daemon/engine: priority prefix ‘<‘ for modules

if the configured modules name is prefixed with ‘<‘ it takes precedence before all others,
e.g. modules = { ‘hints’, ‘<block’ }
means that the ‘hints’ module will be executed in-order (last), and ‘block’ module layer will be called as first in query processing

daemon/engine.c

@@ -434,7 +434,12 @@ int engine_register(struct engine *engine, const char *name)
 	if (engine == NULL || name == NULL) {
 		return kr_error(EINVAL);
 	}
-
+	/* Check priority modules */
+	bool is_priority = false;
+	if (name[0] == '<') {
+		is_priority = true;
+		name += 1;
+	}
 	/* Make sure module is unloaded */
 	(void) engine_unregister(engine, name);
 	/* Attempt to load binary module */
@@ -452,11 +457,16 @@ int engine_register(struct engine *engine, const char *name)
 		free(module);
 		return ret;
 	}
-
 	if (array_push(engine->modules, module) < 0) {
 		engine_unload(engine, module);
 		return kr_error(ENOMEM);
 	}
+	/* Push to front if priority module */
+	if (is_priority) {
+		struct kr_module **arr = engine->modules.at;
+		memmove(&arr[1], &arr[0], sizeof(*arr) * (engine->modules.len - 1));
+		arr[0] = module;
+	}
 
 	/* Register properties */
 	if (module->props) {