fix(UDL): fix listener segfault for modules using new listener API

The new UDL Python API defines modrdn as int instead of the string "1":
(gdb) p *var
$1 = {ob_refcnt = 1000010831, ob_type = 0x7f341f835ae0 <PyLong_Type>}

In UCS 5.0 this was not a problem, as `PyArg_Parse(var, "s", &str1);` worked nevertheless and resulted in:
(gdb) p str1
$2 = 0x7ffef407aea8 "\002"

This seems to have changed in Python 3.11 or earlier and caused:

kernel: [18330.428271] univention-dire[15235]: segfault at 0 ip 00007f23ced86618 sp 00007fff7570ae48 error 4 in libc.so.6[7f23cec45000+155000] likely on CPU 0 (core 0, socket 0)
kernel: [18330.428285] Code: e1 ff ff ff c3 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 00 89 f8 62 a1 fd 00 ef c0 25 ff 0f 00 00 3d e0 0f 00 00 0f 87 38 01 00 00 <62> f3 7d 20 3f 07 00 c5 fb 93 c0 85 c0 74 59 f3 0f bc c0 c3 0f 1f

  #1  0x00007f341f0bd7ee in __GI___strdup (s=0x0) at ./string/strdup.c:41
  #2  0x000055b0683cfd20 in module_get_string (module=<optimized out>, name=name@entry=0x55b0683dbf65 "modrdn") at ./src/handlers.c:158
  #3  0x000055b0683d0580 in handler_import (filename=0x7ffc300a2440 "/usr/lib/univention-directory-listener/system/ldap-cache-baa04df67e7af6bb0769f5cb7e72dba9.py") at ./src/handlers.c:239
  #4  0x000055b0683d0d02 in handlers_load_path (path=0x55b069fcc7b0 "/usr/lib/univention-directory-listener/system") at ./src/handlers.c:516
  #5  0x000055b0683d14a0 in handlers_load_path (path=0x55b069fcc7b0 "/usr/lib/univention-directory-listener/system") at ./src/handlers.c:629
  #6  handlers_load_all_paths () at ./src/handlers.c:535
  #7  handlers_init () at ./src/handlers.c:627
  #8  0x000055b0683ce530 in main (argc=<optimized out>, argv=<optimized out>) at ./src/main.c:564

The modrdn was now changed into a boolean flag. Prior it was evaluated
similar. the pure presence of it (e.g. "0") caused modrdn to be
evaluated as true.

Bug #56533

management/univention-directory-listener/src/handlers.c

@@ -139,17 +139,32 @@ static PyObject *module_get_object(PyObject *module, char *name) {
 }
 
 
+/* Retrieve bool(name) from Python module.  */
+static bool module_get_bool(PyObject *module, char *name) {
+	PyObject *attr;
+	int result;
+	attr = module_get_object(module, name);
+	result = PyObject_IsTrue(attr);
+	Py_XDECREF(attr);
+	return result == 1;
+}
+
+
 /* Retrieve string from Python module. */
 static char *module_get_string(PyObject *module, char *name) {
 	PyObject *var;
 	char *str1, *str2 = NULL;
 
 	if ((var = PyObject_GetAttrString(module, name)) == NULL)
-		goto error0;
-	PyArg_Parse(var, "s", &str1);
+		goto error1;
+	if (!PyUnicode_Check(var))
+	    goto error0;
+	if (PyArg_Parse(var, "s", &str1) != 1)
+	    goto error0;
 	str2 = strdup(str1);
-	Py_XDECREF(var);
 error0:
+	Py_XDECREF(var);
+error1:
 	return str2;
 }
 
@@ -228,11 +243,7 @@ static int handler_import(char *filename) {
 	}
 
 	if (PyObject_HasAttrString(handler->module, "modrdn")) { /* optional */
-		handler->modrdn = module_get_string(handler->module, "modrdn");
-		if (handler->modrdn == NULL) {
-			error_msg = "module_get_string(\"modrdn\")";
-			goto error;
-		}
+		handler->modrdn = module_get_bool(handler->module, "modrdn");
 	}
 	PyErr_Clear();  // Silent error when attribute is not set
 
@@ -333,7 +344,6 @@ static int handler_import(char *filename) {
 	}
 	free(handler->filters);
 	free(handler->description);
-	free(handler->modrdn);
 	free(handler->name);
 	free(handler);
 	univention_debug(UV_DEBUG_LISTENER, UV_DEBUG_ERROR, "import of filename=%s failed in %s", filename, error_msg ? error_msg : "???");

management/univention-directory-listener/src/handlers.h

@@ -66,7 +66,7 @@ struct _Handler {
 	char *description;
 	struct filter **filters;
 	char **attributes;
-	char *modrdn;
+	bool modrdn;
 	bool handle_every_delete;
 	PyObject *handler;
 	PyObject *initialize;