NPE when trying to use CAS on local system tables

patch by David Capwell; reviewed by Ariel Weisberg for CASSANDRA-21112

src/java/org/apache/cassandra/service/consensus/migration/ConsensusRequestRouter.java

@@ -191,7 +191,7 @@ public class ConsensusRequestRouter
         TableMetadata metadata = getTableMetadata(cm, tableId);
         // Non-distributed tables always take the Paxos path
         if (metadata == null)
-            pickPaxos();
+            return pickPaxos();
         return routeAndMaybeMigrate(cm, metadata, key, consistencyLevel, requestTime, timeoutNanos, isForWrite);
     }
 

test/unit/org/apache/cassandra/cql3/SimpleCASTest.java

@@ -0,0 +1,41 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package org.apache.cassandra.cql3;
+
+import org.junit.BeforeClass;
+import org.junit.Test;
+
+public class SimpleCASTest extends CQLTester
+{
+    @BeforeClass
+    public static void setup()
+    {
+        requireNetwork();
+    }
+
+    @Test
+    public void casOnSystemTable()
+    {
+        // in CASSANDRA-21112 there was a NPE caused by a missing return.  This happened when you tried to do
+        // CAS on a local system table, the logic to figure out the protocol is expected to choose paxos but lacked
+        // the return and instead tried to infer from TCM, but local system tables are not in TCM and not allowed
+        // to be used in accord, so failed with a NPE.
+        executeNet("insert into system.peers(peer, data_center) values('0.0.0.0', 'moo') if not exists");
+    }
+}