Function apply_one 

async fn apply_one(
    device_tokens: &Arc<Map>,
    token_owners: &Arc<Map>,
    __arg2: KeyVal<'_>,
    mode: Mode,
) -> Result<bool>

Applies the mode to the token one origin row names, reporting whether the row was acted on.

A false return is a row with nothing to do: one this pass cannot make sense of, a device holding no token here, a token this server does not hold, or a stored value the mode leaves as it is. Only a failed lookup returns an error, because a row that will never decode would otherwise refuse every later boot as well.