added exception handling to clean_up method

will submit a pull request to CherryPy project so we can remove this code when it gets merged

CherryPy.conf

@@ -69,6 +69,7 @@ tools.I18nTool.mo_dir: CherryPyApp.install_dir + '/i18n'
 tools.I18nTool.domain: 'messages'
 
 tools.sessions.on: True
+tools.sessions.storage_class = sessions.RamSession
 #tools.sessions.storage_type = "postgres"
 tools.sessions.table_name = "cherrypy.sessions"
 tools.sessions.timeout: 30

sessions.py

@@ -0,0 +1,38 @@
+from cherrypy.lib.sessions import RamSession as cpRamSession
+
+class RamSession(cpRamSession):
+    def clean_up(self):
+        """Clean up expired sessions."""
+
+        now = self.now()
+        try:
+            for _id, (data, expiration_time) in list(self.cache.items()):
+                if expiration_time <= now:
+                    try:
+                        del self.cache[_id]
+                    except KeyError:
+                        pass
+                    try:
+                        if self.locks[_id].acquire(blocking=False):
+                            lock = self.locks.pop(_id)
+                            lock.release()
+                    except KeyError:
+                        pass
+
+        except RuntimeError:
+            """Under heavy load, list(self.cache.items()) will occasionally raise this error
+            for large session caches with message "dictionary changed size during iteration"
+            Better to pause the cleanup than to let the cleanup thread die.
+            """
+
+            return 
+
+        # added to remove obsolete lock objects
+        for _id in list(self.locks):
+            locked = (
+                _id not in self.cache
+                and self.locks[_id].acquire(blocking=False)
+            )
+            if locked:
+                lock = self.locks.pop(_id)
+                lock.release()