lncfg+lnd: gate usage of tweakless commitments based on local/global feature bits

In this commit, we add a new legacy protocol command line flag:
`committweak`. When set, this forces the node to NOT signal usage of the
new commitment format. This allows us to test that we're able to
properly establish channels with legacy nodes. Within the server, we'll
now gate our signalling of this new feature based on the legacy protocol
config. Finally, when accepting/initiating a new channel funding, we'll
now check both the local and remote global feature bits, only using the
new commitment format if both signal the global feature bit.

fundingmanager.go

@@ -1199,6 +1199,18 @@ func (f *fundingManager) handleFundingOpen(fmsg *fundingOpenMsg) {
 	// reservation attempt may be rejected. Note that since we're on the
 	// responding side of a single funder workflow, we don't commit any
 	// funds to the channel ourselves.
+	//
+	// Before we init the channel, we'll also check to see if we've
+	// negotiated the new tweakless commitment format. This is only the
+	// case if *both* us and the remote peer are signaling the proper
+	// feature bit.
+	localTweakless := fmsg.peer.LocalGlobalFeatures().HasFeature(
+		lnwire.StaticRemoteKeyOptional,
+	)
+	remoteTweakless := fmsg.peer.RemoteGlobalFeatures().HasFeature(
+		lnwire.StaticRemoteKeyOptional,
+	)
+	tweaklessCommitment := localTweakless && remoteTweakless
 	chainHash := chainhash.Hash(msg.ChainHash)
 	req := &lnwallet.InitFundingReserveMsg{
 		ChainHash:        &chainHash,
@@ -1211,6 +1223,7 @@ func (f *fundingManager) handleFundingOpen(fmsg *fundingOpenMsg) {
 		PushMSat:         msg.PushAmount,
 		Flags:            msg.ChannelFlags,
 		MinConfs:         1,
+		Tweakless:        tweaklessCommitment,
 	}
 
 	reservation, err := f.cfg.Wallet.InitChannelReservation(req)
@@ -1246,8 +1259,9 @@ func (f *fundingManager) handleFundingOpen(fmsg *fundingOpenMsg) {
 	}
 
 	fndgLog.Infof("Requiring %v confirmations for pendingChan(%x): "+
-		"amt=%v, push_amt=%v", numConfsReq, fmsg.msg.PendingChannelID,
-		amt, msg.PushAmount)
+		"amt=%v, push_amt=%v, tweakless=%v", numConfsReq,
+		fmsg.msg.PendingChannelID, amt, msg.PushAmount,
+		tweaklessCommitment)
 
 	// Generate our required constraints for the remote party.
 	remoteCsvDelay := f.cfg.RequiredRemoteDelay(amt)
@@ -2440,7 +2454,7 @@ func (f *fundingManager) handleFundingLocked(fmsg *fundingLockedMsg) {
 	// Launch a defer so we _ensure_ that the channel barrier is properly
 	// closed even if the target peer is no longer online at this point.
 	defer func() {
-		// Close the active channel barrier signalling the readHandler
+		// Close the active channel barrier signaling the readHandler
 		// that commitment related modifications to this channel can
 		// now proceed.
 		f.barrierMtx.Lock()
@@ -2748,6 +2762,18 @@ func (f *fundingManager) handleInitFundingMsg(msg *initFundingMsg) {
 	// Initialize a funding reservation with the local wallet. If the
 	// wallet doesn't have enough funds to commit to this channel, then the
 	// request will fail, and be aborted.
+	//
+	// Before we init the channel, we'll also check to see if we've
+	// negotiated the new tweakless commitment format. This is only the
+	// case if *both* us and the remote peer are signaling the proper
+	// feature bit.
+	localTweakless := msg.peer.LocalGlobalFeatures().HasFeature(
+		lnwire.StaticRemoteKeyOptional,
+	)
+	remoteTweakless := msg.peer.RemoteGlobalFeatures().HasFeature(
+		lnwire.StaticRemoteKeyOptional,
+	)
+	tweaklessCommitment := localTweakless && remoteTweakless
 	req := &lnwallet.InitFundingReserveMsg{
 		ChainHash:        &msg.chainHash,
 		NodeID:           peerKey,
@@ -2760,6 +2786,7 @@ func (f *fundingManager) handleInitFundingMsg(msg *initFundingMsg) {
 		PushMSat:         msg.pushAmt,
 		Flags:            channelFlags,
 		MinConfs:         msg.minConfs,
+		Tweakless:        tweaklessCommitment,
 	}
 
 	reservation, err := f.cfg.Wallet.InitChannelReservation(req)
@@ -2828,8 +2855,8 @@ func (f *fundingManager) handleInitFundingMsg(msg *initFundingMsg) {
 	maxValue := f.cfg.RequiredRemoteMaxValue(capacity)
 	maxHtlcs := f.cfg.RequiredRemoteMaxHTLCs(capacity)
 
-	fndgLog.Infof("Starting funding workflow with %v for pendingID(%x)",
-		msg.peer.Address(), chanID)
+	fndgLog.Infof("Starting funding workflow with %v for pendingID(%x), "+
+		"tweakless=%v", msg.peer.Address(), chanID, tweaklessCommitment)
 
 	fundingOpen := lnwire.OpenChannel{
 		ChainHash:            *f.cfg.Wallet.Cfg.NetParams.GenesisHash,