Caches non-existence of ZK node for ZooCache.getChildren (#5143)

* Caches non-existence of ZK node for ZooCache.getChildren

For a nodes data in zookeeper ZooCache was caching when a node did not
exist.  However for getChildren when a node did not exist it would not
cache this and would always go to zookeeper. Also ZooCache was handling
a null return from Zookeeper.getChildren that would never happen.  Fixed
these issues.

fixes #5047

* expire cache entries and remove their watches

* Update test/src/main/java/org/apache/accumulo/test/zookeeper/ZooCacheIT.java

Co-authored-by: Christopher Tubbs <[email protected]>

* code review update

* added test for watch removal and fixed bug with watch removal

* Removes logging added for testing

---------

Co-authored-by: Christopher Tubbs <[email protected]>

core/src/main/java/org/apache/accumulo/core/fate/zookeeper/ZcNode.java

@@ -0,0 +1,142 @@
+/*
+ * 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
+ *
+ *   https://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.accumulo.core.fate.zookeeper;
+
+import java.util.List;
+import java.util.Objects;
+
+import com.google.common.base.Preconditions;
+
+/**
+ * Immutable data class used by zoo cache to hold what it is caching for single zookeeper node. Data
+ * and children are obtained from zookeeper at different times. This class is structured so that
+ * data can be obtained first and then children added later or visa veras.
+ *
+ * <p>
+ * Four distinct states can be cached for a zookeeper node.
+ * <ul>
+ * <li>Can cache that a node does not exist in zookeeper. This state is represented by data, state,
+ * and children all being null.</li>
+ * <li>Can cache only the data for a zookeeper node. For this state data and stat are non-null while
+ * children is null. Calling getChildren on node in this state will throw an exception.</li>
+ * <li>Can cache only the children for a zookeeper node. For this state children is non-null while
+ * data and stat are null. Calling getData or getStat on node in this state will throw an
+ * exception.</li>
+ * <li>Can cache the children and data for a zookeeper node. For this state data,stat, and children
+ * are non-null.</li>
+ * </ul>
+ * <p>
+ *
+ */
+class ZcNode {
+
+  private final byte[] data;
+  private final ZooCache.ZcStat stat;
+  private final List<String> children;
+
+  static final ZcNode NON_EXISTENT = new ZcNode();
+
+  private ZcNode() {
+    this.data = null;
+    this.stat = null;
+    this.children = null;
+  }
+
+  /**
+   * Creates a new ZcNode that combines the data and stat from an existing ZcNode and sets the
+   * children.
+   */
+  ZcNode(List<String> children, ZcNode existing) {
+    Objects.requireNonNull(children);
+    if (existing == null) {
+      this.data = null;
+      this.stat = null;
+    } else {
+      this.data = existing.data;
+      this.stat = existing.stat;
+    }
+
+    this.children = List.copyOf(children);
+  }
+
+  /**
+   * Creates a new ZcNode that combines the children from an existing ZcNode and sets the data and
+   * stat.
+   */
+  ZcNode(byte[] data, ZooCache.ZcStat zstat, ZcNode existing) {
+    this.data = Objects.requireNonNull(data);
+    this.stat = Objects.requireNonNull(zstat);
+    if (existing == null) {
+      this.children = null;
+    } else {
+      this.children = existing.children;
+    }
+  }
+
+  /**
+   * @return the data if the node exists and the data was set OR return null when the node does not
+   *         exist
+   * @throws IllegalStateException in the case where the node exists and the data was never set
+   */
+  byte[] getData() {
+    Preconditions.checkState(cachedData());
+    return data;
+  }
+
+  /**
+   * @return the stat if the node exists and the stat was set OR return null when the node does not
+   *         exist
+   * @throws IllegalStateException in the case where the node exists and the data was never set
+   */
+  ZooCache.ZcStat getStat() {
+    Preconditions.checkState(cachedData());
+    return stat;
+  }
+
+  /**
+   * @return the children if the node exists and the children were set OR return null when the node
+   *         does not exist exists
+   * @throws IllegalStateException in the case where the node exists and the children were never set
+   */
+  List<String> getChildren() {
+    Preconditions.checkState(cachedChildren());
+    return children;
+  }
+
+  /**
+   * @return true if the node does not exists or it exists and children are cached.
+   */
+  boolean cachedChildren() {
+    return children != null || notExists();
+  }
+
+  /**
+   * @return true if the node does not exists or it exists and data and stat cached.
+   */
+  boolean cachedData() {
+    return data != null || notExists();
+  }
+
+  /**
+   * @return true if the node does not exists in zookeeper
+   */
+  boolean notExists() {
+    return stat == null && data == null && children == null;
+  }
+}