twisted reactor执行流程

#reactorbase的主循环
 def mainLoop(self):
        while self._started:
            try:
                while self._started:
                    # Advance simulation time in delayed event
                    # processors.
                    self.runUntilCurrent()
                    t2 = self.timeout()
                    t = self.running and t2
                    self.doIteration(t)
            except:
                log.msg("Unexpected error in main loop.")
                log.err()
            else:
                log.msg('Main loop terminated.')

#posixreactorbase.py
def doPoll(self, timeout):
        """
        Poll the poller for new events.
        """
        if timeout is None:
            timeout = -1  # Wait indefinitely.

        try:
            # Limit the number of events to the number of io objects we're
            # currently tracking (because that's maybe a good heuristic) and
            # the amount of time we block to the value specified by our
            # caller.
            l = self._poller.poll(timeout, len(self._selectables))
        except IOError as err:
            if err.errno == errno.EINTR:
                return
            # See epoll_wait(2) for documentation on the other conditions
            # under which this can fail.  They can only be due to a serious
            # programming error on our part, so let's just announce them
            # loudly.
            raise

        _drdw = self._doReadOrWrite
        for fd, event in l:
            try:
                selectable = self._selectables[fd]
            except KeyError:
                pass
            else:
                log.callWithLogger(selectable, _drdw, selectable, fd, event)

    doIteration = doPoll#类变量的doItertion为doPoll函数
#_drdw为以下类的_doReadOrWrite方法
class _PollLikeMixin(object):
    """
    Mixin for poll-like reactors.

    Subclasses must define the following attributes::

      - _POLL_DISCONNECTED - Bitmask for events indicating a connection was
        lost.
      - _POLL_IN - Bitmask for events indicating there is input to read.
      - _POLL_OUT - Bitmask for events indicating output can be written.

    Must be mixed in to a subclass of PosixReactorBase (for
    _disconnectSelectable).
    """

    def _doReadOrWrite(self, selectable, fd, event):
        """
        fd is available for read or write, do the work and raise errors if
        necessary.
        """
        why = None
        inRead = False
        if event & self._POLL_DISCONNECTED and not (event & self._POLL_IN):
            # Handle disconnection.  But only if we finished processing all
            # the pending input.
            if fd in self._reads:
                # If we were reading from the descriptor then this is a
                # clean shutdown.  We know there are no read events pending
                # because we just checked above.  It also might be a
                # half-close (which is why we have to keep track of inRead).
                inRead = True
                why = CONNECTION_DONE
            else:
                # If we weren't reading, this is an error shutdown of some
                # sort.
                why = CONNECTION_LOST
        else:
            # Any non-disconnect event turns into a doRead or a doWrite.
            try:
                # First check to see if the descriptor is still valid.  This
                # gives fileno() a chance to raise an exception, too.
                # Ideally, disconnection would always be indicated by the
                # return value of doRead or doWrite (or an exception from
                # one of those methods), but calling fileno here helps make
                # buggy applications more transparent.
                if selectable.fileno() == -1:
                    # -1 is sort of a historical Python artifact.  Python
                    # files and sockets used to change their file descriptor
                    # to -1 when they closed.  For the time being, we'll
                    # continue to support this anyway in case applications
                    # replicated it, plus abstract.FileDescriptor.fileno
                    # returns -1.  Eventually it'd be good to deprecate this
                    # case.
                    why = _NO_FILEDESC
                else:
                    if event & self._POLL_IN:
                        # Handle a read event.
                        why = selectable.doRead()#调用selectable的doread方法
                        inRead = True
                    if not why and event & self._POLL_OUT:
                        # Handle a write event, as long as doRead didn't
                        # disconnect us.
                        why = selectable.doWrite()#调用selectable的dowrite方法
                        inRead = False
            except:
                # Any exception from application code gets logged and will
                # cause us to disconnect the selectable.
                why = sys.exc_info()[1]
                log.err()
        if why:
            self._disconnectSelectable(selectable, why, inRead)