LedgerManagerIteratorTest: avoid ledger collision

For the non-LHLM managers, the ledger space seems to be small enough to
allow collisions to occasionally cause a test failure.  Remember
created ledgers to avoid collisions.

(bug W-5104859)
Signed-off-by: Samuel Just <sjustsalesforce.com>

Author: Samuel Just <sjust@salesforce.com>

Reviewers: Charan Reddy Guttapalem <reddycharan18@gmail.com>, Sijie Guo <sijie@apache.org>

This closes #1526 from athanatos/forupstream/wip-ledger-manager-iterator-test

bookkeeper-server/src/test/java/org/apache/bookkeeper/meta/LedgerManagerIteratorTest.java

@@ -52,6 +52,7 @@
 import org.apache.bookkeeper.util.MathUtils;
 import org.apache.bookkeeper.util.ZkUtils;
 import org.apache.bookkeeper.versioning.Version;
+import org.apache.mina.util.ConcurrentHashSet;
 import org.apache.zookeeper.CreateMode;
 import org.apache.zookeeper.ZooDefs;
 import org.junit.After;
@@ -414,6 +415,7 @@ public void checkConcurrentModifications() throws Throwable {
         final long start = MathUtils.nowInNano();
         final CountDownLatch latch = new CountDownLatch(1);
         ArrayList<Thread> threads = new ArrayList<>();
+        final ConcurrentHashSet<Long> createdLedgers = new ConcurrentHashSet<>();
         for (int i = 0; i < numWriters; ++i) {
             Thread thread = new Thread(safeWrapper(() -> {
                 LedgerManager writerLM = getIndependentLedgerManager();
@@ -431,7 +433,7 @@ public void checkConcurrentModifications() throws Throwable {
                         if (!longRange) {
                             candidate %= 1000000;
                         }
-                    } while (mustExist.contains(candidate));
+                    } while (mustExist.contains(candidate) || !createdLedgers.add(candidate));
                     try {
                         createLedger(writerLM, candidate, Optional.empty());
                         removeLedger(writerLM, candidate, Optional.empty());