text: #!/usr/bin/python
  # -*- coding: utf-8 -*-
  #
  # Copyright 2013-2015 clowwindy
  #
  # Licensed under the Apache License, Version 2.0 (the "License"); you may
  # not use this file except in compliance with the License. You may obtain
  # a copy of the License at
  #
  #     http://www.apache.org/licenses/LICENSE-2.0
  #
  # Unless required by applicable law or agreed to in writing, software
  # distributed under the License is distributed on an "AS IS" BASIS, WITHOUT
  # WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. See the
  # License for the specific language governing permissions and limitations
  # under the License.
  
  # from ssloop
  # https://github.com/clowwindy/ssloop
  
  from __future__ import absolute_import, division, print_function, \
      with_statement
  
  import os
  import time
  import socket
  import select
  import errno
  import logging
  from collections import defaultdict
  
  from shadowsocks import shell
  
  
  __all__ = ['EventLoop', 'POLL_NULL', 'POLL_IN', 'POLL_OUT', 'POLL_ERR',
             'POLL_HUP', 'POLL_NVAL', 'EVENT_NAMES']
  
  POLL_NULL = 0x00
  POLL_IN = 0x01
  POLL_OUT = 0x04
  POLL_ERR = 0x08
  POLL_HUP = 0x10
  POLL_NVAL = 0x20
  
  
  EVENT_NAMES = {
      POLL_NULL: 'POLL_NULL',
      POLL_IN: 'POLL_IN',
      POLL_OUT: 'POLL_OUT',
      POLL_ERR: 'POLL_ERR',
      POLL_HUP: 'POLL_HUP',
      POLL_NVAL: 'POLL_NVAL',
  }
  
  # we check timeouts every TIMEOUT_PRECISION seconds
  TIMEOUT_PRECISION = 1
  
  
  class KqueueLoop(object):
  
      MAX_EVENTS = 1024
  
      def __init__(self):
          self._kqueue = select.kqueue()
          self._fds = {}
  
      def _control(self, fd, mode, flags):
          events = []
          if mode & POLL_IN:
              events.append(select.kevent(fd, select.KQ_FILTER_READ, flags))
          if mode & POLL_OUT:
              events.append(select.kevent(fd, select.KQ_FILTER_WRITE, flags))
          for e in events:
              self._kqueue.control([e], 0)
  
      def poll(self, timeout):
          if timeout < 0:
              timeout = None  # kqueue behaviour
          events = self._kqueue.control(None, KqueueLoop.MAX_EVENTS, timeout)
          results = defaultdict(lambda: POLL_NULL)
          for e in events:
              fd = e.ident
              if e.filter == select.KQ_FILTER_READ:
                  results[fd] |= POLL_IN
              elif e.filter == select.KQ_FILTER_WRITE:
                  results[fd] |= POLL_OUT
          return results.items()
  
      def register(self, fd, mode):
          self._fds[fd] = mode
          self._control(fd, mode, select.KQ_EV_ADD)
  
      def unregister(self, fd):
          self._control(fd, self._fds[fd], select.KQ_EV_DELETE)
          del self._fds[fd]
  
      def modify(self, fd, mode):
          self.unregister(fd)
          self.register(fd, mode)
  
      def close(self):
          self._kqueue.close()
  
  
  class SelectLoop(object):
  
      def __init__(self):
          self._r_list = set()
          self._w_list = set()
          self._x_list = set()
  
      def poll(self, timeout):
          r, w, x = select.select(self._r_list, self._w_list, self._x_list,
                                  timeout)
          results = defaultdict(lambda: POLL_NULL)
          for p in [(r, POLL_IN), (w, POLL_OUT), (x, POLL_ERR)]:
              for fd in p[0]:
                  results[fd] |= p[1]
          return results.items()
  
      def register(self, fd, mode):
          if mode & POLL_IN:
              self._r_list.add(fd)
          if mode & POLL_OUT:
              self._w_list.add(fd)
          if mode & POLL_ERR:
              self._x_list.add(fd)
  
      def unregister(self, fd):
          if fd in self._r_list:
              self._r_list.remove(fd)
          if fd in self._w_list:
              self._w_list.remove(fd)
          if fd in self._x_list:
              self._x_list.remove(fd)
  
      def modify(self, fd, mode):
          self.unregister(fd)
          self.register(fd, mode)
  
      def close(self):
          pass
  
  
  class EventLoop(object):
      def __init__(self):
          if hasattr(select, 'epoll'):
              self._impl = select.epoll()
              model = 'epoll'
          elif hasattr(select, 'kqueue'):
              self._impl = KqueueLoop()
              model = 'kqueue'
          elif hasattr(select, 'select'):
              self._impl = SelectLoop()
              model = 'select'
          else:
              raise Exception('can not find any available functions in select '
                              'package')
          self._fdmap = {}  # (f, handler)
          self._last_time = time.time()
          self._periodic_callbacks = []
          self._periodic_callbacks_tick = {}
          self._stopping = False
          logging.debug('using event model: %s', model)
  
      def poll(self, timeout=None):
          events = self._impl.poll(timeout)
          return [(self._fdmap[fd][0], fd, event) for fd, event in events]
  
      def add(self, f, mode, handler):
          fd = f.fileno()
          self._fdmap[fd] = (f, handler)
          self._impl.register(fd, mode)
  
      def remove(self, f):
          fd = f.fileno()
          del self._fdmap[fd]
          self._impl.unregister(fd)
  
      def removefd(self, fd):
          del self._fdmap[fd]
          self._impl.unregister(fd)
  
      def add_periodic(self, callback, tick=2):
          self._periodic_callbacks.append(callback)
          self._periodic_callbacks_tick[callback] = (tick, 0)
  
      def remove_periodic(self, callback):
          self._periodic_callbacks.remove(callback)
          del self._periodic_callbacks_tick[callback]
  
      def modify(self, f, mode):
          fd = f.fileno()
          self._impl.modify(fd, mode)
  
      def stop(self):
          self._stopping = True
  
      def run(self):
          events = []
          while not self._stopping:
              asap = False
              try:
                  if len(self._fdmap) > 0:
                      events = self.poll(TIMEOUT_PRECISION)
                  else:
                      continue
              except (OSError, IOError) as e:
                  if errno_from_exception(e) in (errno.EPIPE, errno.EINTR):
                      # EPIPE: Happens when the client closes the connection
                      # EINTR: Happens when received a signal
                      # handles them as soon as possible
                      asap = True
                      logging.debug('poll:%s', e)
                  else:
                      logging.error('poll:%s', e)
                      import traceback
                      traceback.print_exc()
                      continue
  
              handle = False
              for sock, fd, event in events:
                  handler = self._fdmap.get(fd, None)
                  if handler is not None:
                      handler = handler[1]
                      try:
                          handle = handler.handle_event(sock, fd, event) or handle
                      except (OSError, IOError) as e:
                          shell.print_exception(e)
              now = time.time()
              if asap or now - self._last_time >= TIMEOUT_PRECISION:
                  for callback in self._periodic_callbacks:
                      tick, ticked = self._periodic_callbacks_tick[callback]
                      self._periodic_callbacks_tick[callback] = (tick, (ticked + 1) % (tick + 1))
                      if tick == ticked:
                          callback()
                  self._last_time = now
              if events and not handle:
                  time.sleep(0.001)
  
      def __del__(self):
          self._impl.close()
  
  
  # from tornado
  def errno_from_exception(e):
      """Provides the errno from an Exception object.
  
      There are cases that the errno attribute was not set so we pull
      the errno out of the args but if someone instatiates an Exception
      without any args you will get a tuple error. So this function
      abstracts all that behavior to give you a safe way to get the
      errno.
      """
  
      if hasattr(e, 'errno'):
          return e.errno
      elif e.args:
          return e.args[0]
      else:
          return None
  
  
  # from tornado
  def get_sock_error(sock):
      error_number = sock.getsockopt(socket.SOL_SOCKET, socket.SO_ERROR)
      return socket.error(error_number, os.strerror(error_number))
  