Question

These code is the Server part of my proxy program, and its function is creating the socket and fork four process to accept one by one.

In my program I use gevent model to dispatch all my function and before I alter it to multiple process, my program is all right. but now when I use the second process, the first one stop running, I don’t find where is wrong, maybe the ‘accept’ function or my event is stop dispatch.

It have already bothered me for two days I hope someone can help me.

BTW, my English is poor, I try my best to explain it, hoping you can understand.

 class Client(object):
    def __init__(self, ent, ev):
        ...  

    def receive( self ):
        ...
        if "Content-Length" in dic:
            self.ent_s_send = core.event(core.EV_WRITE,
                                         self.conn.fileno(),
                                         self.ser_send,
                                         [self.conn,self.body]
                                         )
            self.recv_ent = core.event(core.EV_READ, 
                                       self.sock.fileno(),
                                       self.recv_content
                                      )
            self.recv_ent.add()
        ...

    def recv_content(self, ent, ev):
        ...
        self.n = self.sock.recv_into(self.msg,
                                     min(self.total-self.num, 20000),
                                     socket.MSG_DONTWAIT)

        **time.sleep(0.1)**  
        #if i add it here to let the event slow down the problem solved, how it could be? 

        self.num += self.n
        self.msg_buffer.fromstring(self.msg.tostring()[:self.n])
        ...
        if self.total > self.num:  #if not the last msg continue recving and sending...
            self.ent_s_send.add()
            self.recv_ent.add()
        ...

    def ser_send(self, ent, ev):
        ...
        num = self.conn.send(self.msg_buffer,socket.MSG_DONTWAIT)
        ...
        self.msg_buffer = self.msg_buffer[num:]

 ...
 ...

 class Server(object):
    def __init__( self ):
        self.sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
        self.sock.bind(('localhost', 18001))
        self.sock.listen(50)
        self.mutex = multiprocessing.Lock()

    def loop( self, ):

        for i in range(0,4):
            pid = os.fork()
            if pid == 0 or pid == -1:
                break

        if pid == -1:
            print "Fork failed!!!"
            sys.exit()

        elif pid == 0:   **# create four child ps to accept the socket!**
            print "Child  PID =  %d" % os.getpid()
            core.init()
            self.event = core.event(core.EV_READ,
                                self.sock.fileno(),
                                self.onlink)
            self.event.add()
            core.dispatch()

        else:
            os.wait()

    def onlink( self, ent, ev):
        self.mutex.acquire()
        print 'Accept PID = %s' % os.getpid()
        try:
            self.conn, self.addr = self.sock.accept()   
            **#I think 'accept' is the the problem, but I cannot see how.** 

        except socket.error, why:
            if why.args[0] in [ECONNRESET, ENOTCONN, ESHUTDOWN, ECONNABORTED]:
                return
            else:
                raise
        print self.sock,self.conn,self.addr
        self.mutex.release()
        print 'Release PID = %s' % os.getpid()
        cc = Chat( self.conn, self.sock )
        self.event.add()



if __name__ == '__main__':

    s1 = Server()
    s1.loop()

Answer 1

accept() is a blocking call. It’ll wait indefinitely for a client to connect. Holding a mutex over a blocking operation like that is a Bad Idea^TM since you totally lock all other concurrent processes out.

Also, as @Maxim noted in the comments, you don’t really need to lock around accept(). Just let the OS arbitrate dequeuing of incoming connections and dispatch them to your processes.