paymentsdb: verify total amount for last hop in the blinded path

Author: ziggie1984

payments/db/errors.go

@@ -83,6 +83,11 @@ var (
 	// paths or vice versa.
 	ErrMixedBlindedAndNonBlindedPayments = errors.New("mixed blinded and " +
 		"non-blinded payments")
+	// ErrBlindedPaymentMissingTotalAmount is returned if we try to
+	// register a blinded payment attempt where the final hop doesn't set
+	// the total amount.
+	ErrBlindedPaymentMissingTotalAmount = errors.New("blinded payment " +
+		"final hop must set total amount")
 
 	// ErrMPPPaymentAddrMismatch is returned if we try to register an MPP
 	// shard where the payment address doesn't match existing shards.

payments/db/payment.go

@@ -744,6 +744,13 @@ func verifyAttempt(payment *MPPayment, attempt *HTLCAttemptInfo) error {
 	// in the split payment is correct.
 	isBlinded := len(attempt.Route.FinalHop().EncryptedData) != 0
 
+	// For blinded payments, the last hop must set the total amount.
+	if isBlinded {
+		if attempt.Route.FinalHop().TotalAmtMsat == 0 {
+			return ErrBlindedPaymentMissingTotalAmount
+		}
+	}
+
 	// Make sure any existing shards match the new one with regards
 	// to MPP options.
 	mpp := attempt.Route.FinalHop().MPP