Function local_retention_prefix 

pub fn local_retention_prefix(
    segments: &[LocalRetentionSegment],
    size_debt: Option<u64>,
) -> usize

Count the oldest local segments that Kafka’s local retention passes delete.

segments is every local segment, oldest first, and the newest one is the segment the log appends to: Kafka’s deletableSegments walks the active segment too. A result that covers every segment tells the host to roll first and then delete them all, the way Kafka’s deleteSegments does (if (numberOfSegments == numToDelete) roll()); the walk never takes a newest segment that is empty, so the roll always leaves a fresh empty segment behind and never has to recreate the one it deletes.

size_debt is None when Kafka skips the size pass: retention.bytes is unset, the policy does not delete, or the log is smaller than the budget. Otherwise it is the log size minus the budget. The walk never passes a blocked segment.

The result equals local_retention_model from the oldest segment, limited by local_retention_limit. That fold states the Kafka rule. Before that limit, the first kept segment is blocked or not expired. Every selected non-expired segment fits the initial size debt together with every older segment. Before the limit, an unblocked kept segment exceeds that debt. These independent cost bounds reject a selector/reference pair that omits the size pass or deletes beyond the budget.