lnwallet: adopt simple fee structure for commitment transactions

This commit modifies the prior funding workflow to account for fees
when creating the funding output. As a stop gap, the current fee for
the commitment transaction is now hard-coded at 5k satoshis. Once the
fee models are in place this should instead be some high multiple of
the current “average” fee rate within the network, continuing, the
proper fee should be adjusted from the commitment transaction has
outputs are added/removed.

lnwallet/wallet.go

@@ -36,6 +36,8 @@ const (
 	// TODO(roasbeef): should instead be child to make room for future
 	// rotations, etc.
 	identityKeyIndex = hdkeychain.HardenedKeyStart + 2
+
+	commitFee = 5000
 )
 
 var (
@@ -485,7 +487,8 @@ func (l *LightningWallet) InitChannelReservation(capacity,
 // validate a funding reservation request.
 func (l *LightningWallet) handleFundingReserveRequest(req *initFundingReserveMsg) {
 	id := atomic.AddUint64(&l.nextFundingID, 1)
-	reservation := NewChannelReservation(req.capacity, req.fundingAmount,
+	totalCapacity := req.capacity + commitFee
+	reservation := NewChannelReservation(totalCapacity, req.fundingAmount,
 		req.minFeeRate, l, id, req.numConfs)
 
 	// Grab the mutex on the ChannelReservation to ensure thead-safety
@@ -501,10 +504,12 @@ func (l *LightningWallet) handleFundingReserveRequest(req *initFundingReserveMsg
 	// don't need to perform any coin selection. Otherwise, attempt to
 	// obtain enough coins to meet the required funding amount.
 	if req.fundingAmount != 0 {
-		// TODO(roasbeef): consult model for proper fee rate
+		// TODO(roasbeef): consult model for proper fee rate on funding
+		// tx
 		feeRate := uint64(10)
-		if err := l.selectCoinsAndChange(feeRate, req.fundingAmount,
-			ourContribution); err != nil {
+		amt := req.fundingAmount + commitFee
+		err := l.selectCoinsAndChange(feeRate, amt, ourContribution)
+		if err != nil {
 			req.err <- err
 			req.resp <- nil
 			return
@@ -1190,35 +1195,32 @@ out:
 
 // selectCoinsAndChange performs coin selection in order to obtain witness
 // outputs which sum to at least 'numCoins' amount of satoshis. If coin
-// selection is succesful/possible, then the selected coins are available within
-// the passed contribution's inputs. If necessary, a change address will also be
-// generated.
+// selection is succesful/possible, then the selected coins are available
+// within the passed contribution's inputs. If necessary, a change address will
+// also be generated.
 // TODO(roasbeef): remove hardcoded fees and req'd confs for outputs.
 func (l *LightningWallet) selectCoinsAndChange(feeRate uint64, amt btcutil.Amount,
 	contribution *ChannelContribution) error {
 
 	// We hold the coin select mutex while querying for outputs, and
-	// performing coin selection in order to avoid inadvertent double spends
-	// accross funding transactions.
-	// NOTE: We don't use defer her so we can properly release the lock
-	// when we encounter an error condition.
+	// performing coin selection in order to avoid inadvertent double
+	// spends accross funding transactions.
 	l.coinSelectMtx.Lock()
+	defer l.coinSelectMtx.Unlock()
 
 	// Find all unlocked unspent witness outputs with greater than 1
 	// confirmation.
 	// TODO(roasbeef): make num confs a configuration paramter
 	coins, err := l.ListUnspentWitness(1)
 	if err != nil {
-		l.coinSelectMtx.Unlock()
 		return err
 	}
 
 	// Peform coin selection over our available, unlocked unspent outputs
-	// in order to find enough coins to meet the funding amount requirements.
-	// TODO(roasbeef): take in sat/byte
+	// in order to find enough coins to meet the funding amount
+	// requirements.
 	selectedCoins, changeAmt, err := coinSelect(feeRate, amt, coins)
 	if err != nil {
-		l.coinSelectMtx.Unlock()
 		return err
 	}
 
@@ -1254,8 +1256,6 @@ func (l *LightningWallet) selectCoinsAndChange(feeRate uint64, amt btcutil.Amoun
 		}
 	}
 
-	l.coinSelectMtx.Unlock()
-
 	return nil
 }