1) make address map API flexible
Before this patch:
- having a mapping without an actual IP address was an
internal error
- not having a mapping for an IP address was an internal
error
- re-mapping to a new IP address wasn't allowed
After this patch:
- the address map may contain a mapping
without an actual IP address, and the caller must be prepared for it:
find() will return a nullopt. This happens when we first add an entry
to Raft configuration and only later learn its IP address, e.g. via
gossip.
- it is allowed to re-map an existing entry to a new address;
2) subscribe to gossip notifications
Learning IP addresses from gossip allows us to adjust
the address map whenever a node IP address changes.
Gossiper is also the only valid source of re-mapping, other sources
(RPC) should not re-map, since otherwise a packet from a removed
server can remap the id to a wrong address and impact liveness of a Raft
cluster.
3) prompt address map state with app state
Initialize the raft address map with initial
gossip application state, specifically IPs of members
of the cluster. With this, we no longer need to store
these IPs in Raft configuration (and update them when they change).
The obvious drawback of this approach is that a node
may join Raft config before it propagates its IP address
to the cluster via gossip - so the boot process has to
wait until it happens.
Gossip also doesn't tell us which IPs are members of Raft configuration,
so we subscribe to Group0 configuration changes to mark the
members of Raft config "non-expiring" in the address translation
map.
Thanks to the changes above, Raft configuration no longer
stores IP addresses.
We still keep the 'server_info' column in the raft_config system table,
in case we change our mind or decide to store something else in there.
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