tornado 源码分析 之 异步io的实现方式

前言

本文将尝试详细的带大家一步步走完一个异步操作,从而了解tornado是如何实现异步io的.
其实本文是对[上一篇文][1]的实践和复习
主旨在于关注异步io的实现,所以会忽略掉代码中的一些异常处理.文字较多,凑合下吧

接下来只会贴出部分源码,帮助理解,希望有耐心的同学打开tornado源码,一起跟踪一遍吧.

AsyncHTTPClient :

 AsyncHTTPClient 继承 Configurable ,从__new__重看出是单例模式.  
根据 Configurable 的__new__和 AsyncHTTPClient 的 configurable_base 和 configurable_default 得知,
实例化后一定是 SimpleAsyncHTTPClient 的实例

fetch

def fetch(self, request, callback=None, raise_error=True, **kwargs):

        if self._closed:
            raise RuntimeError("fetch() called on closed AsyncHTTPClient")
        if not isinstance(request, HTTPRequest):
            request = HTTPRequest(url=request, **kwargs)
        # We may modify this (to add Host, Accept-Encoding, etc),
        # so make sure we don't modify the caller's object.  This is also
        # where normal dicts get converted to HTTPHeaders objects.
        request.headers = httputil.HTTPHeaders(request.headers)
        request = _RequestProxy(request, self.defaults)
        future = TracebackFuture()
        if callback is not None:
            callback = stack_context.wrap(callback)

            def handle_future(future):
                exc = future.exception()
                if isinstance(exc, HTTPError) and exc.response is not None:
                    response = exc.response
                elif exc is not None:
                    response = HTTPResponse(
                        request, 599, error=exc,
                        request_time=time.time() - request.start_time)
                else:
                    response = future.result()
                self.io_loop.add_callback(callback, response)
            future.add_done_callback(handle_future)
        ##fetch_impl带上handle_response,重点
        def handle_response(response):
            if raise_error and response.error:
                future.set_exception(response.error)
            else:
                future.set_result(response)
        self.fetch_impl(request, handle_response)
        return future

 fetch 中调用 fetch_impl,fetch_impl 中其中一个参数是 callback ,而代码中的 callback 包含了 future 的 set_result ,
所以,当 callback 被调用时,外部的 yield 操作将被激活,程序会在 ioloop 中调用此 callback ,然后回到原函数的 yield 处,
并且原函数返回此次 qeust 的 future 对象,以便在函数外部增加别的 callback 

fetch_impl

def _connection_class(self):
        return _HTTPConnection

def _handle_request(self, request, release_callback, final_callback):
        self._connection_class()(
            self.io_loop, self, request, release_callback,
            final_callback, self.max_buffer_size, self.tcp_client,
            self.max_header_size, self.max_body_size)

在 return 之前,继续查看 fetch_impl 内部是如何处理,根据推测,他一定是将继续处理网络请求,
肯定会将网络请求交由 ioloop 的 epoll 部分处理,设定好处理的 hanlder 再返回
 future.set_result ,接下来继续分析 fetch_impl 内部是如果设置网络请求的.
 fetch_impl 的实现代码中查看,实例化中创建了 tcpclient 对象,这个肯定是关键

根据之前的分析 SimpleAsyncHTTPClient 是单例模式,那他怎么处理各种 http 请求呢?
查看代码得知,他将请求的 request 和 callback 存储在 self.queue 中,
每次 fetch_impl 的时候,一个个 pop 出来处理就好了,这样就能处理n个请求了

一步步跟踪到 _handle_request ,发现最后到了 _HTTPConnection 的实例化中了.
实例化的参数有之前那个包含 future 的 callback .
这样就可以保证 yield 操作可以回到原处了.好了,继续走

_HTTPConnection

class _HTTPConnection(httputil.HTTPMessageDelegate):
    _SUPPORTED_METHODS = set(["GET", "HEAD", "POST", "PUT", "DELETE", "PATCH", "OPTIONS"])

    def __init__(self, io_loop, client, request, release_callback,
                 final_callback, max_buffer_size, tcp_client,
                 max_header_size, max_body_size):
        self.start_time = io_loop.time()
        self.io_loop = io_loop
        self.client = client
        self.request = request
        self.release_callback = release_callback
        self.final_callback = final_callback
        self.max_buffer_size = max_buffer_size
        self.tcp_client = tcp_client
        self.max_header_size = max_header_size
        self.max_body_size = max_body_size
        self.code = None
        self.headers = None
        self.chunks = []
        self._decompressor = None
        # Timeout handle returned by IOLoop.add_timeout
        self._timeout = None
        self._sockaddr = None
        with stack_context.ExceptionStackContext(self._handle_exception):
            self.parsed = urlparse.urlsplit(_unicode(self.request.url))
            if self.parsed.scheme not in ("http", "https"):
                raise ValueError("Unsupported url scheme: %s" %
                                 self.request.url)
            # urlsplit results have hostname and port results, but they
            # didn't support ipv6 literals until python 2.7.
            netloc = self.parsed.netloc
            if "@" in netloc:
                userpass, _, netloc = netloc.rpartition("@")
            host, port = httputil.split_host_and_port(netloc)
            if port is None:
                port = 443 if self.parsed.scheme == "https" else 80
            if re.match(r'^\[.*\]$', host):
                # raw ipv6 addresses in urls are enclosed in brackets
                host = host[1:-1]
            self.parsed_hostname = host  # save final host for _on_connect

            if request.allow_ipv6 is False:
                af = socket.AF_INET
            else:
                af = socket.AF_UNSPEC

            ssl_options = self._get_ssl_options(self.parsed.scheme)

            timeout = min(self.request.connect_timeout, self.request.request_timeout)
            if timeout:
                self._timeout = self.io_loop.add_timeout(
                    self.start_time + timeout,
                    stack_context.wrap(self._on_timeout))
            self.tcp_client.connect(host, port, af=af,
                                    ssl_options=ssl_options,
                                    max_buffer_size=self.max_buffer_size,
                                    callback=self._on_connect)

 _HTTPConnection 的实例化中有一堆成员变量,有点晕,
先不管这么多,关注我们的 callback ,和 tcpclient .

一行行往下看,是 host 和 port 的初始化操作 ,http 和 https 是不一样的嘛,当然得处理一下,

终于到了最后,是 tcpclient.connect ,从 connect 的参数中看到 callback=self._on_connect ,
应该是个重要的方法,出去那些字符串处理,发现 self.connection.write_headers(start_line ,  self.request.headers ) ,
这应该是发送 http 头的操作吧,是网络请求,所以这是处理 connect 这个 url 后,发送 http 头的操作.

还是回头看看是如何 connect 的吧,因为这是异步的关键,搞懂了这个,那剩下来的也是同出一则

TCPClient

转到 tcpclient 的代码去看他的实例化和 connect 操作,看来剩下的路还很长呢
 TCPClient 实例化的代码很短,有个 resolver 对象,先不管

connect

    @gen.coroutine
    def connect(self, host, port, af=socket.AF_UNSPEC, ssl_options=None,
                max_buffer_size=None):
        """Connect to the given host and port.

        Asynchronously returns an `.IOStream` (or `.SSLIOStream` if
        ``ssl_options`` is not None).
        """
        addrinfo = yield self.resolver.resolve(host, port, af)
        connector = _Connector(
            addrinfo, self.io_loop,
            functools.partial(self._create_stream, max_buffer_size))
        af, addr, stream = yield connector.start()
        # TODO: For better performance we could cache the (af, addr)
        # information here and re-use it on subsequent connections to
        # the same host. (http://tools.ietf.org/html/rfc6555#section-4.2)
        if ssl_options is not None:
            stream = yield stream.start_tls(False, ssl_options=ssl_options,
                                            server_hostname=host)
        raise gen.Return(stream)

去到 connect 方法里,发现 coroutine 装饰器,并且调用时设置了 callback=self._on_connect ,
所以当这个 coroutine 的 future 被解决时,会调用 self._on_connect ,
你也可以看到 _on_connect 的参数是 stream ,就是 gen.Return(stream )传过去的. 
因为 gen.coroutine 实现时的代码中,
 send 了 value 后,代码继续走,走到 gen.Return (其实这是个 exception ,
就会走到 gen.coroutine 里的 set_result 了.)

第一个 yield  右边是 self.resolver.resolve ,左边是 addrinfo ,是地址信息,
这个异步操作处理的便是解析 url 的地址信息.此处 tornado 默认使用了阻塞的实现,暂时先不看,
以后在新的篇幅补充,主要内容是 run_on_executor 装饰器的内容,
此处其实是同步返回的,因为默认用的是 BlockingResolver 的代码,直接看下一个 yield 

_Connector

    def __init__(self, addrinfo, io_loop, connect):
        self.io_loop = io_loop
        self.connect = connect

        self.future = Future()
        self.timeout = None
        self.last_error = None
        self.remaining = len(addrinfo)
        self.primary_addrs, self.secondary_addrs = self.split(addrinfo)

 _Connector 实例化,参数有一个 callback ,是本类的 _create_stream ,
并把 self.connect 设置成传过来的 callback 
所以 self.connect 就是 TCPClient._create_stream 了,
成员变量有个 future 实例,我们需要全程高度关注 future 和 callback .

实例化后调用了 start 方法 ,start 内部,调用 try_connect,set_timout ,
根据函数名得知,是 connect 操作和设置超时的操作.最后返回实例化时创建的 future .

try_connect

def start(self, timeout=_INITIAL_CONNECT_TIMEOUT):
        self.try_connect(iter(self.primary_addrs))
        self.set_timout(timeout)
        return self.future

    def try_connect(self, addrs):
        try:
            af, addr = next(addrs)
        except StopIteration:
            # We've reached the end of our queue, but the other queue
            # might still be working.  Send a final error on the future
            # only when both queues are finished.
            if self.remaining == 0 and not self.future.done():
                self.future.set_exception(self.last_error or
                                          IOError("connection failed"))
            return
        future = self.connect(af, addr)
        future.add_done_callback(functools.partial(self.on_connect_done,
                                                   addrs, af, addr))

 future  =  self.connect(af ,  addr ),执行了 TCPClient._create_stream 方法,
返回 future ,并且设置 future 的 callback=on_connect_done 

_create_stream

    def _create_stream(self, max_buffer_size, af, addr):
        # Always connect in plaintext; we'll convert to ssl if necessary
        # after one connection has completed.
        stream = IOStream(socket.socket(af),
                          io_loop=self.io_loop,
                          max_buffer_size=max_buffer_size)
        return stream.connect(addr)

实例化 IOStream ,执行并返回 stream.connect,stream.connect 返回的 future 便是 try_connect 中的 future ,
所以,进去看看 stream.connect 内部是怎么”解决”这个 future 的.

IOStream

connect

def connect(self, address, callback=None, server_hostname=None):
        self._connecting = True
        if callback is not None:
            self._connect_callback = stack_context.wrap(callback)
            future = None
        else:
            future = self._connect_future = TracebackFuture()
        try:
            self.socket.connect(address)
        except socket.error as e:

            if (errno_from_exception(e) not in _ERRNO_INPROGRESS and
                    errno_from_exception(e) not in _ERRNO_WOULDBLOCK):
                if future is None:
                    gen_log.warning("Connect error on fd %s: %s",
                                    self.socket.fileno(), e)
                self.close(exc_info=True)
                return future
        self._add_io_state(self.io_loop.WRITE)
        return future

 self._connecting  =  True  设置此实例正在连接中,连接完毕设置成 false 
如果没有 callback 传入,生成 future 对象, 刚才返回的 future 记录在这个实例的成员变量 self._connect_future 中.
然后执行 socket 的 connect 操作,因为 socket 设置成非阻塞,
所以此处会立即返回,不会阻塞,当连接成功时,缓冲区可写,失败时缓冲区可读可写.这是基础知识,详情百度.
然后调用 self._add_io_state ,返回 future 

_add_io_state

    def _add_io_state(self, state):

        if self.closed():
            # connection has been closed, so there can be no future events
            return
        if self._state is None:
            self._state = ioloop.IOLoop.ERROR | state
            with stack_context.NullContext():
                self.io_loop.add_handler(
                    self.fileno(), self._handle_events, self._state)
        elif not self._state & state:
            self._state = self._state | state
            self.io_loop.update_handler(self.fileno(), self._state)

终于到了这一步,要用 epoll 了!!!根据实例化的代码得知 self._state=None ,
会走 self.io_loop.add_handler 这步,根据我之前发的[文章][2],会将当前的 fd ,当前实例的 _handle_events ,和写,错误操作注册到 epoll 中
接着!!!!!终于走完了这个 yield 的流程了!!!!!!

小总结:

 请一定弄清 future 是怎么传递的,每个 future 管理的 callback 是什么操作.
  _HTTPConnection  中  tcpclient.connect 一个 future   ,callback=self._on_connect  .
 他将在 raise   gen.Return(stream )时被添加到 ioloop 执行. 
  tcpclient.connect 内部的  connector.start 一个 future , 
  callback 是 on_connect_done ,他将在 poll 检测到 write 事件时,被添加到 ioloop 执行

ioloop

    def start(self):
        if self._running:
            raise RuntimeError("IOLoop is already running")
        self._setup_logging()
        if self._stopped:
            self._stopped = False
            return
        old_current = getattr(IOLoop._current, "instance", None)
        IOLoop._current.instance = self
        self._thread_ident = thread.get_ident()
        self._running = True

        old_wakeup_fd = None
        if hasattr(signal, 'set_wakeup_fd') and os.name == 'posix':

            try:
                old_wakeup_fd = signal.set_wakeup_fd(self._waker.write_fileno())
                if old_wakeup_fd != -1:
                    signal.set_wakeup_fd(old_wakeup_fd)
                    old_wakeup_fd = None
            except ValueError:
                old_wakeup_fd = None

        try:
            while True:
                with self._callback_lock:
                    callbacks = self._callbacks
                    self._callbacks = []

                due_timeouts = []

                if self._timeouts:
                    now = self.time()
                    while self._timeouts:
                        if self._timeouts[0].callback is None:

                            heapq.heappop(self._timeouts)
                            self._cancellations -= 1
                        elif self._timeouts[0].deadline <= now:
                            due_timeouts.append(heapq.heappop(self._timeouts))
                        else:
                            break
                    if (self._cancellations > 512
                            and self._cancellations > (len(self._timeouts) >> 1)):

                        self._cancellations = 0
                        self._timeouts = [x for x in self._timeouts
                                          if x.callback is not None]
                        heapq.heapify(self._timeouts)

                for callback in callbacks:
                    self._run_callback(callback)
                for timeout in due_timeouts:
                    if timeout.callback is not None:
                        self._run_callback(timeout.callback)
                callbacks = callback = due_timeouts = timeout = None

                if self._callbacks:
                    poll_timeout = 0.0
                elif self._timeouts:

                    poll_timeout = self._timeouts[0].deadline - self.time()
                    poll_timeout = max(0, min(poll_timeout, _POLL_TIMEOUT))
                else:

                    poll_timeout = _POLL_TIMEOUT

                if not self._running:
                    break

                if self._blocking_signal_threshold is not None:

                    signal.setitimer(signal.ITIMER_REAL, 0, 0)

                try:
                    event_pairs = self._impl.poll(poll_timeout)
                except Exception as e:

                    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)

                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

        finally:
            # reset the stopped flag so another start/stop pair can be issued
            self._stopped = False
            if self._blocking_signal_threshold is not None:
                signal.setitimer(signal.ITIMER_REAL, 0, 0)
            IOLoop._current.instance = old_current
            if old_wakeup_fd is not None:
                signal.set_wakeup_fd(old_wakeup_fd)

接下来 tornado 终于也回到了 ioloop 代码中了(泪奔)!!当连接成功时,该 fd 的缓冲区可写,
 epoll 收到 fd 的 write 操作通知~进入到了 epoll 的 loop 中处理.然后!回到刚才注册的 _handle_events 了!
注意这个 _handle_events 是 IOStream 的实例里的 _handle_events ,他有刚才我们处理的所有信息哦~

接下来看 _handle_events 的代码,看他如果解决掉 future 

IOStream._handle_events

    def _handle_events(self, fd, events):
        if self.closed():
            gen_log.warning("Got events for closed stream %s", fd)
            return
        try:
            if self._connecting:
                # Most IOLoops will report a write failed connect
                # with the WRITE event, but SelectIOLoop reports a
                # READ as well so we must check for connecting before
                # either.
                self._handle_connect()
            if self.closed():
                return
            if events & self.io_loop.READ:
                self._handle_read()
            if self.closed():
                return
            if events & self.io_loop.WRITE:
                self._handle_write()
            if self.closed():
                return
            if events & self.io_loop.ERROR:
                self.error = self.get_fd_error()
                # We may have queued up a user callback in _handle_read or
                # _handle_write, so don't close the IOStream until those
                # callbacks have had a chance to run.
                self.io_loop.add_callback(self.close)
                return
            state = self.io_loop.ERROR
            if self.reading():
                state |= self.io_loop.READ
            if self.writing():
                state |= self.io_loop.WRITE
            if state == self.io_loop.ERROR and self._read_buffer_size == 0:
                # If the connection is idle, listen for reads too so
                # we can tell if the connection is closed.  If there is
                # data in the read buffer we won't run the close callback
                # yet anyway, so we don't need to listen in this case.
                state |= self.io_loop.READ
            if state != self._state:
                assert self._state is not None, \
                    "shouldn't happen: _handle_events without self._state"
                self._state = state
                self.io_loop.update_handler(self.fileno(), self._state)
        except UnsatisfiableReadError as e:
            gen_log.info("Unsatisfiable read, closing connection: %s" % e)
            self.close(exc_info=True)
        except Exception:
            gen_log.error("Uncaught exception, closing connection.",
                          exc_info=True)
            self.close(exc_info=True)
            raise

    def _handle_connect(self):
        err = self.socket.getsockopt(socket.SOL_SOCKET, socket.SO_ERROR)
        if err != 0:
            self.error = socket.error(err, os.strerror(err))
            # IOLoop implementations may vary: some of them return
            # an error state before the socket becomes writable, so
            # in that case a connection failure would be handled by the
            # error path in _handle_events instead of here.
            if self._connect_future is None:
                gen_log.warning("Connect error on fd %s: %s",
                                self.socket.fileno(), errno.errorcode[err])
            self.close()
            return
        if self._connect_callback is not None:
            callback = self._connect_callback
            self._connect_callback = None
            self._run_callback(callback)
        if self._connect_future is not None:
            future = self._connect_future
            self._connect_future = None
            future.set_result(self)
        self._connecting = False

判断是否在连接中,当然是了,刚才我也强调过了,
然后进入 _handle_connect,_handle_connect 先判断 connect 有没成功,
成功了就是设置 _connect_future 的 result,set_result(self ),把 self(iostream )设置进去了!   
然后 _connect_future 的 callbacks 会在下一次循环被 ioloop 消化掉!!

一步步返回看,这个 future 正是我们之前的那个 yiled 操作的右边的返回的 future ,
所以刚才 _Connector.try_connect 设置的 callback   ,on_connect_done 会在 ioloop 的 callback 里执行. 
根据上一篇[文章][3]讲的 coroutine 的源码得知,此 future 里还有 Runner.run 的 callback 哦~
所以 ,run 里 send 了 vaule 到 gen . 
终于终于!!程序回到了刚才的 yield 处了!!!!!

tornado正是如此实现异步io的

感觉一直讲完整个操作不太现实,剩下的大家还是自己跟踪吧,道理跟这个流程类似.
 yield 操作右边,一定是返回一个 future (旧版本貌似是 YieldPoint ,因为没看过旧版,所以不太清楚) ,
然后在返回 future 之前,设置好 fd 的 handler ,和其他的解析工作,然后等待 epoll 检测到关心的 io   event ,
在 io 的 handler 里把 future 解决,从而回到 yield 处~ 核心就是 ioloop 三部分 ,future,gen.coroutine . 
相互配合完成异步操作. 跟踪几遍消化一下,就可以写 tornado 的扩展了. 

祝大家武运亨通