lightningd: insist on payment_secret if bit compulsory on invoices.

This grandfathers in old invoices for the moment.

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

lightningd/htlc_set.c

@@ -1,3 +1,4 @@
+#include <common/features.h>
 #include <common/timeout.h>
 #include <lightningd/htlc_end.h>
 #include <lightningd/htlc_set.h>
@@ -114,6 +115,17 @@ void htlc_set_add(struct lightningd *ld,
 		return;
 	}
 
+	/* 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) {
+		log_debug(ld->log, "Missing payment_secret, but required for %s",
+			  type_to_string(tmpctx, struct sha256,
+					 &hin->payment_hash));
+		local_fail_in_htlc(hin,
+				   take(failmsg_incorrect_or_unknown(NULL, ld, hin)));
+		return;
+	}
+
 	/* BOLT #4:
 	 *  - otherwise, if it supports `basic_mpp`:
 	 *    - MUST add it to the HTLC set corresponding to that `payment_hash`.