Buffered output. Added net.monitor() and node.readline()

Moved monitor() and readline() into net.py and node.py respectively,
which will hopefully be useful for monitoring large sets of hosts,
as is done in udpbwtest.py.

Changed iperf to use interactive command infrastructure (such as it
is), which may make it more reliable. Hopefully it's a bit clearer
as well, although it is slightly more complicated.

examples/udpbwtest.py

@@ -18,7 +18,6 @@ various Mininet configurations, this example:
 
 import os
 import re
-import select
 import sys
 from time import time
 
@@ -30,37 +29,6 @@ from mininet.node import KernelSwitch
 from mininet.topolib import TreeTopo
 from mininet.util import quietRun
 
-# Some useful stuff: buffered readline and host monitoring
-
-def readline( host, buf ):
-    "Read a line from a host, buffering with buffer."
-    buf += host.read( 1024 )
-    if '\n' not in buf:
-        return None, buf
-    pos = buf.find( '\n' )
-    line = buf[ 0 : pos ]
-    rest = buf[ pos + 1: ]
-    return line, rest
-
-def monitor( hosts, seconds ):
-    "Monitor a set of hosts and yield their output."
-    poller = select.poll()
-    Node = hosts[ 0 ] # so we can call class method fdToNode
-    buffers = {}
-    for host in hosts:
-        poller.register( host.stdout )
-        buffers[ host ] = ''
-    quitTime = time() + seconds
-    while time() < quitTime:
-        ready = poller.poll()
-        for fd, event in ready:
-            host = Node.fdToNode( fd )
-            if event & select.POLLIN:
-                line, buffers[ host ] = readline( host, buffers[ host ] )
-                if line:
-                    yield host, line
-    yield None, ''
-
 # bwtest support
 
 def parsebwtest( line,
@@ -111,8 +79,9 @@ def udpbwtest( net, seconds ):
     print
     results = {}
     print "*** Monitoring hosts"
-    output = monitor( hosts, seconds )
-    while True:
+    output = net.monitor( hosts )
+    quitTime = time() + seconds
+    while time() < quitTime:
         host, line = output.next()
         if host is None:
             break