python程序中使用的python select

Question

I was reading a proxy server developed using python

I don't understand the method def _read_write which uses select to write client and server socket.

def _read_write(self):
    time_out_max = self.timeout/3
    socs = [self.client, self.target]
    count = 0
    while 1:
        count += 1
        (recv, _, error) = select.select(socs, [], socs, 3)
        if error:
            break
        if recv:
            for in_ in recv:
                data = in_.recv(BUFLEN)
                if in_ is self.client:
                    out = self.target
                else:
                    out = self.client
                if data:
                    out.send(data)
                    count = 0
        if count == time_out_max:
            break

Please someone help me to understand.

Answer 1

Here is my quick and dirty annotation:

def _read_write(self):

    # This allows us to get multiple
    # lower-level timeouts before we give up.
    # (but see later note about Python 3)

    time_out_max = self.timeout/3

    # We have two sockets we care about

    socs = [self.client, self.target]

    # Loop until error or timeout
    count = 0
    while 1:
        count += 1

        # select is very efficient.  It will let
        # other processes execute until we have
        # data or an error.

        # We only care about receive and error
        # conditions, so we pass in an empty list
        # for transmit, and assign transmit results
        # to the _ variable to ignore.

        # We also pass a timeout of 3 seconds, which
        # is why it's OK to divide the timeout value
        # by 3 above.

        # Note that select doesn't read anything for
        # us -- it just blocks until data is ready.

        (recv, _, error) = select.select(socs, [], socs, 3)

        # If we have an error, break out of the loop

        if error:
            break

        # If we have receive data, it's from the client
        # for the target, or the other way around, or
        # even both.  Loop through and deal with whatever
        # receive data we have and send it to the other
        # port.

        # BTW, "if recv" is redundant here -- (a) in
        # general (except for timeouts) we'll have
        # receive data here, and (b) the for loop won't
        # execute if we don't.

        if recv:
            for in_ in recv:

                # Read data up to a max of BUFLEN,

                data = in_.recv(BUFLEN)

                # Dump the data out the other side.
                # Indexing probably would have been
                # more efficient than this if/else

                if in_ is self.client:
                    out = self.target
                else:
                    out = self.client

                # I think this may be a bug.  IIRC,
                # send is not required to send all the
                # data, but I don't remember and cannot
                # be bothered to look it up right now.

                if data:
                    out.send(data)
                    # Reset the timeout counter.
                    count = 0

        # This is ugly -- should be >=, then it might
        # work even on Python 3...

        if count == time_out_max:
            break

    # We're done with the loop and exit the function on
    # either a timeout or an error.