Function ensure_cert_can_exist 

pub(crate) fn ensure_cert_can_exist(
    epoch: EpochNumber,
    first_epoch: EpochNumber,
    epoch_height: u64,
    decided_height: Option<u64>,
    current_epoch: Option<EpochNumber>,
) -> Result<(), StateCertFetchError>

Fails until this node decides the root block that a state certificate for epoch signs.

decided_height is None while this node’s decided leaf predates epochs. current_epoch is the epoch this node’s consensus is in, None if unknown: once epochs have started, a node whose decided leaf trails its consensus still fetches any epoch that consensus has already left.