What happens in the unlikely case that a Validator has no transactions to nominate but it's time to start a nominating round?

Question

In the case that a Validator's transaction pool is empty, is the following code ever called inside of HerderImpl::triggerNextLedger?:

mHerderSCPDriver.nominate(slotIndex, newProposedValue, proposedSet,
                          lcl.header.scpValue);

I'm wondering about an edge-case. If nominate() is not called when the transaction pool is empty, then the leader selection timer in NominationProtocol::nominate will never be started. And additionally the Validator will not echo any other votes due to the following check in NominationProtocol::processEnvelope:

if (mNominationStarted)
{
    // ... echo any received votes from the SCPEnvelope
}

mNominationStarted will be false, so the Validator will simply not be part of the protocol.

So my question is, if the transaction pool is empty does the Node indeed start the nominating round regardless (because the timer triggering HerderImpl::triggerNextLedger has fired). Or is this edge-case not really considered because in practice there are always non-zero valid transactions in the pool?

Answer 1

It is perfectly fine to nominate an empty transaction set, which is what validators do in case there are no transactions submitted. From the Stellar Consensus Protocol point of view, empty set is still a valid value for the network to agree on.