Function sign_decided_leaves 

async fn sign_decided_leaves<P>(
    consensus_handle: Arc<ConsensusHandle<SeqTypes, Arc<P>>>,
    state_signer: StateSigner<SequencerApiVersion>,
    leaves: Receiver<DecidedLeaf>,
)
where
    P: SequencerPersistence,

Signs and relays the newest leaf of every decide, in order, off the event loop, because a relay post can take as long as its HTTP timeout. The relay needs a threshold of signatures on one height, so every node must sign the same heights. Signing every decide keeps that while posts fall behind, and dropping only the oldest once the queue is full keeps it as well.