davide-armand opened a new pull request, #23196:
URL: https://github.com/apache/kafka/pull/23196
KIP-1312 can unregister a controller only after `metadata.version` reaches
4.4. Old, stopped controller registrations ("ghost controllers") can advertise
only older metadata versions, so feature validation rejects the
`metadata.version` upgrade needed to remove them (deadlock situation).
Bridge that deadlock with `decommission-controller`. Before 4.4-IV2, it
marks a stopped controller that is no longer a quorum voter with
`__decommissioned_controller`. Patched controllers ignore marked registrations
during feature validation, allowing `metadata.version` to reach 4.4.
Afterwards, running the same operation writes an `UnregisterControllerRecord`
and deletes the registration (KIP-1312 behavior).
This retains the useful marker and validation filter from the pre-KIP-1312
implementation (PR #70), but changes its integration with Kafka. PR #70
introduced a fork-local `DecommissionController` protocol and always retained
the marked registration, while also widening its advertised feature ranges.
This port reuses the upstream KIP-1312 UnregisterController API, preserves the
stored feature ranges, and physically removes the registration once 4.4 is
finalized.
Co-authored-by: Pi Agent (gpt-5.6-terra)
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