tornado 的协程调度原理

本文讨论 tornado 的协程实现原理，简单做了一份笔记。

首先看一段最常见的 tornado web 代码：

import tornado
import tornado.web
import tornado.gen
from tornado.gen import coroutine
from tornado.httpclient import AsyncHTTPClient

class GenHandler(tornado.web.RequestHandler):
    @coroutine
    def get(self):
        url = 'http://www.baidu.com'
        http_client = AsyncHTTPClient()
        response = yield http_client.fetch(url)
        yield tornado.gen.sleep(5)
        self.write(response.body)

class MainHanler(tornado.web.RequestHandler):
    def get(self):
        self.write('root')

if __name__ == "__main__":
    application = tornado.web.Application([
        (r"/", MainHanler),
        (r"/gen_async/", GenHandler),
    ], autoreload=True)
    application.listen(8888)
    tornado.ioloop.IOLoop.current().start()

其中最后一行代码 tornado.ioloop.IOLoop.current().start() 启动服务。带着几个问题往下看：

知道 yield 可以暂存执行状态，等「合适的时机」重新恢复执行，那么保存的状态到哪去了？
上一个问题中「合适的时机」是到底是什么时候？
继续接上一个问题，具体是怎么恢复执行的？

IOLoop 类相当于是对多路复用的封装，起到事件循环的作用，调度整个协程执行过程。

查看 IOLoop 的源码，可以看到 IOLoop 继承自 Configurable，PollIOLoop 又继承自 IOLoop。当 IOLoop 启动时，会确定使用哪一种多路复用方式，epoll、kqueue 还是 select？

# IOLoop 类
# IOLoop 中的 configurable_default 方法是重写 Configurable 的
# 这里会确定使用哪种多路复用方式
@classmethod
def configurable_default(cls):
    if hasattr(select, "epoll"):
        from tornado.platform.epoll import EPollIOLoop
        return EPollIOLoop
    if hasattr(select, "kqueue"):
        # Python 2.6+ on BSD or Mac
        from tornado.platform.kqueue import KQueueIOLoop
      return KQueueIOLoop
    from tornado.platform.select import SelectIOLoop
  return SelectIOLoop

# PollIOLoop类
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)

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)

def update_handler(self, fd, events):
    fd, obj = self.split_fd(fd)
    self._impl.modify(fd, events | self.ERROR)

def remove_handler(self, fd):
    fd, obj = self.split_fd(fd)
    self._handlers.pop(fd, None)
    self._events.pop(fd, None)
    try:
        self._impl.unregister(fd)
    except Exception:
        gen_log.debug("Error deleting fd from IOLoop", exc_info=True)

PollIOLoop 中 initalize 方法中调用 add_handler 方法，注册对应事件的处理函数，如 socket 可读时，回调哪个函数去处理。

IOLoop 和协程之间的信使：Future

class Future(object):
    def __init__(self):
        self._result = None
        self._exc_info = None
        self._callbacks = []
        self.running = True
        
    def set_result(self, result):
        ...
        
    def set_exc_info(self, exce_info):
        ...
        
    def result(self):
        ...
    
    def exc_info(self):
        ...
        
    def add_done_callback(self, callback):
        self._callbacks.append(callback)

Future 对象起到“占位符”的作用，协程的执行结果会通过 set_result 方式写入其中，并调用通过 add_done_callback 设置的回调。

恢复唤醒协程的 Runner

class Runner(object):
    def __init__(self, gen, result_future, first_yielded):
        self.gen = gen
        self.result_future = result_future
        self.future = _null_future
        self.yield_point = None
        self.pending_callbacks = None
        self.results = None
        self.running = False
        self.finished = False
        self.had_exception = False
        self.io_loop = IOLoop.current()
        self.stack_context_deactivate = None
        #　上面一堆不需要看的初始化
        if self.handle_yield(first_yielded):
            gen = result_future = first_yielded = None
            self.run()
     
    
    def handle_yield(self, yielded):

        self.future = convert_yielded(yielded)

        if self.future is moment:
            self.io_loop.add_callback(self.run)
            return False
        elif not self.future.done():
            def inner(f):
                # Break a reference cycle to speed GC.
                f = None
                self.run()
            self.io_loop.add_future(
                self.future, inner)
            return False
        return True
    
    def run(self):
        if self.running or self.finished:
            return
        try:
            self.running = True
            while True:
                future = self.future
                if not future.done():
                    return
                self.future = None
                try:
                    orig_stack_contexts = stack_context._state.contexts
                    exc_info = None

                    try:
                        value = future.result()
                    except Exception:
                        self.had_exception = True
                        exc_info = sys.exc_info()
                    future = None
  
                    yielded = self.gen.send(value)

                except (StopIteration, Return) as e:
                    self.finished = True
                    self.future = _null_future
                    if self.pending_callbacks and not self.had_exception:
                        raise LeakedCallbackError(
                            "finished without waiting for callbacks %r" %
                            self.pending_callbacks)
                    future_set_result_unless_cancelled(self.result_future,
_value_from_stopiteration(e))
                    self.result_future = None
                    self._deactivate_stack_context()
                    return
                except Exception:
                    # 一些结束操作
                    return
                if not self.handle_yield(yielded):
                    return
                yielded = None
        finally:
            self.running = False

协程每生成一个 Future，都会生成对应的一个 Runner，并将 Future 初始化注入都其中。Runner 的 run 方法中，通过 self.gen.send(Future) 来启动 Future，当 Future 完成时，将其设置成 done，并回调其预设的 callback。

回答第一个问题：协程的状态保存到哪去了：

IOLoop 中通过 add_future 调用实现类 PollIOLoop 中的 add_callback 方法，其中通过 functools 生成偏函数，放入 _callbacks 列表，等待被回调执行。

# IOLoop 的add_future
def add_future(self, future, callback):
    """Schedules a callback on the ``IOLoop`` when the given
    `.Future` is finished.

    The callback is invoked with one argument, the
    `.Future`.
    """
    assert is_future(future)
    callback = stack_context.wrap(callback)
    future.add_done_callback(
        lambda future: self.add_callback(callback, future))

# PollIOLoop 的add_callback
def add_callback(self, callback, *args, **kwargs):
        if thread.get_ident() != self._thread_ident:
            with self._callback_lock:
                if self._closing:
                    return
                list_empty = not self._callbacks
                self._callbacks.append(functools.partial(
                    stack_context.wrap(callback), *args, **kwargs))
                if list_empty:
                    self._waker.wake()
        else:
            if self._closing:
                return
            self._callbacks.append(functools.partial(
                stack_context.wrap(callback), *args, **kwargs))

第二个问题：「合适的时机」是什么？

IOLoop 实际上就是对多路复用的封装，当底层 epoll_wait 事件发生时，即会通知 IOLoop 主线程。

这一段是 IOLoop 中等待多路复用的事件，以及处理事件。

try:
    # 等待事件
      event_pairs = self._impl.poll(poll_timeout)
except Exception as e:
      print("wait fail")
    
      if errno_from_exception(e) == errno.EINTR:
          continue
      else:
          raise
if self._blocking_signal_threshold is not None:
                    signal.setitimer(signal.ITIMER_REAL,
                                     self._blocking_signal_threshold, 0)
# 处理事件
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:
            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