ss源码学习--工作流程

ss的local端和server端的工作流程相似，因此复用了TCPRelay类和TCPRelayHandler类。
两端均是使用TCPRelay类监听连接，并使用TCPRelayHandler类处理请求。

以server端为例：

# server.py
    ...
    tcp_servers = []
    tcp_servers.append(tcprelay.TCPRelay(a_config, dns_resolver, False))
    ...
class 
    def run_server():
        ...
        try:
            loop = eventloop.EventLoop()
            dns_resolver.add_to_loop(loop)
            list(map(lambda s: s.add_to_loop(loop), tcp_servers + udp_servers))

            daemon.set_user(config.get('user', None))
            loop.run()
        except Exception as e: 
            shell.print_exception(e)
            sys.exit(1)

这里创建了一个TCPRelay对象以及EventLoop,并将所有tcp_server加入eventloop中。

在TCPRelay的构造函数中，打开了一个监听套接字，监听1024端口。

# tcpreply.py

class TCPRelay:
    def __init__(self, config, dns_resolver, is_local, stat_callback=None):
        self._config = config
        # 该标志用来区分该对象是客户端还是服务端
        self._is_local = is_local
        self._dns_resolver = dns_resolver
        self._closed = False
        self._eventloop = None

        # 字典存放的为(fd,handle) 键值对，用来保存每个fd对应的handle函数
        self._fd_to_handlers = {}

        self._timeout = config['timeout']

        self._timeouts = []  # a list for all the handlers
        # we trim the timeouts once a while
        self._timeout_offset = 0   # last checked position for timeout
        self._handler_to_timeouts = {}  # key: handler value: index in timeouts

        if is_local:
            listen_addr = config['local_address']
            listen_port = config['local_port']
        else:
            listen_addr = config['server']
            listen_port = config['server_port']
        self._listen_port = listen_port

        addrs = socket.getaddrinfo(listen_addr, listen_port, 0,
                                   socket.SOCK_STREAM, socket.SOL_TCP)
        if len(addrs) == 0:
            raise Exception("can't get addrinfo for %s:%d" %
                            (listen_addr, listen_port))
        af, socktype, proto, canonname, sa = addrs[0]

        server_socket = socket.socket(af, socktype, proto)
        server_socket.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        server_socket.bind(sa)

        server_socket.setblocking(False)

        if config['fast_open']:
            try:
                server_socket.setsockopt(socket.SOL_TCP, 23, 5)
            except socket.error:
                logging.error('warning: fast open is not available')
                self._config['fast_open'] = False

        server_socket.listen(1024)
        self._server_socket = server_socket
        self._stat_callback = stat_callback

然后执行add_to_loop()将自己加入到eventloop中。

# tcprelay.py

class TCPRelay:
    def add_to_loop(self, loop):
        if self._eventloop:
            raise Exception('already add to loop')
        if self._closed:
            raise Exception('already closed')
        self._eventloop = loop

        # 将事件加入到loop，监听IN和ERR
        self._eventloop.add(self._server_socket,
                            eventloop.POLL_IN | eventloop.POLL_ERR, self)
        self._eventloop.add_periodic(self.handle_periodic)

这里实质上是调用了eventloop的add

# eventloop.py

class EventLoop:
    def add(self, f, mode, handler):
        fd = f.fileno()
        self._fdmap[fd] = (f, handler)
        self._impl.register(fd, mode)

注意这里是将TCPRelay这个类自身作为handler放入_fdmap字典中。这个字典的作用主要是存储每个描述符对应的handler，当事件发生时，从中取出对应的handler对事件进行处理。

接着执行loop.run()开始事件循环。

# eventloop.py
class EventLoop:
    def run(self):
        # 事件集合
        events = []
        while not self._stopping:
            asap = False
            try:
                # 返回所有事件
                events = self.poll(TIMEOUT_PRECISION)
            except (OSError, IOError) as e:
                if errno_from_exception(e) in (errno.EPIPE, errno.EINTR):
                    # EPIPE: Happens when the client closes the connection
                    # EINTR: Happens when received a signal
                    # handles them as soon as possible
                    asap = True
                    logging.debug('poll:%s', e)
                else:
                    logging.error('poll:%s', e)
                    import traceback
                    traceback.print_exc()
                    continue

            for sock, fd, event in events:
                # 对于监听到的事件,获取其对应的handle
                handler = self._fdmap.get(fd, None)
                if handler is not None:
                    # handler[0]为socket
                    handler = handler[1]
                    try:
                        # 使用对应的handle处理这个事件
                        handler.handle_event(sock, fd, event)
                    except (OSError, IOError) as e:
                        shell.print_exception(e)
            now = time.time()
            if asap or now - self._last_time >= TIMEOUT_PRECISION:
                for callback in self._periodic_callbacks:
                    callback()
                self._last_time = now

事件发生时，首先根据fd获取其对应的handler，并调用handler.handle_event()

# tcprelay.py
class TCPRelay:
    def handle_event(self, sock, fd, event):
        # handle events and dispatch to handlers
        if sock:
            logging.log(shell.VERBOSE_LEVEL, 'fd %d %s', fd,
                        eventloop.EVENT_NAMES.get(event, event))
        # 如果为监听套接字
        if sock == self._server_socket:
            if event & eventloop.POLL_ERR:
                # TODO
                raise Exception('server_socket error')
            try:
                logging.debug('accept')
                # 尝试接受客户端的连接
                conn = self._server_socket.accept()
                TCPRelayHandler(self, self._fd_to_handlers,
                                self._eventloop, conn[0], self._config,
                                self._dns_resolver, self._is_local)
            except (OSError, IOError) as e:
                error_no = eventloop.errno_from_exception(e)
                if error_no in (errno.EAGAIN, errno.EINPROGRESS,
                                errno.EWOULDBLOCK):
                    return
                else:
                    shell.print_exception(e)
                    if self._config['verbose']:
                        traceback.print_exc()
        # 不是监听套接字，说明是已连接的客户端有可读操作
        else:
            if sock:
                handler = self._fd_to_handlers.get(fd, None)
                if handler:
                    handler.handle_event(sock, event)
            else:
                logging.warn('poll removed fd')

如果是监听套接字的事件，则说明有新的客户端连接。先通过_fdmap.get()来获取handler(这里为TCPRelay对象) 接着accept，然后创建了一个TCPRelayHandler对象。看一下构造函数:

# tcprelay.py

class TCPRelayHandler(object):
    def __init__(self, server, fd_to_handlers, loop, local_sock, config,
                 dns_resolver, is_local):
        self._server = server
        self._fd_to_handlers = fd_to_handlers
        self._loop = loop
        self._local_sock = local_sock
        self._remote_sock = None
        self._config = config
        self._dns_resolver = dns_resolver

        # TCP Relay works as either sslocal or ssserver
        # if is_local, this is sslocal
        self._is_local = is_local
        self._stage = STAGE_INIT
        self._encryptor = encrypt.Encryptor(config['password'],
                                            config['method'])
        self._fastopen_connected = False
        self._data_to_write_to_local = []
        self._data_to_write_to_remote = []
        self._upstream_status = WAIT_STATUS_READING
        self._downstream_status = WAIT_STATUS_INIT
        self._client_address = local_sock.getpeername()[:2]
        self._remote_address = None
        if 'forbidden_ip' in config:
            self._forbidden_iplist = config['forbidden_ip']
        else:
            self._forbidden_iplist = None
        if is_local:
            self._chosen_server = self._get_a_server()

        # 将自身作为handler加入fd_to_handlers
        fd_to_handlers[local_sock.fileno()] = self
        local_sock.setblocking(False)
        local_sock.setsockopt(socket.SOL_TCP, socket.TCP_NODELAY, 1)

        # 加入loop的handle仍然是TCPRelay
        loop.add(local_sock, eventloop.POLL_IN | eventloop.POLL_ERR,
                 self._server)
        self.last_activity = 0
        self._update_activity()

这里将与客户端连接的套接字加入了loop，handler为self._server即那个监听到该连接的TCPRelay对象。

接着上面事件循环，如果是已连接的套接字产生事件，则通过_fdmap.get()获取到那个监听到它的TCPRelay对象，然后在handle_event的过程中，通过_fd_to_handlers.get()获取对应的TCPRelayHandler对象，再使用TCPRelayHandler.handle_event()来处理具体的读写事件。

ps：描述的好乱，抽时间再整理下吧。

参考：
ss2.8 源码