Two questions regarding Besu's P2P network and snap sync

[thinkAfCod]

First, does Besu currently support discV5?
When I add the --Xv5-discovery-enabled startup parameter, it prints logs:

Discovery Protocol v5 is not available

It makes me feel a bit confused.

And Second, why does selectNewPivotBlock use safeBlockHash instead of directly using HeadBlockHash like go-ethereum?

block := api.eth.BlockChain().GetBlockByHash(update.HeadBlockHash)
if block == nil {
	// If this block was previously invalidated, keep rejecting it here too
	if res := api.checkInvalidAncestor(update.HeadBlockHash, update.HeadBlockHash); res != nil {
		return engine.ForkChoiceResponse{PayloadStatus: *res, PayloadID: nil}, nil
	}
	// If the head hash is unknown (was not given to us in a newPayload request),
	// we cannot resolve the header, so not much to do. This could be extended in
	// the future to resolve from the `eth` network, but it's an unexpected case
	// that should be fixed, not papered over.
	header := api.remoteBlocks.get(update.HeadBlockHash)
	if header == nil {
		log.Warn("Forkchoice requested unknown head", "hash", update.HeadBlockHash)
		return engine.STATUS_SYNCING, nil
	}
	// If the finalized hash is known, we can direct the downloader to move
	// potentially more data to the freezer from the get go.
	finalized := api.remoteBlocks.get(update.FinalizedBlockHash)

	// Header advertised via a past newPayload request. Start syncing to it.
	context := []interface{}{"number", header.Number, "hash", header.Hash()}
	if update.FinalizedBlockHash != (common.Hash{}) {
		if finalized == nil {
			context = append(context, []interface{}{"finalized", "unknown"}...)
		} else {
			context = append(context, []interface{}{"finalized", finalized.Number}...)
		}
	}
	log.Info("Forkchoice requested sync to new head", context...)
	if err := api.eth.Downloader().BeaconSync(api.eth.SyncMode(), header, finalized); err != nil {
		return engine.STATUS_SYNCING, err
	}
	return engine.STATUS_SYNCING, nil
}

The snap sync downloader only needs the head block hash information to start syncing data.
And,In a besu node, when a node is syncing data using snap sync, and there is no data before the sync, when will it update its own safe block?
Or are there other details that I have overlooked?

[jframe]

@thinkAfCod discV5 is not currently available in Besu. There has been some discussion on adding discv5 in the Besu contributors channel in discord.

[jframe]

@thinkAfCod Good question on using a head hash instead of the safe block hash. This is something we discussing internally, will let you know our thoughts.