IOLoop的初始化

初始化过程中选择 epoll 的实现方式，Linux 平台为 epoll，BSD 平台为 kqueue，其他平台如果安装有C模块扩展的 epoll 则使用 tornado对 epoll 的封装，否则退化为 select。

class Configurable(object):
def __new__(cls, *args, **kwargs):
    base = cls.configurable_base()
    init_kwargs = {}
    if cls is base:
        impl = cls.configured_class()
        if base.__impl_kwargs:
            init_kwargs.update(base.__impl_kwargs)
    else:
        impl = cls
    init_kwargs.update(kwargs)
    instance = super(Configurable, cls).__new__(impl)
    # initialize vs __init__ chosen for compatibility with AsyncHTTPClient
    # singleton magic.  If we get rid of that we can switch to __init__
    # here too.
    instance.initialize(*args, **init_kwargs)
    return instance

IOLoop继承Configurable类，会在创建是调用new创建类。作者用的是ubuntu，所以会返回一个继承自IOLoop的PollIOLoop类，并且调用类的初始化initialize函数。如下:

def initialize(self, impl, time_func=None, **kwargs):
      super(PollIOLoop, self).initialize(**kwargs)
      self._impl = impl
      if hasattr(self._impl, 'fileno'):
          set_close_exec(self._impl.fileno())
      self.time_func = time_func or time.time
      self._handlers = {}
      self._events = {}
      self._callbacks = []
      self._callback_lock = threading.Lock()
      self._timeouts = []
      self._cancellations = 0
      self._running = False
      self._stopped = False
      self._closing = False
      self._thread_ident = None
      self._blocking_signal_threshold = None
      self._timeout_counter = itertools.count()

      # Create a pipe that we send bogus data to when we want to wake
      # the I/O loop when it is idle
      self._waker = Waker()
      self.add_handler(self._waker.fileno(),
                       lambda fd, events: self._waker.consume(),
                       self.READ)

# 添加一个socket和它的回调函数
def add_handler(self, fd, handler, events):
    fd, obj = self.split_fd(fd)
    self._handlers[fd] = (obj, stack_context.wrap(handler))
    self._impl.register(fd, events | self.ERROR)

PollIOLoop初始化的过程中创建了一个 Waker 对象,将Waker的fd注册到事件回调函数中，设置相应的回调函数(
这么做是为了可以通过Waker类来唤醒阻塞的循环事件)。

PollIOLoop.start()

PollIOLoop 的核心调度集中在 start() 方法中，可以调用该实例的stop()来停止循环。
由于start()代码太长，作者只取了一些主要片段介绍。

with self._callback_lock:
    callbacks = self._callbacks
    self._callbacks = []

self._callback_lock是一个线程锁，self._callbacks支持多线程操作。self._callbacks主要用来放置异步事件。

for callback in callbacks:
    self._run_callback(callback)

运行回调异步事件和超时事件。

try:
    event_pairs = self._impl.poll(poll_timeout)
except Exception as e:
    # Depending on python version and IOLoop implementation,
    # different exception types may be thrown and there are
    # two ways EINTR might be signaled:
    # * e.errno == errno.EINTR
    # * e.args is like (errno.EINTR, 'Interrupted system call')
    if errno_from_exception(e) == errno.EINTR:
        continue
    else:
        raise

self._imple.poll是一个epoll的操作，循环会阻塞在这等待外部连接进入。tornado定义了一个wake()类，可以通过wake类来唤醒循环。

self._events.update(event_pairs)
while self._events:
    fd, events = self._events.popitem()
    try:
        fd_obj, handler_func = self._handlers[fd]
        handler_func(fd_obj, events)
    except (OSError, IOError) as e:
        if errno_from_exception(e) == errno.EPIPE:
            # Happens when the client closes the connection
            pass
        else:
            self.handle_callback_exception(self._handlers.get(fd))
    except Exception:
        self.handle_callback_exception(self._handlers.get(fd))
fd_obj = handler_func = None

当有事件进入时，通过fd从self._handlers中取出相应的回调函数运行