Added timeout option for monitor().

This allows monitor() to be used for polling. Of course, you
probably want to use poll() first anyway, but it's nice not
to have to block if there's no output.

mininet/node.py

@@ -149,10 +149,11 @@ def stop( self ):
         "Stop node."
         self.terminate()
 
-    def waitReadable( self ):
-        "Wait until node's output is readable."
+    def waitReadable( self, timeoutms=None ):
+        """Wait until node's output is readable.
+           timeoutms: timeout in ms or None to wait indefinitely."""
         if len( self.readbuf ) == 0:
-            self.pollOut.poll()
+            self.pollOut.poll( timeoutms )
 
     def sendCmd( self, cmd, printPid=True ):
         """Send a command, followed by a command to echo a sentinel,
@@ -160,6 +161,9 @@ def sendCmd( self, cmd, printPid=True ):
         assert not self.waiting
         if isinstance( cmd, list ):
             cmd = ' '.join( cmd )
+        if not re.search( r'\w', cmd ):
+            # Replace empty commands with something harmless
+            cmd = 'echo -n'
         if len( cmd ) > 0 and cmd[ -1 ] == '&':
             separator = '&'
             cmd = cmd[ :-1 ]
@@ -180,11 +184,12 @@ def sendInt( self, sig=signal.SIGINT ):
             except Exception:
                 pass
 
-    def monitor( self ):
+    def monitor( self, timeoutms=None ):
         """Monitor and return the output of a command.
-           Set self.waiting to False if command has completed."""
+           Set self.waiting to False if command has completed.
+           timeoutms: timeout in ms or None to wait indefinitely."""
         assert self.waiting
-        self.waitReadable()
+        self.waitReadable( timeoutms )
         data = self.read( 1024 )
         # Look for PID
         marker = chr( 1 ) + r'\d+\n'
@@ -395,10 +400,10 @@ def sendCmd( self, cmd, printPid=False):
             error( '*** Error: %s has execed and cannot accept commands' %
                      self.name )
 
-    def monitor( self ):
-        "Monitor node."
+    def monitor( self, *args, **kwargs ):
+        "Monitor a switch."
         if not self.execed:
-            return Node.monitor( self )
+            return Node.monitor( self, *args, **kwargs )
         else:
             return True, ''