[broker] Fix bug that tenants whose allowed clusters include global cannot be created/updated

[massakam]

Motivation

If the allowed clusters include global, tenant creation and update will fail with the following error:

$ pulsar-admin tenants create -r xxx.xxx.xxx -c global,dev1,dev2 sample-tenant

Clusters do not exist

Reason: Clusters do not exist

However, there is actually the global cluster in ZK.

[zk: localhost:2184(CONNECTED) 14] ls /admin/clusters

[dev1, dev2, global]

As a result of the investigation, I found that global was removed from the cluster list in the configuration store cache after getting the list of clusters by the REST API. This is due to the following:

pulsar/pulsar-broker/src/main/java/org/apache/pulsar/broker/admin/impl/ClustersBase.java

Lines 87 to 91 in e1357a7

	Set<String> clusters = clustersListCache().get();
	// Remove "global" cluster from returned list
	clusters.remove(Constants.GLOBAL_CLUSTER);
	return clusters;

global is removed from the clusters variable, which seems to be not a deep copy but a shallow copy.

Modifications

When getting the list of clusters from the configuration store and deleting global from it, create a new Set instance using stream API.

Set<String> clusters = clustersListCache().get().stream()
        .filter(cluster -> !Constants.GLOBAL_CLUSTER.equals(cluster)).collect(Collectors.toSet());

In addition, allow global to be included in allowed clusters when creating and updating tenants, even if the global cluster does not exist in the list of clusters stored in ZK. Even if the global cluster is not registered in ZK, global topics will work fine.