plugin: Add function to collect featurebits that plugins registered

lightningd/plugin.c

@@ -3,6 +3,7 @@
 #include <ccan/opt/opt.h>
 #include <ccan/tal/str/str.h>
 #include <ccan/utf8/utf8.h>
+#include <common/features.h>
 #include <common/utils.h>
 #include <common/version.h>
 #include <lightningd/json.h>
@@ -46,6 +47,18 @@ struct plugins *plugins_new(const tal_t *ctx, struct log_book *log_book,
 	return p;
 }
 
+u8 *plugins_collect_featurebits(const tal_t *ctx, const struct plugins *plugins,
+				enum plugin_features_type type)
+{
+	struct plugin *p;
+	u8 *res = tal_arr(ctx, u8, 0);
+	list_for_each(&plugins->plugins, p, list) {
+		if (p->featurebits[type])
+			res = featurebits_or(ctx, take(res), p->featurebits[type]);
+	}
+	return res;
+}
+
 static void destroy_plugin(struct plugin *p)
 {
 	plugin_hook_unregister_all(p);

lightningd/plugin.h

@@ -266,4 +266,8 @@ struct log *plugin_get_log(struct plugin *plugin);
 struct plugin_destroyed *plugin_detect_destruction(const struct plugin *plugin);
 bool was_plugin_destroyed(struct plugin_destroyed *destroyed);
 
+/* Gather all the features of the given type that plugins registered. */
+u8 *plugins_collect_featurebits(const tal_t *ctx, const struct plugins *plugins,
+				enum plugin_features_type type);
+
 #endif /* LIGHTNING_LIGHTNINGD_PLUGIN_H */