Fix polling in errRun

It's tricky to get this right, but basically we want to read
if there is something to read; if not, we want to check for
EOF.

mininet/util.py

@@ -91,8 +91,9 @@ def errRun( *cmd, **kwargs ):
         errDone = False
     while not outDone or not errDone:
         readable = poller.poll()
-        for fd, _event in readable:
+        for fd, event in readable:
             f = fdtofile[ fd ]
+            if event & POLLIN:
                 data = f.read( 1024 )
                 if echo:
                     output( data )
@@ -104,6 +105,13 @@ def errRun( *cmd, **kwargs ):
                     err += data
                     if data == '':
                         errDone = True
+            elif event & POLLHUP:
+                if f == popen.stdout:
+                    outDone = True
+                elif f == popen.stderr:
+                    errDone = True
+                poller.unregister( fd )
+
     returncode = popen.wait()
     return out, err, returncode