Python阻塞套接字，发送立即返回

Question

我正在用Python的socket模块写一个多线程的socket应用程序。这个服务器会监听连接，当有连接进来时，它会为这个socket创建一个新线程。

服务器线程会向客户端发送一些数据，但客户端还没准备好接收这些数据。我原以为这会导致服务器等待，直到客户端开始接收数据，但实际上服务器会立即返回。

然后客户端调用recv方法，这个方法是阻塞的，但却始终收不到数据。

客户端socket构造函数

self.__clientSocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
self.__clientSocket.connect((server, port))

服务器socket构造函数

        self.servSock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.servSock.setsockopt(socket.SOL_SOCKET, socket.SO_REUSEADDR, 1)
        #self.servSock.settimeout(None)
        self.servSock.setblocking(1)
        self.servSock.bind((self.addr,self.port))
        self.servSock.listen(5)

监听并接受连接的线程

    try:
        (cs, address) = self.servSock.accept()
    except socket.timeout:
        return
    threadName = '\r\nClient %s:%s\r\n' % (cs, address)
    print threadName
    clientSocketHandler = ClientSocket()
    clientSocketHandler.setClientSocket(cs)
    self.clients.newThread(self.clientFunc, {clientSocketHandler : "1"}, threadName).start()

服务器和客户端在ClientSocket内部的发送/接收方法

receivedData = self.__clientSocket.recv(1024*1024)

self.__clientSocket.send(s)

有没有人知道为什么send()方法会立即返回？

Answer 1

抱歉让你们等了，我在问这个问题后不久就解决了。@Lee，谢谢你的回答，它让我找到了正确的方向。解决办法是发送一个4字节的整数，来说明后面数据的大小。客户端总是会先接收到这四个字节，然后再接收数据的大小。

from commandClass import Command
from commandActionClass import CommandAction
import socket
from time import *
import struct

class ClientSocket():
    instance = None
    __connected = False
    __clientSocket = None

    @staticmethod
    def getInstance():
        if ClientSocket.instance == None:
            ClientSocket.instance = ClientSocket()
        return ClientSocket.instance

    def __init__(self):
        self.__connected = False
        self.receivedData = ''
        self.bufSize = 4096
        self.buffer = ''

    def connect(self, server, port):
        if self.isConnected():
            raise Exception('Already connected.')

        self.__clientSocket = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.__clientSocket.setsockopt(socket.SOL_TCP, socket.TCP_NODELAY, 1)
        self.__clientSocket.connect((server, port))
        self.__clientSocket.setblocking(1)
        self.__connected = True

    def disconnect(self):
        try:
            self.receivedData = ''
            self.buffer = ''
            self.__clientSocket.close()
        except Exception, e:
            print e
        finally:
            self.__connected = False

    def sendString(self,s):
        try:
            if (self.isConnected()):
                self.__clientSocket.send(s)
        except Exception, e:
            print e
            self.disconnect()

    def __pop(self, size):
        data = self.receivedData[:size]
        self.receivedData = self.receivedData[size:]
        return data

    def __recv(self,toRead):
        self.flush()
        while ((len(self.receivedData)<toRead)and(self.isConnected())):
            data = self.__clientSocket.recv(self.bufSize)
            if not data:
                self.disconnect()
            self.receivedData = self.receivedData + data

        return self.__pop(toRead)

    def __sendint(self, x):
        self.__sendall(struct.pack("i", x))

    def __recvint(self):
        data = self.__recv(4)
        if not data:
            raise Exception('Expected to receive buffer size')
        return struct.unpack("i", data)[0]

    def flush(self):
        if len(self.buffer)>0:
            self.__clientSocket.sendall(self.buffer)
        self.buffer = ''

    def __sendall(self, s):
        self.buffer = self.buffer + s

    def send(self,s):
        try:
            if (not self.isConnected()):
                raise Exception('Socket is not connected')
            data = s.pickle()
            self.__sendint(len(data))
            self.__sendall(data)
        except Exception, e:
            self.disconnect()
            raise e

    def sendEOC(self):
        self.send(Command(CommandAction.EOC, time()))#send our system time. can be used for ping

    def receive(self):
        if (not self.isConnected()):
            raise Exception('Socket Error. Not Connected')
        try:
            #first receive the size of packet
            buffsize = self.__recvint()
            #now receive the actual data
            data = self.__recv(buffsize)

            if not data:
                raise Exception('No data to receive')

            command = Command.unpickle(data)
        except Exception, e:
            self.disconnect()
            command = Command(CommandAction.Invalid, None)
            raise e
        #finally?
        return command

    def isConnected(self):
        return self.__connected

    def setClientSocket(self, clientSocket):
        self.__clientSocket = clientSocket
        self.__connected = True #assume its connected

Answer 2

客户端不需要随时准备好接收数据——数据会在套接字的接收缓冲区里排队，直到你准备好用recv()来接收它。发送数据的操作会立刻返回，因为发送缓冲区没有满。如果发送缓冲区满了，send()就会暂停，直到有空间可以发送你想要的数据。

大多数情况下，你根本不会把它填满——这就是你现在遇到的情况。另外，你可能不想在recv调用中使用1024*1024这个数字——这个值有点高。

Answer 3

有没有人知道为什么send()会立刻返回？

send()这个函数的作用就是把数据放进网络缓冲区，然后返回发送的字节数。

如果你想让send()等着再返回，可以让客户端发送一个确认消息回来。