ZOOKEEPER-5051: Avoid NPE closing 4lw connection during startup

Author: PAC-MAJ

zookeeper-server/src/main/java/org/apache/zookeeper/server/ZooKeeperServer.java

@@ -330,7 +330,10 @@ protected enum State {
     private final AuthenticationHelper authHelper = new AuthenticationHelper();
 
     void removeCnxn(ServerCnxn cnxn) {
-        zkDb.removeCnxn(cnxn);
+        ZKDatabase db = zkDb;
+        if (zkDb != null) {
+            zkDb.removeCnxn(cnxn);
+        }
     }
 
     /**

zookeeper-server/src/test/java/org/apache/zookeeper/server/ZooKeeperServerTest.java

@@ -225,4 +225,14 @@ public void testUpdateQuotaExceededMetrics() {
     public void testUpdateQuotaExceededMetrics_nullNamespace() {
         assertDoesNotThrow(() -> ZooKeeperServer.updateQuotaExceededMetrics(null));
     }
+
+    @Test
+    public void testRemoveCnxnBeforeStartData() {
+        // Simulate a connection cleanup triggered before the server has fully started.
+        // During this phase zkDb is still null (before startdata()).
+        // 4-letter commands can hit this path.
+        ZooKeeperServer zks = new ZooKeeperServer();
+        ServerCnxn cnxn = mock(ServerCnxn.class);
+        assertDoesNotThrow(() -> zks.removeCnxn(cnxn));
+    }
 }