ZOOKEEPER-4409 NullPointerException in SendAckRequestProcessor (#2163)

(cherry picked from commit 9b81d10)

Author: zouyonghao

zookeeper-server/src/main/java/org/apache/zookeeper/server/quorum/SendAckRequestProcessor.java

@@ -46,14 +46,7 @@ public void processRequest(Request si) {
                 learner.writePacket(qp, false);
             } catch (IOException e) {
                 LOG.warn("Closing connection to leader, exception during packet send", e);
-                try {
-                    if (!learner.sock.isClosed()) {
-                        learner.sock.close();
-                    }
-                } catch (IOException e1) {
-                    // Nothing to do, we are shutting things down, so an exception here is irrelevant
-                    LOG.debug("Ignoring error closing the connection", e1);
-                }
+                learner.closeSockSync();
             }
         }
     }
@@ -63,14 +56,7 @@ public void flush() throws IOException {
             learner.writePacket(null, true);
         } catch (IOException e) {
             LOG.warn("Closing connection to leader, exception during packet send", e);
-            try {
-                if (!learner.sock.isClosed()) {
-                    learner.sock.close();
-                }
-            } catch (IOException e1) {
-                // Nothing to do, we are shutting things down, so an exception here is irrelevant
-                LOG.debug("Ignoring error closing the connection", e1);
-            }
+            learner.closeSockSync();
         }
     }
 

zookeeper-server/src/test/java/org/apache/zookeeper/server/quorum/SendAckRequestProcessorTest.java

@@ -0,0 +1,46 @@
+/*
+ * 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.zookeeper.server.quorum;
+
+import static org.junit.jupiter.api.Assertions.assertTrue;
+import java.io.IOException;
+import org.apache.zookeeper.ZKTestCase;
+import org.apache.zookeeper.ZooDefs;
+import org.apache.zookeeper.server.Request;
+import org.apache.zookeeper.txn.TxnHeader;
+import org.junit.jupiter.api.Test;
+
+class SendAckRequestProcessorTest extends ZKTestCase {
+    static class FakeLearner extends Learner {
+        public FakeLearner() {
+            sock = null;
+        }
+
+        void writePacket(QuorumPacket pp, boolean flush) throws IOException {
+            throw new IOException();
+        }
+    }
+
+    @Test
+    public void learnerSocketCloseTest() {
+        SendAckRequestProcessor processor = new SendAckRequestProcessor(new FakeLearner());
+        processor.processRequest(new Request(0L, 0, ZooDefs.OpCode.sync, new TxnHeader(), new LearnerInfo(), 0L));
+        assertTrue(true, "should get here without exception");
+    }
+}