Function pull 

fn pull(
    client: &Client,
    dir: &Path,
    listed: &Listed,
    state: &mut State,
    state_path: &Path,
    refetch: &BTreeSet<String>,
    report: &mut Report,
    until: Option<Instant>,
) -> Result<bool, SyncError>

Brings what changed there: by sync token, else by comparing ETags. The state is kept after each batch, so a pull cut short loses one batch at most; the sync token moves once everything came. Brought until until at most (a turn among the account’s collections): whether all came, else the next turn goes on from what was kept.