CASSANDRA-21491: Prevent ClientMetrics NPE during internode reactivation

src/java/org/apache/cassandra/net/AbstractMessageHandler.java

@@ -31,7 +31,6 @@
 import org.slf4j.LoggerFactory;
 
 import org.apache.cassandra.concurrent.ManyToOneConcurrentLinkedQueue;
-import org.apache.cassandra.metrics.ClientMetrics;
 import org.apache.cassandra.net.FrameDecoder.CorruptFrame;
 import org.apache.cassandra.net.FrameDecoder.Frame;
 import org.apache.cassandra.net.FrameDecoder.FrameProcessor;
@@ -292,7 +291,8 @@ private void onGlobalReserveCapacityRegained(Limit globalReserve, long elapsedNa
         onReserveCapacityRegained(endpointReserveCapacity, globalReserve, elapsedNanos);
     }
 
-    private void onReserveCapacityRegained(Limit endpointReserve, Limit globalReserve, long elapsedNanos)
+    @VisibleForTesting
+    void onReserveCapacityRegained(Limit endpointReserve, Limit globalReserve, long elapsedNanos)
     {
         if (isClosed)
             return;
@@ -314,7 +314,7 @@ private void onReserveCapacityRegained(Limit endpointReserve, Limit globalReserv
                 decoder.reactivate();
 
                 if (decoder.isActive())
-                    ClientMetrics.instance.unpauseConnection();
+                    onConnectionUnpaused();
             }
         }
         catch (Throwable t)
@@ -323,6 +323,10 @@ private void onReserveCapacityRegained(Limit endpointReserve, Limit globalReserv
         }
     }
 
+    protected void onConnectionUnpaused()
+    {
+    }
+
     protected abstract void fatalExceptionCaught(Throwable t);
 
     // return true if the handler should be reactivated - if no new hurdles were encountered,

src/java/org/apache/cassandra/transport/CQLMessageHandler.java

@@ -159,6 +159,12 @@ public boolean process(FrameDecoder.Frame frame) throws IOException
         return super.process(frame);
     }
 
+    @Override
+    protected void onConnectionUnpaused()
+    {
+        ClientMetrics.instance.unpauseConnection();
+    }
+
     /**
      * Checks limits on bytes in flight and the request rate limiter (if enabled), then takes one of three actions:
      * 

test/unit/org/apache/cassandra/net/AbstractMessageHandlerTest.java

@@ -0,0 +1,86 @@
+/*
+ * 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.net;
+
+import java.io.IOException;
+import java.util.concurrent.atomic.AtomicReference;
+
+import io.netty.channel.Channel;
+import io.netty.channel.EventLoop;
+import org.junit.Test;
+
+import org.apache.cassandra.net.ResourceLimits.Basic;
+import org.apache.cassandra.net.ResourceLimits.Limit;
+
+import static org.junit.Assert.assertNull;
+import static org.mockito.Mockito.mock;
+import static org.mockito.Mockito.verify;
+import static org.mockito.Mockito.when;
+
+public class AbstractMessageHandlerTest
+{
+    @Test
+    public void internodeReactivationDoesNotUseUninitializedClientMetrics() throws IOException
+    {
+        FrameDecoder decoder = mock(FrameDecoder.class);
+        when(decoder.isActive()).thenReturn(true);
+
+        EventLoop eventLoop = mock(EventLoop.class);
+        when(eventLoop.inEventLoop()).thenReturn(true);
+        Channel channel = mock(Channel.class);
+        when(channel.eventLoop()).thenReturn(eventLoop);
+
+        Limit endpointReserve = new Basic(1);
+        Limit globalReserve = new Basic(1);
+        AtomicReference<Throwable> failure = new AtomicReference<>();
+
+        InboundMessageHandler handler = new InboundMessageHandler(decoder,
+                                                                  ConnectionType.LARGE_MESSAGES,
+                                                                  channel,
+                                                                  null,
+                                                                  null,
+                                                                  MessagingService.current_version,
+                                                                  1,
+                                                                  1,
+                                                                  endpointReserve,
+                                                                  globalReserve,
+                                                                  AbstractMessageHandler.WaitQueue.endpoint(endpointReserve),
+                                                                  AbstractMessageHandler.WaitQueue.global(globalReserve),
+                                                                  ignored -> {},
+                                                                  null,
+                                                                  null)
+        {
+            @Override
+            protected boolean processUpToOneMessage(Limit endpoint, Limit global)
+            {
+                return true;
+            }
+
+            @Override
+            protected void fatalExceptionCaught(Throwable cause)
+            {
+                failure.set(cause);
+            }
+        };
+
+        handler.onReserveCapacityRegained(endpointReserve, globalReserve, 0);
+
+        assertNull(failure.get());
+        verify(decoder).reactivate();
+    }
+}