Function catch_up 

fn catch_up(
    conn: &Connection,
    zone_id: &str,
    name: &str,
    row: &Row,
    behind: &Behind,
) -> Result<Option<(LineCount, bool)>, StoreError>

The epoch behind caught up to row’s size, counting only the bytes from its last open line on, and whether it may keep its generation (catch_up_keeps_gen). None if some of those bytes aren’t stored yet (a build without transactions is mid-write): nothing is counted from bytes that would only read as zeros.