bpo-45315: PyType_FromSpec: Copy spec->name and have the type own the memory for its name (GH-29103)

Include/cpython/object.h

@@ -290,6 +290,7 @@ typedef struct _heaptypeobject {
     PyObject *ht_name, *ht_slots, *ht_qualname;
     struct _dictkeysobject *ht_cached_keys;
     PyObject *ht_module;
+    char *_ht_tpname;  // Storage for "tp_name"; see PyType_FromModuleAndSpec
     /* here are optional user slots, followed by the members. */
 } PyHeapTypeObject;
 

Lib/test/test_sys.py

@@ -1419,7 +1419,7 @@ class SizeofTest(unittest.TestCase):
                   '3P'                  # PyMappingMethods
                   '10P'                 # PySequenceMethods
                   '2P'                  # PyBufferProcs
-                  '5P')
+                  '6P')
         class newstyleclass(object): pass
         # Separate block for PyDictKeysObject with 8 keys and 5 entries
         check(newstyleclass, s + calcsize(DICT_KEY_STRUCT_FORMAT) + 32 + 21*calcsize("n2P"))

Misc/NEWS.d/next/C API/2021-10-20-18-41-17.bpo-29103.CMRLyq.rst

@@ -0,0 +1,3 @@
+:c:func:`PyType_FromSpec* <PyType_FromModuleAndSpec>` now copies the class name
+from the spec to a buffer owned by the class, so the original can be safely
+deallocated. Patch by Petr Viktorin.

Modules/_testcapimodule.c

@@ -1163,6 +1163,126 @@ test_get_type_name(PyObject *self, PyObject *Py_UNUSED(ignored))
 }
 
 
+static PyObject *
+simple_str(PyObject *self) {
+    return PyUnicode_FromString("<test>");
+}
+
+
+static PyObject *
+test_type_from_ephemeral_spec(PyObject *self, PyObject *Py_UNUSED(ignored))
+{
+    // Test that a heap type can be created from a spec that's later deleted
+    // (along with all its contents).
+    // All necessary data must be copied and held by the class
+    PyType_Spec *spec = NULL;
+    char *name = NULL;
+    char *doc = NULL;
+    PyType_Slot *slots = NULL;
+    PyObject *class = NULL;
+    PyObject *instance = NULL;
+    PyObject *obj = NULL;
+    PyObject *result = NULL;
+
+    /* create a spec (and all its contents) on the heap */
+
+    const char NAME[] = "testcapi._Test";
+    const char DOC[] = "a test class";
+
+    spec = PyMem_New(PyType_Spec, 1);
+    if (spec == NULL) {
+        PyErr_NoMemory();
+        goto finally;
+    }
+    name = PyMem_New(char, sizeof(NAME));
+    if (name == NULL) {
+        PyErr_NoMemory();
+        goto finally;
+    }
+    memcpy(name, NAME, sizeof(NAME));
+
+    doc = PyMem_New(char, sizeof(DOC));
+    if (name == NULL) {
+        PyErr_NoMemory();
+        goto finally;
+    }
+    memcpy(doc, DOC, sizeof(DOC));
+
+    spec->name = name;
+    spec->basicsize = sizeof(PyObject);
+    spec->itemsize = 0;
+    spec->flags = Py_TPFLAGS_DEFAULT;
+    slots = PyMem_New(PyType_Slot, 3);
+    if (slots == NULL) {
+        PyErr_NoMemory();
+        goto finally;
+    }
+    slots[0].slot = Py_tp_str;
+    slots[0].pfunc = simple_str;
+    slots[1].slot = Py_tp_doc;
+    slots[1].pfunc = doc;
+    slots[2].slot = 0;
+    slots[2].pfunc = NULL;
+    spec->slots = slots;
+
+    /* create the class */
+
+    class = PyType_FromSpec(spec);
+    if (class == NULL) {
+        goto finally;
+    }
+
+    /* deallocate the spec (and all contents) */
+
+    // (Explicitly ovewrite memory before freeing,
+    // so bugs show themselves even without the debug allocator's help.)
+    memset(spec, 0xdd, sizeof(PyType_Spec));
+    PyMem_Del(spec);
+    spec = NULL;
+    memset(name, 0xdd, sizeof(NAME));
+    PyMem_Del(name);
+    name = NULL;
+    memset(doc, 0xdd, sizeof(DOC));
+    PyMem_Del(doc);
+    doc = NULL;
+    memset(slots, 0xdd, 3 * sizeof(PyType_Slot));
+    PyMem_Del(slots);
+    slots = NULL;
+
+    /* check that everything works */
+
+    PyTypeObject *class_tp = (PyTypeObject *)class;
+    PyHeapTypeObject *class_ht = (PyHeapTypeObject *)class;
+    assert(strcmp(class_tp->tp_name, "testcapi._Test") == 0);
+    assert(strcmp(PyUnicode_AsUTF8(class_ht->ht_name), "_Test") == 0);
+    assert(strcmp(PyUnicode_AsUTF8(class_ht->ht_qualname), "_Test") == 0);
+    assert(strcmp(class_tp->tp_doc, "a test class") == 0);
+
+    // call and check __str__
+    instance = PyObject_CallNoArgs(class);
+    if (instance == NULL) {
+        goto finally;
+    }
+    obj = PyObject_Str(instance);
+    if (obj == NULL) {
+        goto finally;
+    }
+    assert(strcmp(PyUnicode_AsUTF8(obj), "<test>") == 0);
+    Py_CLEAR(obj);
+
+    result = Py_NewRef(Py_None);
+  finally:
+    PyMem_Del(spec);
+    PyMem_Del(name);
+    PyMem_Del(doc);
+    PyMem_Del(slots);
+    Py_XDECREF(class);
+    Py_XDECREF(instance);
+    Py_XDECREF(obj);
+    return result;
+}
+
+
 static PyObject *
 test_get_type_qualname(PyObject *self, PyObject *Py_UNUSED(ignored))
 {
@@ -5804,6 +5924,7 @@ static PyMethodDef TestMethods[] = {
     {"test_get_statictype_slots", test_get_statictype_slots,     METH_NOARGS},
     {"test_get_type_name",        test_get_type_name,            METH_NOARGS},
     {"test_get_type_qualname",   test_get_type_qualname,       METH_NOARGS},
+    {"test_type_from_ephemeral_spec", test_type_from_ephemeral_spec, METH_NOARGS},
     {"get_kwargs", (PyCFunction)(void(*)(void))get_kwargs,
       METH_VARARGS|METH_KEYWORDS},
     {"getargs_tuple",           getargs_tuple,                   METH_VARARGS},

Objects/typeobject.c

@@ -2778,6 +2778,7 @@ type_new_alloc(type_new_ctx *ctx)
 
     et->ht_name = Py_NewRef(ctx->name);
     et->ht_module = NULL;
+    et->_ht_tpname = NULL;
 
     return type;
 }
@@ -3435,25 +3436,42 @@ PyType_FromModuleAndSpec(PyObject *module, PyType_Spec *spec, PyObject *bases)
         goto fail;
     }
 
-    /* Set the type name and qualname */
-    const char *s = strrchr(spec->name, '.');
-    if (s == NULL)
-        s = spec->name;
-    else
-        s++;
-
     type = &res->ht_type;
     /* The flags must be initialized early, before the GC traverses us */
     type->tp_flags = spec->flags | Py_TPFLAGS_HEAPTYPE;
-    res->ht_name = PyUnicode_FromString(s);
-    if (!res->ht_name)
-        goto fail;
-    res->ht_qualname = res->ht_name;
-    Py_INCREF(res->ht_qualname);
-    type->tp_name = spec->name;
 
-    Py_XINCREF(module);
+    /* Set the type name and qualname */
-    res->ht_module = module;
+    const char *s = strrchr(spec->name, '.');
+    if (s == NULL) {
+        s = spec->name;
+    }
+    else {
+        s++;
+    }
+
+    res->ht_name = PyUnicode_FromString(s);
+    if (!res->ht_name) {
+        goto fail;
+    }
+    res->ht_qualname = Py_NewRef(res->ht_name);
+
+    /* Copy spec->name to a buffer we own.
+    *
+    * Unfortunately, we can't use tp_name directly (with some
+    * flag saying that it should be deallocated with the type),
+    * because tp_name is public API and may be set independently
+    * of any such flag.
+    * So, we use a separate buffer, _ht_tpname, that's always
+    * deallocated with the type (if it's non-NULL).
+    */
+    Py_ssize_t name_buf_len = strlen(spec->name) + 1;
+    res->_ht_tpname = PyMem_Malloc(name_buf_len);
+    if (res->_ht_tpname == NULL) {
+        goto fail;
+    }
+    type->tp_name = memcpy(res->_ht_tpname, spec->name, name_buf_len);
+
+    res->ht_module = Py_XNewRef(module);
 
     /* Adjust for empty tuple bases */
     if (!bases) {
@@ -4082,6 +4100,7 @@ type_dealloc(PyTypeObject *type)
         _PyDictKeys_DecRef(et->ht_cached_keys);
     }
     Py_XDECREF(et->ht_module);
+    PyMem_Free(et->_ht_tpname);
     Py_TYPE(type)->tp_free((PyObject *)type);
 }
 
@@ -4273,10 +4292,12 @@ type_traverse(PyTypeObject *type, visitproc visit, void *arg)
     Py_VISIT(type->tp_base);
     Py_VISIT(((PyHeapTypeObject *)type)->ht_module);
 
-    /* There's no need to visit type->tp_subclasses or
+    /* There's no need to visit others because they can't be involved
-       ((PyHeapTypeObject *)type)->ht_slots, because they can't be involved
+       in cycles:
-       in cycles; tp_subclasses is a list of weak references,
+       type->tp_subclasses is a list of weak references,
-       and slots is a tuple of strings. */
+       ((PyHeapTypeObject *)type)->ht_slots is a tuple of strings,
+       ((PyHeapTypeObject *)type)->ht_*name are strings.
+       */
 
     return 0;
 }