lnwallet: disallow creating a funding reservation with zero total satoshis

lnwallet/reservation.go

@@ -140,17 +140,25 @@ func NewChannelReservation(capacity, fundingAmt btcutil.Amount, minFeeRate btcut
 	var ourBalance btcutil.Amount
 	var theirBalance btcutil.Amount
 
+	// If we're the responder to a single-funder reservation, then we have
+	// no initial balance in the channel.
 	if fundingAmt == 0 {
 		ourBalance = 0
 		theirBalance = capacity - commitFee
 	} else {
 		// TODO(roasbeef): need to rework fee structure in general and
 		// also when we "unlock" dual funder within the daemon
-		if capacity == fundingAmt+commitFee { // Single funder
+
+		// If we're initiating a single funder workflow, then we pay
+		// all the initial fees within the commitment transaction.
+		if capacity == fundingAmt+commitFee {
 			ourBalance = capacity - commitFee
+			// Otherwise, this is a dual funder workflow where both slides
+			// split the amount funded and the commitment fee.
 		} else {
 			ourBalance = fundingAmt - commitFee
 		}
+
 		theirBalance = capacity - fundingAmt - commitFee
 	}
 

lnwallet/wallet.go

@@ -495,6 +495,14 @@ func (l *LightningWallet) InitChannelReservation(capacity,
 // handleFundingReserveRequest processes a message intending to create, and
 // validate a funding reservation request.
 func (l *LightningWallet) handleFundingReserveRequest(req *initFundingReserveMsg) {
+	// It isn't possible to create a channel with zero funds committed.
+	if req.fundingAmount+req.capacity == 0 {
+		req.err <- fmt.Errorf("cannot have channel with zero " +
+			"satoshis funded")
+		req.resp <- nil
+		return
+	}
+
 	id := atomic.AddUint64(&l.nextFundingID, 1)
 	totalCapacity := req.capacity + commitFee
 	reservation := NewChannelReservation(totalCapacity, req.fundingAmount,
@@ -612,11 +620,11 @@ func (l *LightningWallet) handleFundingCancelRequest(req *fundingReserveCancelMs
 	req.err <- nil
 }
 
-// handleFundingCounterPartyFunds processes the second workflow step for the
-// lifetime of a channel reservation. Upon completion, the reservation will
-// carry a completed funding transaction (minus the counterparty's input
-// signatures), both versions of the commitment transaction, and our signature
-// for their version of the commitment transaction.
+// handleContributionMsg processes the second workflow step for the lifetime of
+// a channel reservation. Upon completion, the reservation will carry a
+// completed funding transaction (minus the counterparty's input signatures),
+// both versions of the commitment transaction, and our signature for their
+// version of the commitment transaction.
 func (l *LightningWallet) handleContributionMsg(req *addContributionMsg) {
 	l.limboMtx.Lock()
 	pendingReservation, ok := l.fundingLimbo[req.pendingFundingID]