, client: AsyncFD]]) =
      assert future.finished
      if future.failed:
        retFut.fail(future.error)
      else:
        retFut.complete(future.read.client)
  return retFut

# -- Await Macro
include asyncmacro

proc readAll*(future: FutureStream[string]): Future[string] {.async.} =
  ## Returns a future that will complete when all the string data from the
  ## specified future stream is retrieved.
  result = ""
  while true:
    let (hasValue, value) = await future.read()
    if hasValue:
      result.add(value)
    else:
      break

proc recvLine*(socket: AsyncFD): Future[string] {.async.} =
  ## Reads a line of data from ``socket``. Returned future will complete once
  ## a full line is read or an error occurs.
  ##
  ## If a full line is read ``\r\L`` is not
  ## added to ``line``, however if solely ``\r\L`` is read then ``line``
  ## will be set to it.
  ##
  ## If the socket is disconnected, ``line`` will be set to ``""``.
  ##
  ## If the socket is disconnected in the middle of a line (before ``\r\L``
  ## is read) then line will be set to ``""``.
  ## The partial line **will be lost**.
  ##
  ## **Warning**: This assumes that lines are delimited by ``\r\L``.
  ##
  ## **Note**: This procedure is mostly used for testing. You likely want to
  ## use ``asyncnet.recvLine`` instead.

  template addNLIfEmpty(): typed =
    if result.len == 0:
      result.add("\c\L")

  result = ""
  var c = ""
  while true:
    c = await recv(socket, 1)
    if c.len == 0:
      return ""
    if c == "\r":
      c = await recv(socket, 1)
      assert c == "\l"
      addNLIfEmpty()
      return
    elif c == "\L":
      addNLIfEmpty()
      return
    add(result, c)

proc callSoon*(cbproc: proc ()) =
  ## Schedule `cbproc` to be called as soon as possible.
  ## The callback is called when control returns to the event loop.
  getGlobalDispatcher().callbacks.addLast(cbproc)

proc runForever*() =
  ## Begins a never ending global dispatcher poll loop.
  while true:
    poll()

proc waitFor*[T](fut: Future[T]): T =
  ## **Blocks** the current thread until the specified future completes.
  while not fut.finished:
    poll()

  fut.read