invoicesrpc: add ability to generate AMP invoices

lnrpc/invoicesrpc/addinvoice.go

@@ -103,6 +103,11 @@ type AddInvoiceData struct {
 	// immediately upon receiving the payment.
 	HodlInvoice bool
 
+	// Amp signals whether or not to create an AMP invoice.
+	//
+	// NOTE: Preimage should always be set to nil when this value is true.
+	Amp bool
+
 	// RouteHints are optional route hints that can each be individually used
 	// to assist in reaching the invoice's destination.
 	RouteHints [][]zpay32.HopHint
@@ -111,6 +116,12 @@ type AddInvoiceData struct {
 // paymentHashAndPreimage returns the payment hash and preimage for this invoice
 // depending on the configuration.
 //
+// For AMP invoices (when Amp flag is true), this method always returns a nil
+// preimage. The hash value can be set externally by the user using the Hash
+// field, or one will be generated randomly. The payment hash here only serves
+// as a unique identifier for insertion into the invoice index, as there is
+// no universal preimage for an AMP payment.
+//
 // For MPP invoices (when Amp flag is false), this method may return nil
 // preimage when create a hodl invoice, but otherwise will always return a
 // non-nil preimage and the corresponding payment hash. The valid combinations
@@ -121,6 +132,42 @@ type AddInvoiceData struct {
 func (d *AddInvoiceData) paymentHashAndPreimage() (
 	*lntypes.Preimage, lntypes.Hash, error) {
 
+	if d.Amp {
+		return d.ampPaymentHashAndPreimage()
+	}
+
+	return d.mppPaymentHashAndPreimage()
+}
+
+// ampPaymentHashAndPreimage returns the payment hash to use for an AMP invoice.
+// The preimage will always be nil.
+func (d *AddInvoiceData) ampPaymentHashAndPreimage() (*lntypes.Preimage, lntypes.Hash, error) {
+	switch {
+
+	// Preimages cannot be set on AMP invoice.
+	case d.Preimage != nil:
+		return nil, lntypes.Hash{},
+			errors.New("preimage set on AMP invoice")
+
+	// If a specific hash was requested, use that.
+	case d.Hash != nil:
+		return nil, *d.Hash, nil
+
+	// Otherwise generate a random hash value, just needs to be unique to be
+	// added to the invoice index.
+	default:
+		var paymentHash lntypes.Hash
+		if _, err := rand.Read(paymentHash[:]); err != nil {
+			return nil, lntypes.Hash{}, err
+		}
+
+		return nil, paymentHash, nil
+	}
+}
+
+// mppPaymentHashAndPreimage returns the payment hash and preimage to use for an
+// MPP invoice.
+func (d *AddInvoiceData) mppPaymentHashAndPreimage() (*lntypes.Preimage, lntypes.Hash, error) {
 	var (
 		paymentPreimage *lntypes.Preimage
 		paymentHash     lntypes.Hash
@@ -332,7 +379,12 @@ func AddInvoice(ctx context.Context, cfg *AddInvoiceConfig,
 	}
 
 	// Set our desired invoice features and add them to our list of options.
-	invoiceFeatures := cfg.GenInvoiceFeatures()
+	var invoiceFeatures *lnwire.FeatureVector
+	if invoice.Amp {
+		invoiceFeatures = cfg.GenAmpInvoiceFeatures()
+	} else {
+		invoiceFeatures = cfg.GenInvoiceFeatures()
+	}
 	options = append(options, zpay32.Features(invoiceFeatures))
 
 	// Generate and set a random payment address for this invoice. If the