libplugin: expose helpers to start and end a jsonrpc response

The usual json_stream starters and command_result enders.

plugins/libplugin.c

@@ -150,6 +150,37 @@ static struct json_stream *jsonrpc_stream_start(struct command *cmd)
 	return js;
 }
 
+struct json_stream *jsonrpc_stream_success(struct command *cmd)
+{
+	struct json_stream *js = jsonrpc_stream_start(cmd);
+
+	json_object_start(js, "result");
+	return js;
+}
+
+struct json_stream *jsonrpc_stream_fail(struct command *cmd,
+					int code,
+					const char *err)
+{
+	struct json_stream *js = jsonrpc_stream_start(cmd);
+
+	json_object_start(js, "error");
+	json_add_member(js, "code", false, "%d", code);
+	json_add_string(js, "message", err);
+
+	return js;
+}
+
+struct json_stream *jsonrpc_stream_fail_data(struct command *cmd,
+					     int code,
+					     const char *err)
+{
+	struct json_stream *js = jsonrpc_stream_fail(cmd, code, err);
+
+	json_object_start(js, "data");
+	return js;
+}
+
 static struct command_result *command_complete(struct command *cmd,
 					       struct json_stream *result)
 {
@@ -162,6 +193,15 @@ static struct command_result *command_complete(struct command *cmd,
 	return &complete;
 }
 
+struct command_result *WARN_UNUSED_RESULT
+command_finished(struct command *cmd, struct json_stream *response)
+{
+	/* "result" or "error" object */
+	json_object_end(response);
+
+	return command_complete(cmd, response);
+}
+
 struct json_out *json_out_obj(const tal_t *ctx,
 			      const char *fieldname,
 			      const char *str)

plugins/libplugin.h

@@ -70,6 +70,25 @@ struct plugin_hook {
 	                                 const jsmntok_t *params);
 };
 
+/* Helper to create a JSONRPC2 response stream with a "result" object. */
+struct json_stream *jsonrpc_stream_success(struct command *cmd);
+
+/* Helper to create a JSONRPC2 response stream with an "error" object. */
+struct json_stream *jsonrpc_stream_fail(struct command *cmd,
+					int code,
+					const char *err);
+
+/* Helper to create a JSONRPC2 response stream with an "error" object,
+ * to which will be added a "data" object. */
+struct json_stream *jsonrpc_stream_fail_data(struct command *cmd,
+					     int code,
+					     const char *err);
+
+/* This command is finished, here's the response (the content of the
+ * "result" or "error" field) */
+struct command_result *WARN_UNUSED_RESULT
+command_finished(struct command *cmd, struct json_stream *response);
+
 /* Helper to create a zero or single-value JSON object; if @str is NULL,
  * object is empty. */
 struct json_out *json_out_obj(const tal_t *ctx,