small refactor: put function to ensure root in util

Two benefits:
- One place to change if in the future, a more granular method of
  root access is used (like the BigSwitch patch).
- Makes this reusable by stuff like examples/baresshd.py that use
  the low-level Mininet API.

mininet/net.py

@@ -96,7 +96,7 @@ from mininet.cli import CLI
 from mininet.log import info, error, debug, output
 from mininet.node import Host, OVSKernelSwitch, Controller
 from mininet.link import Link, Intf
-from mininet.util import quietRun, fixLimits, numCores
+from mininet.util import quietRun, fixLimits, numCores, ensureRoot
 from mininet.util import macColonHex, ipStr, ipParse, netParse, ipAdd
 from mininet.term import cleanUpScreens, makeTerms
 
@@ -571,12 +571,7 @@ class Mininet( object ):
         "Initialize Mininet"
         if cls.inited:
             return
-        if os.getuid() != 0:
+        ensureRoot()
-            # Note: this script must be run as root
-            # Probably we should only sudo when we need
-            # to as per Big Switch's patch
-            print "*** Mininet must run as root."
-            exit( 1 )
         fixLimits()
         cls.inited = True
 

mininet/util.py

@@ -9,6 +9,7 @@ from subprocess import call, check_call, Popen, PIPE, STDOUT
 import re
 from fcntl import fcntl, F_GETFL, F_SETFL
 from os import O_NONBLOCK
+import os
 
 # Command execution support
 
@@ -456,3 +457,13 @@ def buildTopo( topos, topoStr ):
     if topo not in topos:
         raise Exception( 'Invalid topo name %s' % topo )
     return topos[ topo ]( *args, **kwargs )
+
+def ensureRoot():
+    """Ensure that we are running as root.
+
+    Probably we should only sudo when needed as per Big Switch's patch.
+    """
+    if os.getuid() != 0:
+        print "*** Mininet must run as root."
+        exit( 1 )
+    return