Python gevent协程切换实现详解

def recv(self, *args):
  while 1:
    try:
      return self._sock.recv(*args)
    except error as ex:
      if ex.args[0] != EWOULDBLOCK or self.timeout == 0.0:
        raise
      # QQQ without clearing exc_info test__refcount.test_clean_exit fails
      sys.exc_clear()
    self._wait(self._read_event)

# Suitable to be bound as an instance method
def wait_on_socket(socket, watcher, timeout_exc=None):
  if socket is None or watcher is None:
    # test__hub TestCloseSocketWhilePolling, on Python 2; Python 3
    # catches the EBADF differently.
    raise ConcurrentObjectUseError("The socket has already been closed by another greenlet")
  _primitive_wait(watcher, socket.timeout,
          timeout_exc if timeout_exc is not None else _NONE,
          socket.hub)

def _primitive_wait(watcher, timeout, timeout_exc, hub):
  if watcher.callback is not None:
    raise ConcurrentObjectUseError('This socket is already used by another greenlet: %r'
                    % (watcher.callback, ))

  if hub is None:
    hub = get_hub()

  if timeout is None:
    hub.wait(watcher)
    return

  timeout = Timeout._start_new_or_dummy(
    timeout,
    (timeout_exc
     if timeout_exc is not _NONE or timeout is None
     else _timeout_error('timed out')))

  with timeout:
    hub.wait(watcher)

class WaitOperationsGreenlet(SwitchOutGreenletWithLoop): # pylint:disable=undefined-variable

  def wait(self, watcher):
    """
    Wait until the *watcher* (which must not be started) is ready.

    The current greenlet will be unscheduled during this time.
    """
    waiter = Waiter(self) # pylint:disable=undefined-variable
    watcher.start(waiter.switch, waiter)
    try:
      result = waiter.get()
      if result is not waiter:
        raise InvalidSwitchError(
          'Invalid switch into %s: got %r (expected %r; waiting on %r with %r)' % (
            getcurrent(), # pylint:disable=undefined-variable
            result,
            waiter,
            self,
            watcher
          )
        )
    finally:
      watcher.stop()

def get(self):
  """If a value/an exception is stored, return/raise it. Otherwise until switch() or throw() is called."""
  if self._exception is not _NONE:
    if self._exception is None:
      return self.value
    getcurrent().throw(*self._exception) # pylint:disable=undefined-variable
  else:
    if self.greenlet is not None:
      raise ConcurrentObjectUseError('This Waiter is already used by %r' % (self.greenlet, ))
    self.greenlet = getcurrent() # pylint:disable=undefined-variable
    try:
      return self.hub.switch()
    finally:
      self.greenlet = None