Fix paginated getChildren returning no children for the root path

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

@@ -976,8 +976,10 @@ private void buildChildrenPathWithStat(DataNode n, String path, Stat stat, long
             if (stat != null) {
                 n.copyStat(stat);
             }
+            // The root's children are keyed as "/child", not "//child"
+            final String prefix = "/".equals(path) ? path : path + "/";
             for (String child : n.getChildren()) {
-                DataNode childNode = nodes.get(path + "/" + child);
+                DataNode childNode = nodes.get(prefix + child);
                 if (null != childNode) {
                     final long czxId = childNode.stat.getCzxid();
 

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

@@ -33,6 +33,7 @@
 import java.io.StringWriter;
 import java.lang.reflect.Field;
 import java.util.ArrayList;
+import java.util.Arrays;
 import java.util.List;
 import java.util.Map;
 import java.util.concurrent.CompletableFuture;
@@ -415,6 +416,57 @@ public void testGetChildrenPaginated() throws NodeExistsException, NoNodeExcepti
         assertEquals(0, nextPage.getMinCzxidOffset());
     }
 
+    @Test(timeout = 60000)
+    public void testGetChildrenPaginatedOnRoot() throws NodeExistsException, NoNodeException {
+        final int firstCzxId = 1000;
+        final int countNodes = 10;
+
+        DataTree dt = new DataTree();
+        // Create the children in reverse lexical order so that ordering by czxid (correct) and
+        // ordering by name (incorrect) produce different results, and the test can tell them apart.
+        List<String> childrenCreated = new ArrayList<>(countNodes);
+        for (int i = 0; i < countNodes; ++i) {
+            final String name = "test-" + (countNodes - 1 - i);
+            dt.createNode("/" + name, new byte[0], null, 0, dt.getNode("/").stat.getCversion() + 1, firstCzxId + i, 1);
+            childrenCreated.add(name);
+        }
+
+        // "/zookeeper" has czxid 0, so it sorts ahead of everything created above.
+        List<String> expected = new ArrayList<>();
+        expected.add("zookeeper");
+        expected.addAll(childrenCreated);
+
+        // Bounded pages on "/" must return every child, in creation order
+        List<String> collected = new ArrayList<>();
+        PaginationNextPage nextPage = new PaginationNextPage();
+        long minCzxid = -1;
+        int offset = 0;
+        final int watchCount = dt.getWatchCount();
+        boolean reachedLastPage = false;
+        for (int i = 0; i < countNodes + 2; ++i) {
+            List<String> page = dt.getPaginatedChildren("/", null, new DummyWatcher(), 3, minCzxid, offset, nextPage);
+            collected.addAll(page);
+            if (nextPage.getMinCzxid() == ZooDefs.GetChildrenPaginated.lastPageMinCzxid) {
+                reachedLastPage = true;
+                assertEquals("The watch must be set on the last page", watchCount + 1, dt.getWatchCount());
+                break;
+            }
+            // Asserting this on every intermediate page is what catches the "//child" bug: with the
+            // bug the very first page looks like a last page and registers the watch immediately.
+            assertEquals("The watch must not be set before the last page", watchCount, dt.getWatchCount());
+            minCzxid = nextPage.getMinCzxid();
+            offset = nextPage.getMinCzxidOffset();
+        }
+        assertTrue("Pagination never reported a last page", reachedLastPage);
+        assertEquals(expected, collected);
+
+        // minCzxId > 0 skips the fast path even with an unbounded page size
+        List<String> result = dt.getPaginatedChildren("/", null, null, Integer.MAX_VALUE, firstCzxId + countNodes - 2, 0, nextPage);
+        assertEquals(Arrays.asList("test-1", "test-0"), result);
+        assertEquals("The remainder fits in one page, so it is the last page",
+                ZooDefs.GetChildrenPaginated.lastPageMinCzxid, nextPage.getMinCzxid());
+    }
+
     @Test(timeout = 60000)
     public void testGetChildrenPaginatedWithOffset() throws NodeExistsException, NoNodeException {
         final String rootPath = "/children";

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

@@ -19,6 +19,7 @@
 package org.apache.zookeeper.test;
 
 import static org.junit.Assert.fail;
+import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.HashSet;
 import java.util.List;
@@ -127,6 +128,38 @@ public void testPaginationIterator() throws Exception {
         Assert.assertEquals(createdChildrenMetadata.keySet(), readChildrenMetadata);
     }
 
+    @Test(timeout = 30000)
+    public void testPaginationIteratorOnRoot() throws Exception {
+        final String prefix = "/testPaginationRoot-" + UUID.randomUUID();
+        List<String> expected = new ArrayList<String>();
+        for (int i = 0; i < 10; i++) {
+            zk.create(prefix + i, new byte[0], Ids.OPEN_ACL_UNSAFE, CreateMode.PERSISTENT);
+            expected.add(prefix.substring(1) + i);
+        }
+
+        // Collect into a List (not a Set) so duplicated or out-of-order pages are caught.
+        List<String> actual = new ArrayList<String>();
+        RemoteIterator<String> childrenIterator = zk.getChildrenIterator("/", null, 3, -1);
+        while (childrenIterator.hasNext()) {
+            actual.add(childrenIterator.next());
+        }
+
+        Assert.assertEquals("Root pagination returned duplicates: " + actual,
+                new HashSet<String>(actual).size(), actual.size());
+        Assert.assertTrue("Missing children of root: " + expected + " vs " + actual, actual.containsAll(expected));
+        Assert.assertTrue("Missing /zookeeper: " + actual, actual.contains("zookeeper"));
+
+        // The nodes were created in order, so they must come back in that order. Other tests may
+        // leave unrelated nodes at the root, so compare only the ones this test created.
+        List<String> actualOurs = new ArrayList<String>();
+        for (String child : actual) {
+            if (child.startsWith(prefix.substring(1))) {
+                actualOurs.add(child);
+            }
+        }
+        Assert.assertEquals("Root children returned out of creation order", expected, actualOurs);
+    }
+
     /*
      * This test validates a known list of children is returned by the iterator despite server failures.
      * After the iterator is created successfully, the following logic drives the rest of the test: