chanfunding: allow to set a reserved amount not used for funding

lnwallet/chanfunding/assembler.go

@@ -78,6 +78,12 @@ type Request struct {
 	// responder as part of the initial channel creation.
 	PushAmt btcutil.Amount
 
+	// WalletReserve is a reserved amount that is not used to fund the
+	// channel when a maximum amount defined by FundUpToMaxAmt is set. This
+	// is useful when a reserved wallet balance must stay available due to
+	// e.g. anchor channels.
+	WalletReserve btcutil.Amount
+
 	// MinConfs controls how many confirmations a coin need to be eligible
 	// to be used as an input to the funding transaction. If this value is
 	// set to zero, then zero conf outputs may be spent.

lnwallet/chanfunding/coin_select.go

@@ -255,11 +255,12 @@ func CoinSelectSubtractFees(feeRate chainfee.SatPerKWeight, amt,
 }
 
 // CoinSelectUpToAmount attempts to select coins such that we'll select up to
-// maxAmount exclusive of fees if sufficient funds are available. If
-// insufficient funds are available this method selects all available coins.
+// maxAmount exclusive of fees and optional reserve if sufficient funds are
+// available. If insufficient funds are available this method selects all
+// available coins.
 func CoinSelectUpToAmount(feeRate chainfee.SatPerKWeight, minAmount, maxAmount,
-	dustLimit btcutil.Amount, coins []Coin) ([]Coin, btcutil.Amount,
-	btcutil.Amount, error) {
+	reserved, dustLimit btcutil.Amount, coins []Coin) ([]Coin,
+	btcutil.Amount, btcutil.Amount, error) {
 
 	var (
 		// selectSubtractFee is tracking if our coin selection was
@@ -269,6 +270,13 @@ func CoinSelectUpToAmount(feeRate chainfee.SatPerKWeight, minAmount, maxAmount,
 		outputAmount      = maxAmount
 	)
 
+	// Get total balance from coins which we need for reserve considerations
+	// and fee santiy checks.
+	var totalBalance btcutil.Amount
+	for _, coin := range coins {
+		totalBalance += btcutil.Amount(coin.Value)
+	}
+
 	// First we try to select coins to create an output of the specified
 	// maxAmount with or without a change output that covers the miner fee.
 	selected, changeAmt, err := CoinSelect(
@@ -276,25 +284,42 @@ func CoinSelectUpToAmount(feeRate chainfee.SatPerKWeight, minAmount, maxAmount,
 	)
 
 	var errInsufficientFunds *ErrInsufficientFunds
-	if errors.As(err, &errInsufficientFunds) {
+	if err == nil { //nolint:gocritic,ifElseChain
+		// If the coin selection succeeds we check if our total balance
+		// covers the selected set of coins including fees plus an
+		// optional anchor reserve.
+
+		// First we sum up the value of all selected coins.
+		var sumSelected btcutil.Amount
+		for _, coin := range selected {
+			sumSelected += btcutil.Amount(coin.Value)
+		}
+
+		// We then subtract the change amount from the value of all
+		// selected coins to obtain the actual amount that is selected.
+		sumSelected -= changeAmt
+
+		// Next we check if our total balance can cover for the selected
+		// output plus the optional anchor reserve.
+		if totalBalance-sumSelected < reserved {
+			// If our local balance is insufficient to cover for the
+			// reserve we try to select an output amount that uses
+			// our total balance minus reserve and fees.
+			selectSubtractFee = true
+		}
+	} else if errors.As(err, &errInsufficientFunds) {
 		// If the initial coin selection fails due to insufficient funds
 		// we select our total available balance minus fees.
 		selectSubtractFee = true
-	} else if err != nil {
+	} else {
 		return nil, 0, 0, err
 	}
 
-	// If we determined that our local balance is insufficient we check our
-	// total balance minus fees.
+	// If we determined that our local balance is insufficient we check
+	// our total balance minus fees and optional reserve.
 	if selectSubtractFee {
-		// Get balance from coins.
-		var totalBalance btcutil.Amount
-		for _, coin := range coins {
-			totalBalance += btcutil.Amount(coin.Value)
-		}
-
 		selected, outputAmount, changeAmt, err = CoinSelectSubtractFees(
-			feeRate, totalBalance, dustLimit, coins,
+			feeRate, totalBalance-reserved, dustLimit, coins,
 		)
 		if err != nil {
 			return nil, 0, 0, err

lnwallet/chanfunding/coin_select_test.go

@@ -550,6 +550,7 @@ func TestCoinSelectUpToAmount(t *testing.T) {
 		name     string
 		minValue btcutil.Amount
 		maxValue btcutil.Amount
+		reserved btcutil.Amount
 		coins    []Coin
 
 		expectedInput      []btcutil.Amount
@@ -688,6 +689,26 @@ func TestCoinSelectUpToAmount(t *testing.T) {
 		expectedInput:      []btcutil.Amount{1 * coin},
 		expectedFundingAmt: 1*coin - fundingFee(feeRate, 1, false) - 1,
 		expectedChange:     0,
+	}, {
+		// This test makes sure that if a reserved value is required
+		// then it is handled correctly by leaving exactly the reserved
+		// value as change and still maxing out the funding amount.
+		name: "sanity check for correct reserved amount subtract " +
+			"from total",
+		coins: []Coin{{
+			TxOut: wire.TxOut{
+				PkScript: p2wkhScript,
+				Value:    1 * coin,
+			},
+		}},
+		minValue: minValue,
+		maxValue: 1*coin - 9000,
+		reserved: 10000,
+
+		expectedInput: []btcutil.Amount{1 * coin},
+		expectedFundingAmt: 1*coin -
+			fundingFee(feeRate, 1, true) - 10000,
+		expectedChange: 10000,
 	}}
 
 	for _, test := range testCases {
@@ -698,7 +719,7 @@ func TestCoinSelectUpToAmount(t *testing.T) {
 			selected, localFundingAmt, changeAmt,
 				err := CoinSelectUpToAmount(
 				feeRate, test.minValue, test.maxValue,
-				dustLimit, test.coins,
+				test.reserved, dustLimit, test.coins,
 			)
 			if len(test.expectErr) == 0 && err != nil {
 				t.Fatalf(err.Error())

lnwallet/chanfunding/wallet_assembler.go

@@ -308,7 +308,7 @@ func (w *WalletAssembler) ProvisionChannel(r *Request) (Intent, error) {
 			selectedCoins, localContributionAmt, changeAmt,
 				err = CoinSelectUpToAmount(
 				r.FeeRate, r.MinFundAmt, r.FundUpToMaxAmt,
-				w.cfg.DustLimit, coins,
+				r.WalletReserve, w.cfg.DustLimit, coins,
 			)
 			if err != nil {
 				return err

lnwallet/wallet.go

@@ -836,6 +836,7 @@ func (l *LightningWallet) handleFundingReserveRequest(req *InitFundingReserveMsg
 
 	localFundingAmt := req.LocalFundingAmt
 	remoteFundingAmt := req.RemoteFundingAmt
+	hasAnchors := req.CommitType.HasAnchors()
 
 	var (
 		fundingIntent chanfunding.Intent
@@ -855,9 +856,33 @@ func (l *LightningWallet) handleFundingReserveRequest(req *InitFundingReserveMsg
 	// funder in the attached request to provision the inputs/outputs
 	// that'll ultimately be used to construct the funding transaction.
 	if !ok {
+		var err error
+		var numAnchorChans int
+
+		// Get the number of anchor channels to determine if there is a
+		// reserved value that must be respected when funding up to the
+		// maximum amount. Since private channels (most likely) won't be
+		// used for routing other than the last hop, they bear a smaller
+		// risk that we must force close them in order to resolve a HTLC
+		// up/downstream. Hence we exclude them from the count of anchor
+		// channels in order to attribute the respective anchor amount
+		// to the channel capacity.
+		if req.FundUpToMaxAmt > 0 && req.MinFundAmt > 0 {
+			numAnchorChans, err = l.CurrentNumAnchorChans()
+			if err != nil {
+				req.err <- err
+				req.resp <- nil
+				return
+			}
+
+			isPublic := req.Flags&lnwire.FFAnnounceChannel != 0
+			if hasAnchors && isPublic {
+				numAnchorChans++
+			}
+		}
+
 		// Coin selection is done on the basis of sat/kw, so we'll use
 		// the fee rate passed in to perform coin selection.
-		var err error
 		fundingReq := &chanfunding.Request{
 			RemoteAmt:         req.RemoteFundingAmt,
 			LocalAmt:          req.LocalFundingAmt,
@@ -867,6 +892,9 @@ func (l *LightningWallet) handleFundingReserveRequest(req *InitFundingReserveMsg
 			PushAmt: lnwire.MilliSatoshi.ToSatoshis(
 				req.PushMSat,
 			),
+			WalletReserve: l.RequiredReserve(
+				uint32(numAnchorChans),
+			),
 			MinConfs:     req.MinConfs,
 			SubtractFees: req.SubtractFees,
 			FeeRate:      req.FundingFeePerKw,
@@ -939,7 +967,6 @@ func (l *LightningWallet) handleFundingReserveRequest(req *InitFundingReserveMsg
 	// funding tx ready, so this will always pass.  We'll do another check
 	// when the PSBT has been verified.
 	isPublic := req.Flags&lnwire.FFAnnounceChannel != 0
-	hasAnchors := req.CommitType.HasAnchors()
 	if enforceNewReservedValue {
 		err = l.enforceNewReservedValue(fundingIntent, isPublic, hasAnchors)
 		if err != nil {
@@ -1160,10 +1187,7 @@ func (l *LightningWallet) CheckReservedValue(in []wire.OutPoint,
 	}
 
 	// We reserve a given amount for each anchor channel.
-	reserved := btcutil.Amount(numAnchorChans) * AnchorChanReservedValue
-	if reserved > MaxAnchorChanReservedValue {
-		reserved = MaxAnchorChanReservedValue
-	}
+	reserved := l.RequiredReserve(uint32(numAnchorChans))
 
 	if walletBalance < reserved {
 		walletLog.Debugf("Reserved value=%v above final "+