gh-111178: fix UBSan failures in Modules/_dbmmodule.c (#129775)

This fixes UBSan failures for `dbmobject`.

In addition, we perform some minor cleanup changes such as renaming
some `args` parameter to `dummy` in some `METH_NOARGS` methods and
suppressing an unused return value in `_dbm_module_free`.

Author: picnixz

Modules/_dbmmodule.c

@@ -64,6 +64,8 @@ typedef struct {
     DBM *di_dbm;
 } dbmobject;
 
+#define dbmobject_CAST(op)  ((dbmobject *)(op))
+
 #include "clinic/_dbmmodule.c.h"
 
 #define check_dbmobject_open(v, err)                                \
@@ -94,15 +96,16 @@ newdbmobject(_dbm_state *state, const char *file, int flags, int mode)
 
 /* Methods */
 static int
-dbm_traverse(dbmobject *dp, visitproc visit, void *arg)
+dbm_traverse(PyObject *dp, visitproc visit, void *arg)
 {
     Py_VISIT(Py_TYPE(dp));
     return 0;
 }
 
 static void
-dbm_dealloc(dbmobject *dp)
+dbm_dealloc(PyObject *self)
 {
+    dbmobject *dp = dbmobject_CAST(self);
     PyObject_GC_UnTrack(dp);
     if (dp->di_dbm) {
         dbm_close(dp->di_dbm);
@@ -113,8 +116,9 @@ dbm_dealloc(dbmobject *dp)
 }
 
 static Py_ssize_t
-dbm_length(dbmobject *dp)
+dbm_length(PyObject *self)
 {
+    dbmobject *dp = dbmobject_CAST(self);
     _dbm_state *state = PyType_GetModuleState(Py_TYPE(dp));
     assert(state != NULL);
     if (dp->di_dbm == NULL) {
@@ -135,8 +139,9 @@ dbm_length(dbmobject *dp)
 }
 
 static int
-dbm_bool(dbmobject *dp)
+dbm_bool(PyObject *self)
 {
+    dbmobject *dp = dbmobject_CAST(self);
     _dbm_state *state = PyType_GetModuleState(Py_TYPE(dp));
     assert(state != NULL);
 
@@ -166,10 +171,11 @@ dbm_bool(dbmobject *dp)
 }
 
 static PyObject *
-dbm_subscript(dbmobject *dp, PyObject *key)
+dbm_subscript(PyObject *self, PyObject *key)
 {
     datum drec, krec;
     Py_ssize_t tmp_size;
+    dbmobject *dp = dbmobject_CAST(self);
     _dbm_state *state = PyType_GetModuleState(Py_TYPE(dp));
     assert(state != NULL);
     if (!PyArg_Parse(key, "s#", &krec.dptr, &tmp_size)) {
@@ -192,10 +198,11 @@ dbm_subscript(dbmobject *dp, PyObject *key)
 }
 
 static int
-dbm_ass_sub(dbmobject *dp, PyObject *v, PyObject *w)
+dbm_ass_sub(PyObject *self, PyObject *v, PyObject *w)
 {
     datum krec, drec;
     Py_ssize_t tmp_size;
+    dbmobject *dp = dbmobject_CAST(self);
 
     if ( !PyArg_Parse(v, "s#", &krec.dptr, &tmp_size) ) {
         PyErr_SetString(PyExc_TypeError,
@@ -305,7 +312,7 @@ _dbm_dbm_keys_impl(dbmobject *self, PyTypeObject *cls)
 static int
 dbm_contains(PyObject *self, PyObject *arg)
 {
-    dbmobject *dp = (dbmobject *)self;
+    dbmobject *dp = dbmobject_CAST(self);
     datum key, val;
     Py_ssize_t size;
 
@@ -452,15 +459,16 @@ _dbm_dbm_clear_impl(dbmobject *self, PyTypeObject *cls)
 }
 
 static PyObject *
-dbm__enter__(PyObject *self, PyObject *args)
+dbm__enter__(PyObject *self, PyObject *Py_UNUSED(dummy))
 {
     return Py_NewRef(self);
 }
 
 static PyObject *
-dbm__exit__(PyObject *self, PyObject *args)
+dbm__exit__(PyObject *self, PyObject *Py_UNUSED(args))
 {
-    return _dbm_dbm_close_impl((dbmobject *)self);
+    dbmobject *dp = dbmobject_CAST(self);
+    return _dbm_dbm_close_impl(dp);
 }
 
 static PyMethodDef dbm_methods[] = {
@@ -610,7 +618,7 @@ _dbm_module_clear(PyObject *module)
 static void
 _dbm_module_free(void *module)
 {
-    _dbm_module_clear((PyObject *)module);
+    (void)_dbm_module_clear((PyObject *)module);
 }
 
 static PyModuleDef_Slot _dbmmodule_slots[] = {