KIP-111 — Kafka should preserve the Principal generated by the PrincipalBuilder while processing the request received on socket channel, on the broker.

Discarded SecurityBroker

KIP-111 changes the broker to preserve the full `Principal` object produced by a custom `PrincipalBuilder` throughout request processing (stored in `Session`), rather than extracting only the principal name into a plain `KafkaPrincipal`. Without this, custom `Authorizer` implementations that need fields beyond `name` (e.g. group membership, certificate attributes) cannot access them because the richer object is discarded at the socket-read boundary.

Details