Function backfill 

async fn backfill(
    session: &mut Imap,
    state: &mut SyncState,
    state_path: &Path,
    key: &str,
    known: FolderState,
    root: &Path,
    validity: u32,
    report: &mut Report,
) -> Result<(), SyncError>

The older messages of a folder, those not here yet, a round at a time, newest first, each round sized first so the disk keeps its reserve; the folder remembers once it holds everything.