common/onion_decode: put the path_id into onion_payload->payment_secret.

And check it in invoice.c, insead of a hack where we compare against invhash.
Restore checking, too.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/onion_decode.c

@@ -285,9 +285,14 @@ struct onion_payload *onion_decode(const tal_t *ctx,
 				goto field_bad;
 		}
 
-		/* Blinded paths have no payment secret or metadata:
+		/* We stash path_id (if present and valid!) in payment_secret */
-		 * we use the path_id for that. */
+		if (tal_bytelen(enc->path_id) == sizeof(*p->payment_secret)) {
+			p->payment_secret = tal_steal(p,
+						      (struct secret *)enc->path_id);
+		} else
 			p->payment_secret = NULL;
+
+		/* FIXME: if we supported metadata, it would also be in path_id */
 		p->payment_metadata = NULL;
 		return p;
 	}

lightningd/htlc_set.c

@@ -116,12 +116,9 @@ void htlc_set_add(struct lightningd *ld,
 		return;
 	}
 
-	/* FIXME! */
-	bool ignore_secret = details->invstring && strstarts(details->invstring, "lni1");
-
 	/* If we insist on a payment secret, it must always have it */
 	if (feature_is_set(details->features, COMPULSORY_FEATURE(OPT_PAYMENT_SECRET))
-	    && !payment_secret && !ignore_secret) {
+	    && !payment_secret) {
 		log_debug(ld->log, "Missing payment_secret, but required for %s",
 			  type_to_string(tmpctx, struct sha256,
 					 &hin->payment_hash));

lightningd/invoice.c

@@ -142,27 +142,18 @@ static void invoice_secret(const struct preimage *payment_preimage,
 	memcpy(payment_secret->data, secret.u.u8, sizeof(secret.u.u8));
 }
 
-/* FIXME: This is a hack.  The real secret should be a signature of some
+/* FIXME: The spec should require a *real* secret: a signature of the
- * onion key, using the payer_id */
+ * payment_hash using the payer_id key.  This just means they've
+ * *seen* the invoice! */
 static void invoice_secret_bolt12(struct lightningd *ld,
-				  const char *invstring,
+				  const struct sha256 *payment_hash,
 				  struct secret *payment_secret)
 {
-	char *fail;
+	const void *path_id = invoice_path_id(tmpctx,
-	struct tlv_invoice *inv;
+					      &ld->invoicesecret_base,
-	struct sha256 merkle;
+					      payment_hash);
-
+	assert(tal_bytelen(path_id) == sizeof(*payment_secret));
-	inv = invoice_decode(tmpctx, invstring, strlen(invstring),
+	memcpy(payment_secret, path_id, sizeof(*payment_secret));
-			     NULL, NULL, &fail);
-	if (!inv) {
-		log_broken(ld->log, "Unable to decode our invoice %s",
-			   invstring);
-		return;
-	}
-
-	merkle_tlv(inv->fields, &merkle);
-	BUILD_ASSERT(sizeof(*payment_secret) == sizeof(merkle));
-	memcpy(payment_secret, &merkle, sizeof(merkle));
 }
 
 struct invoice_payment_hook_payload {
@@ -398,8 +389,6 @@ invoice_check_payment(const tal_t *ctx,
 	}
 
 	details = wallet_invoice_details(ctx, ld->wallet, invoice);
-	/* FIXME! */
-	bool ignore_secret = details->invstring && strstarts(details->invstring, "lni1");
 
 	/* BOLT #4:
 	 *  - if the `payment_secret` doesn't match the expected value for that
@@ -408,17 +397,17 @@ invoice_check_payment(const tal_t *ctx,
 	 *    - MUST fail the HTLC.
 	 */
 	if (feature_is_set(details->features, COMPULSORY_FEATURE(OPT_VAR_ONION))
-	    && !payment_secret && !ignore_secret) {
+	    && !payment_secret) {
 		log_debug(ld->log, "Attept to pay %s without secret",
 			  type_to_string(tmpctx, struct sha256, &details->rhash));
 		return tal_free(details);
 	}
 
-	if (payment_secret && !ignore_secret) {
+	if (payment_secret) {
 		struct secret expected;
 
 		if (details->invstring && strstarts(details->invstring, "lni1"))
-			invoice_secret_bolt12(ld, details->invstring, &expected);
+			invoice_secret_bolt12(ld, payment_hash, &expected);
 		else
 			invoice_secret(&details->r, &expected);
 		if (!secret_eq_consttime(payment_secret, &expected)) {