Make ZooCache close() and clear() more idempotent (apache#5338)

* Return early in clear() and close() if this ZooCache has already been closed. This avoids throwing an exception

Author: DomGarguilo

core/src/main/java/org/apache/accumulo/core/zookeeper/ZooCache.java

@@ -349,7 +349,7 @@ private ZcInterruptedException(InterruptedException e) {
    * @return children list, or null if node has no children or does not exist
    */
   public List<String> getChildren(final String zPath) {
-    Preconditions.checkState(!closed);
+    Preconditions.checkState(!closed, "Operation not allowed: ZooCache is already closed.");
     ensureWatched(zPath);
     ZooRunnable<List<String>> zr = new ZooRunnable<>() {
 
@@ -409,7 +409,7 @@ public byte[] get(final String zPath) {
    * @return path data, or null if non-existent
    */
   public byte[] get(final String zPath, final ZcStat status) {
-    Preconditions.checkState(!closed);
+    Preconditions.checkState(!closed, "Operation not allowed: ZooCache is already closed.");
     ensureWatched(zPath);
     ZooRunnable<byte[]> zr = new ZooRunnable<>() {
 
@@ -474,7 +474,7 @@ public byte[] run() throws KeeperException, InterruptedException {
    * @param cachedStat cached statistic, that is or will be cached
    */
   protected void copyStats(ZcStat userStat, ZcStat cachedStat) {
-    Preconditions.checkState(!closed);
+    Preconditions.checkState(!closed, "Operation not allowed: ZooCache is already closed.");
     if (userStat != null && cachedStat != null) {
       userStat.set(cachedStat);
     }
@@ -484,13 +484,18 @@ protected void copyStats(ZcStat userStat, ZcStat cachedStat) {
    * Clears this cache.
    */
   protected void clear() {
-    Preconditions.checkState(!closed);
+    if (closed) {
+      return;
+    }
     nodeCache.clear();
     updateCount.incrementAndGet();
     log.trace("{} cleared all from cache", cacheId);
   }
 
   public void close() {
+    if (closed) {
+      return;
+    }
     clear();
     closed = true;
   }
@@ -500,7 +505,7 @@ public void close() {
    * count is the same, then it means cache did not change.
    */
   public long getUpdateCount() {
-    Preconditions.checkState(!closed);
+    Preconditions.checkState(!closed, "Operation not allowed: ZooCache is already closed.");
     return updateCount.get();
   }
 
@@ -534,7 +539,7 @@ public boolean childrenCached(String zPath) {
    * Removes all paths in the cache match the predicate.
    */
   public void clear(Predicate<String> pathPredicate) {
-    Preconditions.checkState(!closed);
+    Preconditions.checkState(!closed, "Operation not allowed: ZooCache is already closed.");
     Predicate<String> pathPredicateWrapper = path -> {
       boolean testResult = pathPredicate.test(path);
       if (testResult) {