Add version field to auth cookie

auth.py

@@ -3,6 +3,7 @@ import hashlib
 import ssl
 from datetime import datetime
 
+FORMAT_VERSION = 1
 
 class User:
     def __init__(self, username, groups, all_groups, timestamp=None,
@@ -22,19 +23,26 @@ class User:
     def summarize(self):
         return ":".join((
             self.username, ",".join(self.groups), ",".join(self.all_groups),
-            str(self.timestamp.timestamp()), str(self.permanent)))
+            str(self.timestamp.timestamp()), str(self.permanent),
+            str(FORMAT_VERSION)))
 
     @staticmethod
     def from_summary(summary):
-        parts = summary.split(":", 4)
-        if len(parts) != 5:
+        parts = summary.split(":", 5)
+        if len(parts) != 6:
+            return None
+        (name, group_str, all_group_str, timestamp_str,
+            permanent_str, version_str) = parts
+        try:
+            if int(version_str) != FORMAT_VERSION:
                 return None
-        (name, group_str, all_group_str, timestamp_str, permanent_str) = parts
             timestamp = datetime.fromtimestamp(float(timestamp_str))
             groups = group_str.split(",")
             all_groups = all_group_str.split(",")
             permanent = permanent_str == "True"
             return User(name, groups, all_groups, timestamp, permanent)
+        except ValueError:
+            return None
 
     @staticmethod
     def from_hashstring(secure_string):