Fixed problem with initializing db's latest state field

lib/zodb.py

@@ -179,19 +179,18 @@ def zconn_at(zconn): # -> tid
 
         raise AssertionError("zconn_at: TODO: add support for zstor %r" % zstor)
 
-    # ZODB3, ZODB4
-    #   
-    # If historic connection, convert and return zconn.before. Otherwise
-    # returns tid of last transaction, if viewing latest db state, or if  
-    # later db state exists, returns tid of db state previous to later
-    # state   
-    else:
+    elif zmajor == 4:
+        if not zconn.before 
         zconn._inv_lock.acquire()
         try:
             viewed_state = zconn.viewed_db_state
         finally:
             zconn._inv_lock.release()
         return viewed_state
+    
+    # TODO ZODB3  
+    else: 
+        pass
 
 # before2at converts tid that specifies database state as "before" into tid that
 # specifies database state as "at".
@@ -342,7 +341,9 @@ if zmajor == 4:
         self.history(z64)
         self._a()
         try:
-            if self.latest_state is None:
+            # initialize latest_state with last_tid if not yet initialized
+            # during invalidation process
+            if not hasattr(self, 'latest_state'):
                 self.latest_state = last_tid
         finally:
             self._r()