ZOOKEEPER-5012: guard zkDb close in QuorumPeer shutdown

Reviewers: PDavid, anmolnar
Author: seekskyworld
Closes #2345 from seekskyworld/fix/5012-quorumpeer-shutdown-npe

Author: seekskyworld

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

@@ -1705,10 +1705,12 @@ public void shutdown() {
             this.interrupt();
             getElectionAlg().shutdown();
         }
-        try {
-            zkDb.close();
-        } catch (IOException ie) {
-            LOG.warn("Error closing logs ", ie);
+        if (zkDb != null) {
+            try {
+                zkDb.close();
+            } catch (IOException ie) {
+                LOG.warn("Error closing logs ", ie);
+            }
         }
     }
 

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

@@ -18,6 +18,7 @@
 
 package org.apache.zookeeper.server.quorum;
 
+import static org.junit.jupiter.api.Assertions.assertDoesNotThrow;
 import static org.junit.jupiter.api.Assertions.assertEquals;
 import static org.junit.jupiter.api.Assertions.assertFalse;
 import static org.junit.jupiter.api.Assertions.assertTrue;
@@ -80,6 +81,12 @@ public void testQuorumPeerListendOnSpecifiedClientIP(@TempDir File dataDir) thro
         peer2.shutdown();
     }
 
+    @Test
+    public void testShutdownWithoutZkDbDoesNotThrow() throws Exception {
+        QuorumPeer peer = new QuorumPeer();
+        assertDoesNotThrow(peer::shutdown);
+    }
+
     @Test
     public void testProcessReconfigWillRecreateSocketAddresses() throws IOException {
         QuorumPeerConfig.setReconfigEnabled(true);