plugin: Allow custom features only if the plugin is not dynamic

This is in order to avoid having to update featurebits as plugins get
activated and deactivated.

lightningd/plugin.c

@@ -836,6 +836,7 @@ bool plugin_parse_getmanifest_response(const char *buffer,
                                        struct plugin *plugin)
 {
 	const jsmntok_t *resulttok, *dynamictok, *featurestok, *tok;
+	bool have_featurebits = false;
 	u8 *featurebits;
 
 	resulttok = json_get_member(buffer, toks, "result");
@@ -849,6 +850,7 @@ bool plugin_parse_getmanifest_response(const char *buffer,
 			    json_tok_full(buffer, dynamictok));
 
 	featurestok = json_get_member(buffer, resulttok, "featurebits");
+
 	if (featurestok) {
 		for (int i = 0; i < NUM_PLUGIN_FEATURES_TYPE; i++) {
 			tok = json_get_member(buffer, featurestok,
@@ -860,6 +862,8 @@ bool plugin_parse_getmanifest_response(const char *buffer,
 			featurebits =
 			    json_tok_bin_from_hex(plugin, buffer, tok);
 
+			have_featurebits |= tal_bytelen(featurebits) > 0;
+
 			if (featurebits) {
 				plugin->featurebits[i] = featurebits;
 			} else {
@@ -873,6 +877,16 @@ bool plugin_parse_getmanifest_response(const char *buffer,
 		}
 	}
 
+	if (plugin->dynamic && have_featurebits) {
+		plugin_kill(plugin,
+			    "Custom featurebits only allows for non-dynamic "
+			    "plugins: dynamic=%d, featurebits=%.*s",
+			    plugin->dynamic,
+			    featurestok->end - featurestok->start,
+			    buffer + featurestok->start);
+		return true;
+	}
+
 	if (!plugin_opts_add(plugin, buffer, resulttok) ||
 	    !plugin_rpcmethods_add(plugin, buffer, resulttok) ||
 	    !plugin_subscriptions_add(plugin, buffer, resulttok) ||

tests/plugins/feature-test.py

@@ -5,6 +5,7 @@ from pyln.client import Plugin
 # Register a different set feature of feature bits for each location so we can
 # later check that they are being passed correctly.
 plugin = Plugin(
+    dynamic=False,
     init_features=1 << 101,
     node_features=1 << 103,
     invoice_features=1 << 105,