Try to fix iperf race condition

This is more complicated than it should be. We are also relying on
the fact that waitOutput should eat extra prompts most of the time.
Still not perfect - it's hard to get this exactly right, and we
should try to make it easier!

Fixes #589

mininet/net.py

@@ -767,8 +767,12 @@ class Mininet( object ):
         cliout = client.cmd( iperfArgs + '-t %d -c ' % seconds +
                              server.IP() + ' ' + bwArgs )
         debug( 'Client output: %s\n' % cliout )
+        servout = ''
+        # We need the second *b/sec from the iperf server output
+        while len( re.findall( '/sec', servout ) ) < 2:
+            servout += server.monitor( timeoutms=5000 )
         server.sendInt()
-        servout = server.waitOutput()
+        server.waitOutput()
         debug( 'Server output: %s\n' % servout )
         result = [ self._parseIperf( servout ), self._parseIperf( cliout ) ]
         if l4Type == 'UDP':