session.r2py

# This module wraps communications in a signaling protocol.   The purpose is to
# overlay a connection-based protocol with explicit message signaling.   
#
# The protocol is to send the size of the message followed by \n and then the
# message itself.   The size of a message must be able to be stored in 
# sessionmaxdigits.   A size of -1 indicates that this side of the connection
# should be considered closed.
#
# Note that the client will block while sending a message, and the receiver 
# will block while recieving a message.   
#
# While it should be possible to reuse the connectionbased socket for other 
# tasks so long as it does not overlap with the time periods when messages are 
# being sent, this is inadvisable.

class SessionEOF(Exception):
  pass

sessionmaxdigits = 20

# get the next message off of the socket...
def session_recvmessage(socketobj):

  messagesizestring = ''
  # first, read the number of characters...
  for junkcount in range(sessionmaxdigits):
    while True:
      try:
        currentbyte = socketobj.recv(1)
        break
      except SocketWouldBlockError:
        sleep(0.01)

    if currentbyte == '\n':
      break
    
    # not a valid digit
    if currentbyte not in '0123456789' and messagesizestring != '' and currentbyte != '-':
      raise ValueError, "Incorrect value in message size header. Found character '%s' in the header." % str(currentbyte)
     
    messagesizestring = messagesizestring + currentbyte

  else:
    # too large
    raise ValueError, "The header message size exceeds the maximum limit of %d." % sessionmaxdigits

  try:
    messagesize = int(messagesizestring)
  except ValueError:
    raise ValueError, "Unable to convert the message size '%s' to int." % messagesizestring
  
  # nothing to read...
  if messagesize == 0:
    return ''

  # end of messages
  if messagesize == -1:
    raise SessionEOF, "Connection Closed. Received a messagesize of -1"

  if messagesize < 0:
    raise ValueError, "Received a negative message size '%d'" % messagesize

  data = ''
  while len(data) < messagesize:
    try:
      chunk =  socketobj.recv(messagesize-len(data))
      if chunk == '': 
        raise SessionEOF, "Received an empty string when performing socketobj.recv(). Socket possibly closed."
      data = data + chunk
    except SocketWouldBlockError:
      sleep(0.01)

  return data

# a private helper function
def session_sendhelper(socketobj,data):
  sentlength = 0
  # if I'm still missing some, continue to send (I could have used sendall
  # instead but this isn't supported in repy currently)
  while sentlength < len(data):
    try:
      thissent = socketobj.send(data[sentlength:])
      sentlength = sentlength + thissent
    except SocketWouldBlockError:
      sleep(0.01)


# send the message 
def session_sendmessage(socketobj,data):
  header = str(len(data)) + '\n'
  # Sending these piecemeal does not accomplish anything, and can contribute 
  # to timeout issues when run by constantly overloaded machines.
  # session_sendhelper(socketobj,header)

  # Concatenate the header and data, rather than sending both separately.
  complete_packet = header + data

  # session_sendhelper(socketobj,data)

  session_sendhelper(socketobj, complete_packet)