Function compact_recheck 

pub(super) fn compact_recheck(
    copied_at: u64,
    current: Option<u64>,
) -> CompactionVerdict

Pure decision: the compacted copy was built under a read guard at content generation copied_at; installing it is only sound if nothing advanced the generation in between. Single-writer invariant: this volume thread is the index’s only writer, so Abort means that invariant broke somewhere and installing would silently lose the in-between mutations — the copy is discarded loudly instead.