df-feerates: mimic old behavior if there's no plugin handling

We delegate the decision about what to do about 'out of bound' feerates
to the plugin (if one exists), however in the case that the plugin
doesnt exist or doesnt want to figure it out, we default to using the
'best' as their intended feerate, and rejecting if it's out of bounds.

lightningd/dual_open_control.c

@@ -434,8 +434,29 @@ openchannel2_hook_cb(struct openchannel2_payload *payload STEALS)
 	/* If there's no plugin, the funding_feerate_per_kw will be zero.
 	 * In this case, we set the funding_feerate_per_kw to the default,
 	 * the 'best' */
-	if (payload->funding_feerate_per_kw == 0)
-		payload->funding_feerate_per_kw = payload->funding_feerate_best;
+	if (payload->funding_feerate_per_kw == 0) {
+		u32 best_feerate
+			= payload->funding_feerate_per_kw
+			= payload->funding_feerate_best;
+
+		/* We use the old checks here now, against the base feerate */
+		if (best_feerate < payload->feerate_our_min) {
+			msg = towire_dualopend_fail(NULL, tal_fmt(tmpctx,
+						    "feerate_per_kw %u below"
+						    " minimum %u",
+						    best_feerate,
+						    payload->feerate_our_min));
+			return subd_send_msg(dualopend, take(msg));
+		}
+		if (best_feerate > payload->feerate_our_max) {
+			msg = towire_dualopend_fail(NULL, tal_fmt(tmpctx,
+						    "feerate_per_kw %u above"
+						    " maximum %u",
+						    best_feerate,
+						    payload->feerate_our_max));
+			return subd_send_msg(dualopend, take(msg));
+		}
+	}
 
 	/* If there's no plugin, the psbt will be NULL. We should pass an empty
 	 * PSBT over, in this case */
@@ -443,6 +464,7 @@ openchannel2_hook_cb(struct openchannel2_payload *payload STEALS)
 					       payload->funding_feerate_per_kw,
 					       payload->psbt,
 					       payload->our_shutdown_scriptpubkey);
+
 	subd_send_msg(dualopend, take(msg));
 }