tweak: do not crash when trying to remove all nodes

neo/master/handlers/administration.py

@@ -197,9 +197,14 @@ class AdministrationHandler(MasterHandler):
     @__change_pt_rpc
     def tweakPartitionTable(self, conn, uuid_list):
         app = self.app
-        app.broadcastPartitionChanges(app.pt.tweak([node
-            for node in app.nm.getStorageList()
-            if node.getUUID() in uuid_list or not node.isRunning()]))
+        drop_list = [node for node in app.nm.getStorageList()
+            if node.getUUID() in uuid_list or not node.isRunning()]
+        try:
+            changed_list = app.pt.tweak(drop_list)
+        except PartitionTableException, e:
+            raise AnswerDenied(str(e))
+        else:
+            app.broadcastPartitionChanges(changed_list)
         conn.answer(Errors.Ack(''))
 
     @check_state(ClusterStates.RUNNING)

neo/master/pt.py

@@ -230,6 +230,8 @@ class PartitionTable(neo.lib.pt.PartitionTable):
         # Collect some data in a usable form for the rest of the method.
         node_list = {node: {} for node in self.count_dict
                               if node not in drop_list}
+        if not node_list:
+            raise neo.lib.pt.PartitionTableException("Can't remove all nodes.")
         drop_list = defaultdict(list)
         for offset, row in enumerate(self.partition_list):
             for cell in row: