ZOOKEEPER-4325: Fix bug when list "/" with ZkUtil::listSubTreeBFS

Fix bug [ZOOKEEPER-4325](https://issues.apache.org/jira/browse/ZOOKEEPER-4325), re-create PR from [#1719](https://github.com/apache/zookeeper/pull/1719)

Author: autumind <shen.zhibing@mobcb.com>

Reviewers: maoling <maoling@apache.org>

Closes #1729 from autumind/master

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

@@ -193,7 +193,8 @@ public class ZKUtil {
             String node = queue.poll();
             List<String> children = zk.getChildren(node, false);
             for (final String child : children) {
-                final String childPath = node + "/" + child;
+                // Fix IllegalArgumentException when list "/".
+                final String childPath = (node.equals("/") ? "" : node) + "/" + child;
                 queue.add(childPath);
                 tree.add(childPath);
             }

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

@@ -20,15 +20,20 @@ package org.apache.zookeeper;
 
 import static org.junit.Assume.assumeTrue;
 import static org.junit.jupiter.api.Assertions.assertEquals;
+import static org.junit.jupiter.api.Assertions.assertIterableEquals;
 import static org.junit.jupiter.api.Assertions.assertNotNull;
 import static org.junit.jupiter.api.Assertions.assertNull;
 import java.io.File;
 import java.io.IOException;
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.List;
 import java.util.UUID;
+import org.apache.zookeeper.test.ClientBase;
 import org.junit.jupiter.api.BeforeAll;
 import org.junit.jupiter.api.Test;
 
-public class ZKUtilTest {
+public class ZKUtilTest extends ClientBase {
 
     private static final File testData = new File(System.getProperty("test.data.dir", "build/test/data"));
 
@@ -85,4 +90,33 @@ public class ZKUtilTest {
         assertEquals(expectedMessage, error);
     }
 
+    @Test
+    public void testListRootPathSuccess() throws IOException, InterruptedException, KeeperException {
+        TestableZooKeeper zk = createClient();
+        zk.setData("/", "some".getBytes(), -1);
+        zk.create("/a", "some".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
+        zk.create("/a/b", "some".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
+
+        List<String> list = ZKUtil.listSubTreeBFS(zk, "/");
+        list.remove(Quotas.procZookeeper);
+        list.remove(Quotas.quotaZookeeper);
+        list.remove(ZooDefs.CONFIG_NODE);
+        assertEquals(3, list.size());
+        assertIterableEquals(Arrays.asList("/", "/a", "/a/b"), list);
+    }
+
+    @Test
+    public void testListNoneRootPathSuccess() throws IOException, InterruptedException, KeeperException {
+        TestableZooKeeper zk = createClient();
+        zk.setData("/", "some".getBytes(), -1);
+        zk.create("/a", "some".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
+        zk.create("/a/b", "some".getBytes(), ZooDefs.Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
+        List<String> aList = ZKUtil.listSubTreeBFS(zk, "/a");
+        assertEquals(2, aList.size());
+        assertIterableEquals(Arrays.asList("/a", "/a/b"), aList);
+
+        List<String> bList = ZKUtil.listSubTreeBFS(zk, "/a/b");
+        assertEquals(1, bList.size());
+        assertIterableEquals(Collections.singletonList("/a/b"), bList);
+    }
 }