fix "which" calls always returning true

The redirection of stderr towards stdout means calls to "which program"
always return something even when "program" is not installed.

This patch checks for the return value instead before returning the
value.

See #814.

mininet/node.py

@@ -63,7 +63,7 @@ from time import sleep
 from mininet.log import info, error, warn, debug
 from mininet.util import ( quietRun, errRun, errFail, moveIntf, isShellBuiltin,
                            numCores, retry, mountCgroups, BaseString, decode,
-                           encode, Python3 )
+                           encode, Python3, which )
 from mininet.moduledeps import moduleDeps, pathCheck, TUN
 from mininet.link import Link, Intf, TCIntf, OVSIntf
 from re import findall
@@ -1447,7 +1447,7 @@ class Controller( Node ):
     @classmethod
     def isAvailable( cls ):
         "Is controller available?"
-        return quietRun( 'which controller' )
+        return which( 'controller' )
 
 
 class OVSController( Controller ):
@@ -1459,9 +1459,9 @@ class OVSController( Controller ):
 
     @classmethod
     def isAvailable( cls ):
-        return ( quietRun( 'which ovs-controller' ) or
-                 quietRun( 'which test-controller' ) or
-                 quietRun( 'which ovs-testcontroller' ) ).strip()
+        return (which( 'ovs-controller' ) or
+                which( 'test-controller' ) or
+                which( 'ovs-testcontroller' ))
 
 class NOX( Controller ):
     "Controller to run a NOX application."

mininet/util.py

@@ -171,6 +171,11 @@ def quietRun( cmd, **kwargs ):
     "Run a command and return merged stdout and stderr"
     return errRun( cmd, stderr=STDOUT, **kwargs )[ 0 ]
 
+def which(cmd, **kwargs ):
+    "Run a command and return merged stdout and stderr"
+    out, _, ret = errRun( ["which", cmd], stderr=STDOUT, **kwargs )
+    return out.rstrip() if ret == 0 else None
+
 # pylint: enable=maybe-no-member
 
 def isShellBuiltin( cmd ):