`services.hyperhive.domain` and `swarm.hives.<hiveName>.domain` were two
homes for one value with nothing asserting they agreed. The failure that
buys is the worst shape a config defect has: it evaluates cleanly, and
the symptom ("the other hives can't reach me") appears on a machine
other than the misconfigured one.
The directory is now the single source. `hives.<name>.domain` gains the
`<name>.<swarm.domain>` default -- a derivation from two values an
operator had to state explicitly, not a guess -- so a conventional swarm
is a list of names and a hive addressed by something else says so in the
one place every other hive reads. `services.hyperhive.domain` reads its
own entry; the direct formula is deleted rather than kept as a fallback,
which would have restored the second path (and, reading `swarm.domain`
itself, a second path that can disagree).
Setting it directly still wins, with a deprecation warning: nothing
breaks today, but a value written only there is invisible to the swarm.
The self-entry assertion now fires on an EMPTY directory too. Since
`swarm.domain` became required, every hive is in a swarm -- a swarm of
one is still a swarm -- and this host's address is read out of the
directory, so the entry is missing either way and the precise message
should be the one that fires.
Upgrading costs one line on hives that never listed themselves:
`services.hyperhive.swarm.hives.<hiveName> = { };`, no value.
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