gh-122728: Fix SystemError in PyEval_GetLocals()

Fix PyEval_GetLocals() to avoid SystemError ("bad argument to
internal function"). Don't redefine the 'ret' variable in the if
block.

Add an unit test on PyEval_GetLocals().

Author: vstinner

Lib/test/test_capi/test_misc.py

@@ -1157,6 +1157,16 @@ def genf(): yield
         gen = genf()
         self.assertEqual(_testcapi.gen_get_code(gen), gen.gi_code)
 
+    def test_pyeval_getlocals(self):
+        # Test PyEval_GetLocals()
+        x = 1
+        y = 2
+        local_vars = _testcapi.pyeval_getlocals()
+        self.assertEqual(local_vars,
+            {'self': self,
+             'x': 1,
+             'y': 2})
+
 
 @requires_limited_api
 class TestHeapTypeRelative(unittest.TestCase):

Misc/NEWS.d/next/C_API/2024-08-06-14-23-11.gh-issue-122728.l-fQ-v.rst

@@ -0,0 +1,2 @@
+Fix :c:func:`PyEval_GetLocals` to avoid :exc:`SystemError` ("bad argument to
+internal function"). Patch by Victor Stinner.

Modules/_testcapimodule.c

@@ -3341,6 +3341,12 @@ test_critical_sections(PyObject *module, PyObject *Py_UNUSED(args))
     Py_RETURN_NONE;
 }
 
+static PyObject *
+pyeval_getlocals(PyObject *module, PyObject *Py_UNUSED(args))
+{
+    return Py_XNewRef(PyEval_GetLocals());
+}
+
 static PyMethodDef TestMethods[] = {
     {"set_errno",               set_errno,                       METH_VARARGS},
     {"test_config",             test_config,                     METH_NOARGS},
@@ -3483,6 +3489,7 @@ static PyMethodDef TestMethods[] = {
     {"test_weakref_capi", test_weakref_capi, METH_NOARGS},
     {"function_set_warning", function_set_warning, METH_NOARGS},
     {"test_critical_sections", test_critical_sections, METH_NOARGS},
+    {"pyeval_getlocals", pyeval_getlocals, METH_NOARGS},
     {NULL, NULL} /* sentinel */
 };
 

Python/ceval.c

@@ -2499,7 +2499,7 @@ PyEval_GetLocals(void)
         PyFrameObject *f = _PyFrame_GetFrameObject(current_frame);
         PyObject *ret = f->f_locals_cache;
         if (ret == NULL) {
-            PyObject *ret = PyDict_New();
+            ret = PyDict_New();
             if (ret == NULL) {
                 Py_DECREF(locals);
                 return NULL;