Improve the reliability of the refresh process due to the loss of slots

[ljfuyuan]

When the cluster slots information has not yet been allocated, getNodeAddrs may return empty data, causing the masters and replicas information to be cleared and unable to be restored to the correct state.

In addition, we should regard the getNodeAddrs parameter preferReplicas, such as EachNode's replicas or getRandomConn's readonly, we cannot simply fill in the address of the replicas directly in getNodeAddrs.

[mna]

Note that this does not return all node addresses, it will return the replicas if it knows about any, otherwise the masters. I'm not sure I like switching the refresh to get cluster slots from replicas - it should be ok, but it might need a READONLY call to make the replica accept the command (I'm not sure since this is not serving a key, it's an administration command), we'd have to test that and it might differ based on redis version...

I'll come back to review it when I have a bit more time to get back in context, but I think we should address that in getNodeAddrs instead and reuse the StartupNodes only if both c.masters and c.replicas are empty, and here in refresh fall back to using the replicas only if getNodeAddrs(false) returns an empty list of addresses.

[mna]

here in refresh fall back to using the replicas only if getNodeAddrs(false) returns an empty list of addresses.

And making sure the replicas do serve the CLUSTER SLOTS command without a READONLY request first.

[ljfuyuan]

Indeed, I made the mistake of getNodeAddrs(true) will not return all nodes.

And making sure the replicas do serve the CLUSTER SLOTS command without a READONLY request first

I did tests on redis 5.0.8 and 6.2.13 clusters. The replica node can correctly return cluster slots information without READONLY

So, based on the above, can we try to do this? At the same time, the semantics of preferReplicas can be preserved .

        addrs, _ := c.getNodeAddrs(false)
	if len(addrs) == 0 {
		if addrs, _ = c.getNodeAddrs(true); len(addrs) == 0 {
			addrs = c.StartupNodes
		}
	}

[mna]

Yes, I think that would be good.

[mna]

Thanks for your contribution!