mint: support paying amountless invoices

Author: elnosh

cashu/cashu.go

@@ -502,6 +502,7 @@ const (
 	AmountLimitExceeded            CashuErrCode = 11006
 	DuplicateInputErrCode          CashuErrCode = 11007
 	DuplicateOutputErrCode         CashuErrCode = 11008
+	AmountlessMismatchErrCode      CashuErrCode = 11011
 
 	UnknownKeysetErrCode  CashuErrCode = 12001
 	InactiveKeysetErrCode CashuErrCode = 12002
@@ -514,8 +515,7 @@ const (
 	LightningPaymentErrCode     CashuErrCode = 20004
 	MeltQuotePendingErrCode     CashuErrCode = 20005
 	MeltQuoteAlreadyPaidErrCode CashuErrCode = 20006
-
-	MeltQuoteErrCode CashuErrCode = 20009
+	MeltQuoteErrCode            CashuErrCode = 20010
 )
 
 var (
@@ -531,6 +531,8 @@ var (
 	MintQuoteAlreadyIssued       = Error{Detail: "quote already issued", Code: MintQuoteAlreadyIssuedErrCode}
 	MintingDisabled              = Error{Detail: "minting is disabled", Code: MintingDisabledErrCode}
 	MintAmountExceededErr        = Error{Detail: "max amount for minting exceeded", Code: AmountLimitExceeded}
+	MeltAmountlessMismatchErr    = Error{Detail: "invoice amount and amountless option do not match", Code: AmountlessMismatchErrCode}
+	InvoiceAmountMissingErr      = Error{Detail: "amountless option not specified for amountless invoice request", Code: AmountlessMismatchErrCode}
 	MintQuoteInvalidSigErr       = Error{Detail: "Mint quote with pubkey but no valid signature provided.", Code: MintQuoteInvalidSigErrCode}
 	OutputsOverQuoteAmountErr    = Error{Detail: "sum of the output amounts is greater than quote amount", Code: StandardErrCode}
 	ProofAlreadyUsedErr          = Error{Detail: "proof already used", Code: ProofAlreadyUsedErrCode}

cashu/nuts/nut05/nut05.go

@@ -44,13 +44,22 @@ func StringToState(state string) State {
 }
 
 type PostMeltQuoteBolt11Request struct {
-	Request string               `json:"request"`
-	Unit    string               `json:"unit"`
-	Options map[string]MppOption `json:"options,omitempty"`
+	Request string      `json:"request"`
+	Unit    string      `json:"unit"`
+	Options MeltOptions `json:"options,omitempty"`
 }
 
-type MppOption struct {
-	AmountMsat uint64 `json:"amount"`
+type MeltOptions struct {
+	MppOption        *Amount     `json:"mpp,omitempty"`
+	AmountlessOption *AmountMsat `json:"amountless,omitempty"`
+}
+
+type Amount struct {
+	Amount uint64 `json:"amount"`
+}
+
+type AmountMsat struct {
+	AmountMsat uint64 `json:"amount_msat"`
 }
 
 type PostMeltQuoteBolt11Response struct {

cashu/nuts/nut06/nut06.go

@@ -69,10 +69,11 @@ type NutSetting struct {
 }
 
 type MethodSetting struct {
-	Method    string `json:"method"`
-	Unit      string `json:"unit"`
-	MinAmount uint64 `json:"min_amount,omitempty"`
-	MaxAmount uint64 `json:"max_amount,omitempty"`
+	Method     string `json:"method"`
+	Unit       string `json:"unit"`
+	MinAmount  uint64 `json:"min_amount,omitempty"`
+	MaxAmount  uint64 `json:"max_amount,omitempty"`
+	Amountless bool   `json:"amountless,omitempty"`
 }
 
 type Supported struct {

mint/lightning/fakebackend.go

@@ -81,7 +81,12 @@ func (fb *FakeBackend) InvoiceStatus(hash string) (Invoice, error) {
 	return fb.Invoices[invoiceIdx].ToInvoice(), nil
 }
 
-func (fb *FakeBackend) SendPayment(ctx context.Context, request string, maxFee uint64) (PaymentStatus, error) {
+func (fb *FakeBackend) SendPayment(
+	ctx context.Context,
+	request string,
+	amount uint64,
+	maxFee uint64,
+) (PaymentStatus, error) {
 	invoice, err := decodepay.Decodepay(request)
 	if err != nil {
 		return PaymentStatus{}, fmt.Errorf("error decoding invoice: %v", err)

mint/lightning/lightning.go

@@ -7,7 +7,8 @@ type Client interface {
 	ConnectionStatus() error
 	CreateInvoice(amount uint64) (Invoice, error)
 	InvoiceStatus(hash string) (Invoice, error)
-	SendPayment(ctx context.Context, request string, maxFee uint64) (PaymentStatus, error)
+	// amount used only for supporting amountless invoices in melt requests
+	SendPayment(ctx context.Context, request string, amount uint64, maxFee uint64) (PaymentStatus, error)
 	PayPartialAmount(ctx context.Context, request string, amountMsat uint64, maxFee uint64) (PaymentStatus, error)
 	OutgoingPaymentStatus(ctx context.Context, hash string) (PaymentStatus, error)
 	FeeReserve(amount uint64) uint64

mint/lightning/lnd.go

@@ -12,6 +12,7 @@ import (
 	"github.com/lightningnetwork/lnd/lnrpc/invoicesrpc"
 	"github.com/lightningnetwork/lnd/lnrpc/routerrpc"
 	"github.com/lightningnetwork/lnd/macaroons"
+	decodepay "github.com/nbd-wtf/ln-decodepay"
 	"google.golang.org/grpc"
 	"google.golang.org/grpc/credentials"
 )
@@ -112,12 +113,27 @@ func (lnd *LndClient) InvoiceStatus(hash string) (Invoice, error) {
 	return invoice, nil
 }
 
-func (lnd *LndClient) SendPayment(ctx context.Context, request string, maxFee uint64) (PaymentStatus, error) {
+func (lnd *LndClient) SendPayment(
+	ctx context.Context,
+	request string,
+	amount uint64,
+	maxFee uint64,
+) (PaymentStatus, error) {
 	feeLimit := &lnrpc.FeeLimit{Limit: &lnrpc.FeeLimit_Fixed{Fixed: int64(maxFee)}}
 	sendPaymentRequest := lnrpc.SendRequest{
 		PaymentRequest: request,
 		FeeLimit:       feeLimit,
 	}
+
+	bolt11, err := decodepay.Decodepay(request)
+	if err != nil {
+		return PaymentStatus{}, err
+	}
+	// if this is an amountless invoice, pay the amount specified
+	if bolt11.MSatoshi == 0 {
+		sendPaymentRequest.Amt = int64(amount)
+	}
+
 	sendPaymentResponse, err := lnd.grpcClient.SendPaymentSync(ctx, &sendPaymentRequest)
 	if err != nil {
 		// if context deadline is exceeded (1 min), mark payment as pending

mint/mint.go

@@ -571,6 +571,7 @@ func (m *Mint) RequestMeltQuote(meltQuoteRequest nut05.PostMeltQuoteBolt11Reques
 		return storage.MeltQuote{}, cashu.BuildCashuError(errmsg, cashu.UnitErrCode)
 	}
 
+	var quoteAmount uint64
 	// check invoice passed is valid
 	request := meltQuoteRequest.Request
 	bolt11, err := decodepay.Decodepay(request)
@@ -579,10 +580,23 @@ func (m *Mint) RequestMeltQuote(meltQuoteRequest nut05.PostMeltQuoteBolt11Reques
 		return storage.MeltQuote{}, cashu.BuildCashuError(errmsg, cashu.MeltQuoteErrCode)
 	}
 	if bolt11.MSatoshi == 0 {
-		return storage.MeltQuote{}, cashu.BuildCashuError("invoice has no amount", cashu.MeltQuoteErrCode)
+		// if amountless invoice, check amount specified in options
+		if meltQuoteRequest.Options.AmountlessOption == nil {
+			return storage.MeltQuote{}, cashu.InvoiceAmountMissingErr
+		} else {
+			amountless := meltQuoteRequest.Options.AmountlessOption
+			quoteAmount = amountless.AmountMsat / 1000
+		}
+	} else {
+		quoteAmount = uint64(bolt11.MSatoshi) / 1000
+
+		// if amountless option passed, check that amounts matched
+		if meltQuoteRequest.Options.AmountlessOption != nil {
+			if meltQuoteRequest.Options.AmountlessOption.AmountMsat != uint64(bolt11.MSatoshi) {
+				return storage.MeltQuote{}, cashu.MeltAmountlessMismatchErr
+			}
+		}
 	}
-	invoiceSatAmount := uint64(bolt11.MSatoshi) / 1000
-	quoteAmount := invoiceSatAmount
 
 	// check if a mint quote exists with the same invoice.
 	_, err = m.db.GetMintQuoteByPaymentHash(bolt11.PaymentHash)
@@ -594,31 +608,31 @@ func (m *Mint) RequestMeltQuote(meltQuoteRequest nut05.PostMeltQuoteBolt11Reques
 	isMpp := false
 	var amountMsat uint64 = 0
 	// check mpp option
-	if len(meltQuoteRequest.Options) > 0 {
-		mpp, ok := meltQuoteRequest.Options["mpp"]
-		if ok {
-			if m.mppEnabled {
-				// if this is an internal invoice, reject MPP request
-				if isInternal {
-					return storage.MeltQuote{},
-						cashu.BuildCashuError("mpp for internal invoice is not allowed", cashu.MeltQuoteErrCode)
-				}
+	if meltQuoteRequest.Options.MppOption != nil {
+		if m.mppEnabled {
+			// if this is an internal invoice, reject MPP request
+			if isInternal {
+				return storage.MeltQuote{},
+					cashu.BuildCashuError("mpp for internal invoice is not allowed", cashu.MeltQuoteErrCode)
+			}
 
-				// check mpp msat amount is less than invoice amount
-				if mpp.AmountMsat >= uint64(bolt11.MSatoshi) {
-					return storage.MeltQuote{},
-						cashu.BuildCashuError("mpp amount is not less than amount in invoice",
-							cashu.MeltQuoteErrCode)
-				}
-				isMpp = true
-				amountMsat = mpp.AmountMsat
-				quoteAmount = amountMsat / 1000
-				m.logInfof("got melt quote request to pay partial amount '%v' of invoice with amount '%v'",
-					quoteAmount, invoiceSatAmount)
-			} else {
+			// check mpp msat amount is less than invoice amount
+			mppAmount := meltQuoteRequest.Options.MppOption.Amount
+			if mppAmount >= uint64(bolt11.MSatoshi) {
 				return storage.MeltQuote{},
-					cashu.BuildCashuError("MPP is not supported", cashu.MeltQuoteErrCode)
+					cashu.BuildCashuError("mpp amount is not less than amount in invoice", cashu.MeltQuoteErrCode)
+			}
+			if mppAmount > 0 && bolt11.MSatoshi == 0 {
+				return storage.MeltQuote{}, cashu.BuildCashuError("invalid invoice for MPP option", cashu.MeltQuoteErrCode)
 			}
+			isMpp = true
+			amountMsat = mppAmount
+			quoteAmount = amountMsat / 1000
+			m.logInfof("got melt quote request to pay partial amount '%v' of invoice with amount '%v'",
+				quoteAmount, bolt11.MSatoshi/1000)
+		} else {
+			return storage.MeltQuote{},
+				cashu.BuildCashuError("MPP is not supported", cashu.MeltQuoteErrCode)
 		}
 	}
 
@@ -661,8 +675,8 @@ func (m *Mint) RequestMeltQuote(meltQuoteRequest nut05.PostMeltQuoteBolt11Reques
 		AmountMsat:     amountMsat,
 	}
 
-	m.logInfof("got melt quote request for invoice of amount '%v'. Setting fee reserve to %v",
-		invoiceSatAmount, meltQuote.FeeReserve)
+	m.logInfof("got melt quote request for amount '%v'. Setting fee reserve to %v",
+		quoteAmount, meltQuote.FeeReserve)
 
 	if err := m.db.SaveMeltQuote(meltQuote); err != nil {
 		errmsg := fmt.Sprintf("error saving melt quote to db: %v", err)
@@ -861,7 +875,12 @@ func (m *Mint) MeltTokens(ctx context.Context, meltTokensRequest nut05.PostMeltB
 			)
 		} else {
 			m.logInfof("attempting to pay invoice: %v", meltQuote.InvoiceRequest)
-			sendPaymentResponse, err = m.lightningClient.SendPayment(ctx, meltQuote.InvoiceRequest, meltQuote.Amount)
+			sendPaymentResponse, err = m.lightningClient.SendPayment(
+				ctx,
+				meltQuote.InvoiceRequest,
+				meltQuote.Amount,
+				meltQuote.FeeReserve,
+			)
 		}
 		if err != nil {
 			// if SendPayment failed do not return yet, an extra check will be done
@@ -1625,10 +1644,11 @@ func (m *Mint) SetMintInfo(mintInfo MintInfo) {
 		Nut05: nut06.NutSetting{
 			Methods: []nut06.MethodSetting{
 				{
-					Method:    cashu.BOLT11_METHOD,
-					Unit:      cashu.Sat.String(),
-					MinAmount: m.limits.MeltingSettings.MinAmount,
-					MaxAmount: m.limits.MeltingSettings.MaxAmount,
+					Method:     cashu.BOLT11_METHOD,
+					Unit:       cashu.Sat.String(),
+					MinAmount:  m.limits.MeltingSettings.MinAmount,
+					MaxAmount:  m.limits.MeltingSettings.MaxAmount,
+					Amountless: true,
 				},
 			},
 			Disabled: false,