Updating check_switch_started in p4runtime_switch.py (#85)

The previous implementation had a race conditiona between the
socket check and the BMv2 startup. Sometimes, if the check happen
just before (or as) BMv2 was trying to bind the port, it would
not be able to bind it. This solution uses psutil's equivalent
to 'netstat' to determine whether is BMv2 has bound the port yet.

Also, some minor README and comment improvements.

P4D2_2017_Fall/utils/p4runtime_switch.py

@@ -25,6 +25,14 @@ from mininet.log import info, error, debug
 sys.path.append('/home/vagrant/behavioral-model/mininet')
 from p4_mininet import P4Switch
 
+SWITCH_START_TIMEOUT = 10 # seconds
+
+import psutil
+def check_listening_on_port(port):
+    for c in psutil.net_connections(kind='inet'):
+        if c.status == 'LISTEN' and c.laddr[1] == port:
+            return True
+    return False
 
 class P4RuntimeSwitch(P4Switch):
     "BMv2 switch with gRPC support"
@@ -59,6 +67,10 @@ class P4RuntimeSwitch(P4Switch):
             self.grpc_port = P4RuntimeSwitch.next_grpc_port
             P4RuntimeSwitch.next_grpc_port += 1
 
+        if check_listening_on_port(self.grpc_port):
+            error('%s cannot bind port %d because it is bound by another process\n' % (self.name, self.grpc_port))
+            exit(1)
+
         self.verbose = verbose
         logfile = "/tmp/p4s.{}.log".format(self.name)
         self.output = open(logfile, 'w')
@@ -75,17 +87,12 @@ class P4RuntimeSwitch(P4Switch):
 
 
     def check_switch_started(self, pid):
-        while True:
+        for _ in range(SWITCH_START_TIMEOUT * 2):
             if not os.path.exists(os.path.join("/proc", str(pid))):
                 return False
-            sock = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
-            sock.settimeout(0.5)
-            result = sock.connect_ex(("localhost", self.grpc_port))
-            if result == 0:
-                # TODO It seems like sometimes BMv2 can hang if multiple instances start up too quickly;
-                #      this sleep might also do nothing...
-                sleep(0.5)
+            if check_listening_on_port(self.grpc_port):
                 return True
+            sleep(0.5)
 
     def start(self, controllers):
         info("Starting P4 switch {}.\n".format(self.name))