port.py

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#! /usr/bin/env python
# -*- coding: utf-8 -*-
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#            ROBOTICS™
#
#
#  Copyright © 2012 Clearpath Robotics, Inc. 
#  All Rights Reserved
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# modification, are permitted provided that the following conditions are met:
#     * Redistributions of source code must retain the above copyright
#       notice, this list of conditions and the following disclaimer.
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#       documentation and/or other materials provided with the distribution.
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#       names of its contributors may be used to endorse or promote products
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# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" AND
# ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED
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# DISCLAIMED. IN NO EVENT SHALL CLEARPATH ROBOTICS, INC. BE LIABLE FOR ANY
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# (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
# LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND
# ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
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import applanix_msgs.msg as msg

# Node source 
from translator import Translator

# Python
import threading
import socket
import struct
from cStringIO import StringIO


class Port(threading.Thread):
  """ Common base class for DataPort and ControlPort. Provides functionality to
      recv/send Applanix-formatted packets from the socket. Could in future 
      support LoggingPort and DisplayPort."""
  checksum_struct = struct.Struct("<hh")

  def __init__(self, sock, **opts):
    super(Port, self).__init__()
    self.sock = sock
    self.opts = opts
    self.daemon = False
    self.finish = threading.Event()

    # These are only for receiving. 
    self.header = msg.CommonHeader()
    self.footer = msg.CommonFooter()

  def recv(self, d=False):
    """ Receive a packet from the port's socket.
        Returns (pkt_id, pkt_str), where pkt_id is ("$GRP"|"$MSG", num)
        Returns None, None when no data. """
    try:
      header_str = self.sock.recv(self.header.translator().size)
    except socket.timeout:
      return None, None

    header_data = StringIO(header_str)
    self.header.translator().deserialize(header_data)
    pkt_id = (str(self.header.start).encode('string_escape'), self.header.id)

    # Initial sanity check.
    if pkt_id[0] not in (msg.CommonHeader.START_GROUP, msg.CommonHeader.START_MESSAGE):
      raise ValueError("Bad header %s.%d" % pkt_id)

    # Special case for a troublesome undocumented packet.
    if pkt_id == ("$GRP", 20015):
      self.sock.recv(135)
      return None, None

    # Receive remainder of packet from data socket. 
    pkt_str = self.sock.recv(self.header.length)

    # Check package footer.
    footer_data = StringIO(pkt_str[-self.footer.translator().size:])
    self.footer.translator().deserialize(footer_data)
    if str(self.footer.end) != msg.CommonFooter.END:
      raise("Bad footer from pkt %s.%d" % pkt_id)

    # Check package checksum.
    if self._checksum(StringIO(header_str + pkt_str)) != 0:
      raise("Bad checksum from pkt %s.%d: %%d" % pkt_id % checksum)

    return pkt_id, pkt_str 

  def send(self, header, message):
    """ Sends a header/msg/footer out the socket. Takes care of computing
        length field for header and checksum field for footer. """
    msg_buff = StringIO()
    message.translator().preserialize()
    message.translator().serialize(msg_buff)
    pad_count = -msg_buff.tell() % 4
    msg_buff.write("\x00" * pad_count)

    footer = msg.CommonFooter(end=msg.CommonFooter.END)
    header.length = msg_buff.tell() + footer.translator().size

    # Write header and message to main buffer.
    buff = StringIO()
    header.translator().serialize(buff)
    buff.write(msg_buff.getvalue())
     
    # Write footer.
    footer_start = buff.tell()
    footer.translator().serialize(buff) 

    # Compute checksum.
    buff.seek(0)
    footer.checksum = 65536 - self._checksum(buff)

    # Rewrite footer with correct checksum.
    buff.seek(footer_start)
    footer.translator().serialize(buff) 

    #print buff.getvalue().encode("string_escape")
    self.sock.send(buff.getvalue())

  @classmethod
  def _checksum(cls, buff):
    """ Compute Applanix checksum. Expects a StringIO with a 
      size that is a multiple of four bytes. """
    checksum = 0
    while True:
      data = buff.read(cls.checksum_struct.size)
      if len(data) == 0:
        break
      if len(data) < 4:
        raise ValueError("Checksum data length is not a multiple of 4.")
      c1, c2 = cls.checksum_struct.unpack(data)
      checksum += c1 + c2
    return checksum % 65536