[1165] recover the strong exception guarantee in update_config_data()

(I first thought it could be deferred, but since the exception can happen
just due to incorrect config input, it should be guaranteed from the
beginning.)

Also made sure that both the default ACL and zone config are changed
atomically under the protection of lock.

src/bin/xfrout/xfrout.py.in

@@ -431,6 +431,8 @@ class UnixSockServer(socketserver_mixin.NoPollMixIn,
         '''Initialization shared with the mock server class used for tests'''
         self._lock = threading.Lock()
         self._transfers_counter = 0
+        self._zone_config = {}
+        self._acl = None # this will be initialized in update_config_data()
 
     def _receive_query_message(self, sock):
         ''' receive request message from sock'''
@@ -536,10 +538,13 @@ class UnixSockServer(socketserver_mixin.NoPollMixIn,
 
         This method creates a XfroutSession object.
         '''
+        self._lock.acquire()
+        acl = self._acl
+        zone_config = self._zone_config
+        self._lock.release()
         self.RequestHandlerClass(sock_fd, request_data, self,
                                  self.tsig_key_ring,
-                                 self._guess_remote(sock_fd), self._acl,
-                                 self._zone_config)
+                                 self._guess_remote(sock_fd), acl, zone_config)
 
     def _remove_unused_sock_file(self, sock_file):
         '''Try to remove the socket file. If the file is being used
@@ -583,24 +588,30 @@ class UnixSockServer(socketserver_mixin.NoPollMixIn,
     def update_config_data(self, new_config):
         '''Apply the new config setting of xfrout module.
 
-        Note: this method does not provide strong exception guarantee;
-        if an exception is raised in the middle of parsing and building the
-        given config data, the incomplete set of new configuration will
-        remain.  This should be fixed.
         '''
-        logger.info(XFROUT_NEW_CONFIG)
-        if 'transfer_acl' in new_config:
-            try:
-                self._acl = REQUEST_LOADER.load(new_config['transfer_acl'])
-            except LoaderError as e:
-                raise XfroutConfigError('Failed to parse transfer_acl: ' +
-                                        str(e))
-        zone_config = new_config.get('zone_config')
-        if zone_config is not None:
-            self._zone_config = self.__create_zone_config(zone_config)
         self._lock.acquire()
-        self._max_transfers_out = new_config.get('transfers_out')
-        self.set_tsig_key_ring(new_config.get('tsig_key_ring'))
+        try:
+            logger.info(XFROUT_NEW_CONFIG)
+            new_acl = self._acl
+            if 'transfer_acl' in new_config:
+                try:
+                    new_acl = REQUEST_LOADER.load(new_config['transfer_acl'])
+                except LoaderError as e:
+                    raise XfroutConfigError('Failed to parse transfer_acl: ' +
+                                            str(e))
+
+            new_zone_config = self._zone_config
+            zconfig_data = new_config.get('zone_config')
+            if zconfig_data is not None:
+                new_zone_config = self.__create_zone_config(zconfig_data)
+
+            self._acl = new_acl
+            self._zone_config = new_zone_config
+            self._max_transfers_out = new_config.get('transfers_out')
+            self.set_tsig_key_ring(new_config.get('tsig_key_ring'))
+        except Exception as e:
+            self._lock.release()
+            raise e
         self._lock.release()
         logger.info(XFROUT_NEW_CONFIG_DONE)