lnwallet/wallet: define SubtractFees for InitFundingReserveMsg

This commit adds a SubtractFees option to the funding request, letting
the caller specify that the fees should be deducted from the funding
amount. This paves the way for letting the funding manager spend up to a
given amount when creating a channel, like the rest of the funds in the
wallet.

lnwallet/wallet.go

@@ -69,6 +69,13 @@ type InitFundingReserveMsg struct {
 	// workflow.
 	NodeAddr net.Addr
 
+	// SubtractFees should be set if we intend to spend exactly
+	// LocalFundingAmt when opening the channel, subtracting the fees from
+	// the funding output. This can be used for instance to use all our
+	// remaining funds to open the channel, since it will take fees into
+	// account.
+	SubtractFees bool
+
 	// LocalFundingAmt is the amount of funds requested from us for this
 	// channel.
 	LocalFundingAmt btcutil.Amount
@@ -450,7 +457,6 @@ func (l *LightningWallet) handleFundingReserveRequest(req *InitFundingReserveMsg
 		return
 	}
 
-	capacity := req.LocalFundingAmt + req.RemoteFundingAmt
 	localFundingAmt := req.LocalFundingAmt
 
 	var (
@@ -468,14 +474,21 @@ func (l *LightningWallet) handleFundingReserveRequest(req *InitFundingReserveMsg
 		var err error
 		selected, err = l.selectCoinsAndChange(
 			req.FundingFeePerKw, req.LocalFundingAmt, req.MinConfs,
+			req.SubtractFees,
 		)
 		if err != nil {
 			req.err <- err
 			req.resp <- nil
 			return
 		}
+
+		localFundingAmt = selected.fundingAmt
 	}
 
+	// The total channel capacity will be the size of the funding output we
+	// created plus the remote contribution.
+	capacity := localFundingAmt + req.RemoteFundingAmt
+
 	id := atomic.AddUint64(&l.nextFundingID, 1)
 	reservation, err := NewChannelReservation(
 		capacity, localFundingAmt, req.CommitFeePerKw, l, id,
@@ -1289,6 +1302,7 @@ func (l *LightningWallet) WithCoinSelectLock(f func() error) error {
 type coinSelection struct {
 	coins       []*wire.TxIn
 	change      []*wire.TxOut
+	fundingAmt  btcutil.Amount
 	unlockCoins func()
 }
 
@@ -1296,10 +1310,12 @@ type coinSelection struct {
 // outputs which sum to at least 'amt' amount of satoshis. If necessary,
 // a change address will also be generated. If coin selection is
 // successful/possible, then the selected coins and change outputs are
-// returned. This method locks the selected outputs, and a function closure to
+// returned, and the value of the resulting funding output. This method locks
-// unlock them in case of an error is returned.
+// the selected outputs, and a function closure to unlock them in case of an
+// error is returned.
 func (l *LightningWallet) selectCoinsAndChange(feeRate SatPerKWeight,
-	amt btcutil.Amount, minConfs int32) (*coinSelection, error) {
+	amt btcutil.Amount, minConfs int32, subtractFees bool) (
+	*coinSelection, error) {
 
 	// We hold the coin select mutex while querying for outputs, and
 	// performing coin selection in order to avoid inadvertent double
@@ -1317,14 +1333,38 @@ func (l *LightningWallet) selectCoinsAndChange(feeRate SatPerKWeight,
 		return nil, err
 	}
 
+	var (
+		selectedCoins []*Utxo
+		fundingAmt    btcutil.Amount
+		changeAmt     btcutil.Amount
+	)
+
 	// Perform coin selection over our available, unlocked unspent outputs
 	// in order to find enough coins to meet the funding amount
 	// requirements.
-	selectedCoins, changeAmt, err := coinSelect(feeRate, amt, coins)
+	switch {
+	// In case this request want the fees subtracted from the local amount,
+	// we'll call the specialized method for that. This ensures that we
+	// won't deduct more that the specified balance from our wallet.
+	case subtractFees:
+		dustLimit := l.Cfg.DefaultConstraints.DustLimit
+		selectedCoins, fundingAmt, changeAmt, err = coinSelectSubtractFees(
+			feeRate, amt, dustLimit, coins,
+		)
 		if err != nil {
 			return nil, err
 		}
 
+	// Ótherwise do a normal coin selection where we target a given funding
+	// amount.
+	default:
+		fundingAmt = amt
+		selectedCoins, changeAmt, err = coinSelect(feeRate, amt, coins)
+		if err != nil {
+			return nil, err
+		}
+	}
+
 	// Record any change output(s) generated as a result of the coin
 	// selection, but only if the addition of the output won't lead to the
 	// creation of dust.
@@ -1374,6 +1414,7 @@ func (l *LightningWallet) selectCoinsAndChange(feeRate SatPerKWeight,
 	return &coinSelection{
 		coins:       inputs,
 		change:      changeOutputs,
+		fundingAmt:  fundingAmt,
 		unlockCoins: unlock,
 	}, nil
 }