multi: add explicit hodl invoice flag to invoice

Previously it wasn't possible to store a preimage in the invoice
database and signal that a payment should not be settled right away. The
only way to hold a payment was to insert the magic UnknownPreimage value
in the invoice database. This commit introduces a distinct flag to
signal that an invoice is a hold invoice and thereby allows the preimage
to be present in the database already.

Preparation for (key send) hodl invoices for which we already know the
preimage.

invoices/invoiceregistry.go

@@ -652,12 +652,6 @@ func (i *InvoiceRegistry) processKeySend(ctx invoiceUpdateCtx) error {
 		return errors.New("invalid keysend preimage")
 	}
 
-	// Don't accept zero preimages as those have a special meaning in our
-	// database for hodl invoices.
-	if preimage == channeldb.UnknownPreimage {
-		return errors.New("invalid keysend preimage")
-	}
-
 	// Only allow keysend for non-mpp payments.
 	if ctx.mpp != nil {
 		return errors.New("no mpp keysend supported")
@@ -688,7 +682,7 @@ func (i *InvoiceRegistry) processKeySend(ctx invoiceUpdateCtx) error {
 		Terms: channeldb.ContractTerm{
 			FinalCltvDelta:  finalCltvDelta,
 			Value:           amt,
-			PaymentPreimage: preimage,
+			PaymentPreimage: &preimage,
 			Features:        features,
 		},
 	}
@@ -948,7 +942,7 @@ func (i *InvoiceRegistry) SettleHodlInvoice(preimage lntypes.Preimage) error {
 		return &channeldb.InvoiceUpdateDesc{
 			State: &channeldb.InvoiceStateUpdateDesc{
 				NewState: channeldb.ContractSettled,
-				Preimage: preimage,
+				Preimage: &preimage,
 			},
 		}, nil
 	}

invoices/test_utils_test.go

@@ -92,7 +92,7 @@ var (
 	testInvoiceAmt = lnwire.MilliSatoshi(100000)
 	testInvoice    = &channeldb.Invoice{
 		Terms: channeldb.ContractTerm{
-			PaymentPreimage: testInvoicePreimage,
+			PaymentPreimage: &testInvoicePreimage,
 			Value:           testInvoiceAmt,
 			Expiry:          time.Hour,
 			Features:        testFeatures,
@@ -102,12 +102,12 @@ var (
 
 	testHodlInvoice = &channeldb.Invoice{
 		Terms: channeldb.ContractTerm{
-			PaymentPreimage: channeldb.UnknownPreimage,
-			Value:           testInvoiceAmt,
-			Expiry:          time.Hour,
-			Features:        testFeatures,
+			Value:    testInvoiceAmt,
+			Expiry:   time.Hour,
+			Features: testFeatures,
 		},
 		CreationDate: testInvoiceCreationDate,
+		HodlInvoice:  true,
 	}
 )
 
@@ -225,7 +225,7 @@ func newTestInvoice(t *testing.T, preimage lntypes.Preimage,
 
 	return &channeldb.Invoice{
 		Terms: channeldb.ContractTerm{
-			PaymentPreimage: preimage,
+			PaymentPreimage: &preimage,
 			PaymentAddr:     payAddr,
 			Value:           testInvoiceAmount,
 			Expiry:          expiry,

invoices/update.go

@@ -81,7 +81,7 @@ func updateInvoice(ctx *invoiceUpdateCtx, inv *channeldb.Invoice) (
 
 		case channeldb.HtlcStateSettled:
 			return nil, ctx.settleRes(
-				inv.Terms.PaymentPreimage,
+				*inv.Terms.PaymentPreimage,
 				ResultReplayToSettled,
 			), nil
 
@@ -187,8 +187,7 @@ func updateMpp(ctx *invoiceUpdateCtx,
 
 	// Check to see if we can settle or this is an hold invoice and
 	// we need to wait for the preimage.
-	holdInvoice := inv.Terms.PaymentPreimage == channeldb.UnknownPreimage
-	if holdInvoice {
+	if inv.HodlInvoice {
 		update.State = &channeldb.InvoiceStateUpdateDesc{
 			NewState: channeldb.ContractAccepted,
 		}
@@ -201,7 +200,7 @@ func updateMpp(ctx *invoiceUpdateCtx,
 	}
 
 	return &update, ctx.settleRes(
-		inv.Terms.PaymentPreimage, ResultSettled,
+		*inv.Terms.PaymentPreimage, ResultSettled,
 	), nil
 }
 
@@ -269,14 +268,13 @@ func updateLegacy(ctx *invoiceUpdateCtx,
 
 	case channeldb.ContractSettled:
 		return &update, ctx.settleRes(
-			inv.Terms.PaymentPreimage, ResultDuplicateToSettled,
+			*inv.Terms.PaymentPreimage, ResultDuplicateToSettled,
 		), nil
 	}
 
 	// Check to see if we can settle or this is an hold invoice and we need
 	// to wait for the preimage.
-	holdInvoice := inv.Terms.PaymentPreimage == channeldb.UnknownPreimage
-	if holdInvoice {
+	if inv.HodlInvoice {
 		update.State = &channeldb.InvoiceStateUpdateDesc{
 			NewState: channeldb.ContractAccepted,
 		}
@@ -290,6 +288,6 @@ func updateLegacy(ctx *invoiceUpdateCtx,
 	}
 
 	return &update, ctx.settleRes(
-		inv.Terms.PaymentPreimage, ResultSettled,
+		*inv.Terms.PaymentPreimage, ResultSettled,
 	), nil
 }