Function read_via_index 

pub(crate) fn read_via_index(
    fs: &FileStore,
    zone: &str,
    offset: u64,
    limit: u64,
) -> Option<IndexedRead>

Read lines [offset, offset + limit) of output through a fresh output.idx, with the freshness check, the index entries and the output bytes all from ONE database snapshot (Codex on #3634): validating in one read and fetching entries and bytes in later ones would let another srv instance replace output and rebuild the index in between — the answer would mix the new file’s lines with the old file’s total.

None when the index doesn’t describe output in that snapshot (missing, stale, another generation’s) or a read hits bytes not stored yet: the caller rebuilds the index and retries once, or takes its slow path.