Unit Test getAllChildrenPaginated for transaction children deleted case

[huizhilu]

Description

There is a retry logic for API getAllChildrenPaginated() when there is deletion in the transaction children(children created by a multi transaction operation) and the new page's start minCzxidOffset shifts. Current implementation uses the last child as a flag to represent if there is offset shifting.

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

Lines 2899 to 2918 in 4ca8b70

	} else {
	// If a child with the same czxid is returned in the previous page, and then deleted
	// on the server, the children not in the previous page but offset is less than czxidOffset
	// would be missed in the next page. Use the last child in the previous page to determine whether or not
	// the deletion case happens. If it happens, retry fetching the page starting from offset 0.
	String lastChild = childrenPage.isEmpty() ? "" : childrenPage.get(childrenPage.size() - 1);
	childrenPage = getChildren(path, watcher, Integer.MAX_VALUE, minCzxid,
	nextPage.getMinCzxidOffset() - 1, null, nextPage);
	if (!lastChild.equals(childrenPage.get(0))) {
	if (retryCount++ >= 5) {
	LOG.warn("Failed to fetch children pages because of znode deletion and retries exceeding 5");
	throw KeeperException.create(Code.OPERATIONTIMEOUT);
	}
	LOG.info("Due to znode deletion, retry fetching children from offset 0 for minCzxid={}, retries={}",
	minCzxid, retryCount);
	childrenPage = getChildren(path, watcher, Integer.MAX_VALUE, minCzxid, 0, null, nextPage);
	}
	}
	allChildrenSet.addAll(childrenPage);
	} while (nextPage.getMinCzxid() != ZooDefs.GetChildrenPaginated.lastPageMinCzxid);

We want to add a unit test for the code logic. As we could not simulate the read/write race condition on the server, we will need to mock the states in the zookeeper api for the uni test.

Related to #8