lightningd: add override amt to invoice_check_payment

Adds `expected_msat_override` to the `invoice_check_payment` check. If
it's set, it will be used to override the invoice amount as the expected
amount of the payment check.

This enables us to charge a different amount for a payment than the
amount stated on the invoice.

Signed-off-by: Peter Neuroth <pet.v.ne@gmail.com>

Author: nepet

lightningd/htlc_set.c

@@ -150,8 +150,8 @@ void htlc_set_add_(struct lightningd *ld,
 	 *     [Failure Messages](#failure-messages)
 	 *     - Note: "amount paid" specified there is the `total_msat` field.
 	 */
-	details = invoice_check_payment(tmpctx, ld, payment_hash,
-					total_msat, payment_secret, &err);
+	details = invoice_check_payment(tmpctx, ld, payment_hash, total_msat,
+					NULL, payment_secret, &err);
 	if (!details) {
 		log_debug(log, "payment failed: %s", err);
 		fail(arg, take(failmsg_incorrect_or_unknown(NULL, ld, msat)));

lightningd/invoice.c

@@ -332,6 +332,7 @@ invoice_check_payment(const tal_t *ctx,
 		      struct lightningd *ld,
 		      const struct sha256 *payment_hash,
 		      const struct amount_msat msat,
+		      const struct amount_msat *expected_msat_override,
 		      const struct secret *payment_secret,
 		      const char **err)
 {
@@ -408,15 +409,19 @@ invoice_check_payment(const tal_t *ctx,
 	if (details->msat != NULL) {
 		struct amount_msat twice;
 
-		if (amount_msat_less(msat, *details->msat)) {
+		/* Override the expected amount. */
+		struct amount_msat expected_msat =
+			expected_msat_override ? *expected_msat_override : *details->msat;
+
+		if (amount_msat_less(msat, expected_msat)) {
 			*err = tal_fmt(ctx, "Attempt to pay %s with amount %s < %s",
 				       fmt_sha256(tmpctx, &details->rhash),
 				       fmt_amount_msat(tmpctx, msat),
-				       fmt_amount_msat(tmpctx, *details->msat));
+				       fmt_amount_msat(tmpctx, expected_msat));
 			return tal_free(details);
 		}
 
-		if (amount_msat_add(&twice, *details->msat, *details->msat)
+		if (amount_msat_add(&twice, expected_msat, expected_msat)
 		    && amount_msat_greater(msat, twice)) {
 			*err = tal_fmt(ctx, "Attempt to pay %s with amount %s > %s",
 				       fmt_sha256(tmpctx, &details->rhash),

lightningd/invoice.h

@@ -50,6 +50,7 @@ struct invoice_details {
  * @ld: lightningd
  * @payment_hash: hash of preimage they want.
  * @msat: amount they offer to pay.
+ * @expected_msat_override: if set: overrides the amount we expect to be payed.
  * @payment_secret: they payment secret they sent, if any.
  * @err: error string if it returns NULL.
  *
@@ -59,6 +60,7 @@ const struct invoice_details *invoice_check_payment(const tal_t *ctx,
 						    struct lightningd *ld,
 						    const struct sha256 *payment_hash,
 						    const struct amount_msat msat,
+						    const struct amount_msat *expected_msat_override,
 						    const struct secret *payment_secret,
 						    const char **err);
 

lightningd/pay.c

@@ -1482,7 +1482,7 @@ static struct command_result *self_payment(struct lightningd *ld,
 				     local_invreq_id);
 
 	/* Now, resolve the invoice */
-	inv = invoice_check_payment(tmpctx, ld, rhash, msat, payment_secret, &err);
+	inv = invoice_check_payment(tmpctx, ld, rhash, msat, NULL, payment_secret, &err);
 	if (!inv) {
 		struct routing_failure *fail;
 		wallet_payment_set_status(ld->wallet, rhash, partid, groupid,

wallet/test/run-wallet.c

@@ -391,6 +391,7 @@ const struct invoice_details *invoice_check_payment(const tal_t *ctx UNNEEDED,
 						    struct lightningd *ld UNNEEDED,
 						    const struct sha256 *payment_hash UNNEEDED,
 						    const struct amount_msat msat UNNEEDED,
+						    const struct amount_msat *expected_msat_override UNNEEDED,
 						    const struct secret *payment_secret UNNEEDED,
 						    const char **err UNNEEDED)
 { fprintf(stderr, "invoice_check_payment called!\n"); abort(); }