In one of the following patches, we introduce support for zero-token
nodes. From that point, getting all nodes and getting all token
owners isn't equivalent. In this patch, we ensure that we consider
only token owners when we want to consider only token owners (for
example, in the replication logic), and we consider all nodes when
we want to consider all nodes (for example, in the topology logic).
The main purpose of this patch is to make the PR introducing
zero-token nodes easier to review. The patch that introduces
zero-token nodes is already complicated. We don't want trivial
changes from this patch to make noise there.
This patch introduces changes needed for zero-token nodes only in the
Raft-based topology and in the recovery mode. Zero-token nodes are
unsupported in the gossip-based topology outside recovery.
Some functions added to `token_metadata` and `topology` are
inefficient because they compute a new data structure in every call.
They are never called in the hot path, so it's not a serious problem.
Nevertheless, we should improve it somehow. Note that it's not
obvious how to do it because we don't want to make `token_metadata`
store topology-related data. Similarly, we don't want to make
`topology` store token-related data. We can think of an improvement
in a follow-up.
We don't remove unused `topology::get_datacenter_rack_nodes` and
`topology::get_datacenter_nodes`. These function can be useful in the
future. Also, `topology::_dc_nodes` is used internally in `topology`.
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