Function replica_fetch_mutation 

pub fn replica_fetch_mutation(facts: ReplicaFetchFacts) -> ReplicaFetchMutation

Fence one follower Fetch response row against its in-flight request epoch, its current metadata target and any reported leader identity; then select one exclusive response action.

A follower’s request covers every partition it follows on one leader, so the caller applies this once per row of the answer, with that row’s own partition configuration and the epoch it was asked under. The guarantee is therefore per partition and does not weaken as a response carries more of them: a fenced row mutates nothing, whatever the rows beside it say. An unfenced row is retried on an error, truncates on a KIP-320 divergence (which Kafka sends with no records), and appends otherwise.