Properly untrack Python GC objects during deallocation.

Add PyObject_GC_UnTrack() in deallocation functions for Python types that
have PyTPFLAGS_HAVE_GC set, either explicitly or by inheriting from a type
with GC set. Not untracking before clearing instance data introduces
potential race conditions (if GC happens to run between the partial clearing
and the actual deallocation) and produces a warning under Python 3.11.

(The warning then triggered an assertion failure, which only showed up when
building in Py_DEBUG mode; this therefor also fixes that assertion failure.)

PiperOrigin-RevId: 579827001

python/descriptor.c

@@ -197,10 +197,25 @@ static PyObject* PyUpb_DescriptorBase_CopyToProto(PyObject* _self,
 }
 
 static void PyUpb_DescriptorBase_Dealloc(PyUpb_DescriptorBase* base) {
+  // This deallocator can be called on different types (which, despite
+  // 'Base' in the name of one of them, do not inherit from each other).
+  // Some of these types are GC types (they have Py_TPFLAGS_HAVE_GC set),
+  // which means Python's GC can visit them (via tp_visit and/or tp_clear
+  // methods) at any time. This also means we *must* stop GC from tracking
+  // instances of them before we start destructing the object. In Python
+  // 3.11, failing to do so would raise a runtime warning.
+  if (PyType_HasFeature(Py_TYPE(base), Py_TPFLAGS_HAVE_GC)) {
+    PyObject_GC_UnTrack(base);
+  }
   PyUpb_ObjCache_Delete(base->def);
-  Py_XDECREF(base->message_meta);
-  Py_DECREF(base->pool);
-  Py_XDECREF(base->options);
+  // In addition to being visited by GC, instances can also (potentially) be
+  // accessed whenever arbitrary code is executed. Destructors can execute
+  // arbitrary code, so any struct members we DECREF should be set to NULL
+  // or a new value *before* calling Py_DECREF on them. The Py_CLEAR macro
+  // (and Py_SETREF in Python 3.8+) takes care to do this safely.
+  Py_CLEAR(base->message_meta);
+  Py_CLEAR(base->pool);
+  Py_CLEAR(base->options);
   PyUpb_Dealloc(base);
 }
 
@@ -256,9 +271,13 @@ PyObject* PyUpb_Descriptor_GetClass(const upb_MessageDef* m) {
 
 void PyUpb_Descriptor_SetClass(PyObject* py_descriptor, PyObject* meta) {
   PyUpb_DescriptorBase* base = (PyUpb_DescriptorBase*)py_descriptor;
-  Py_XDECREF(base->message_meta);
-  base->message_meta = meta;
   Py_INCREF(meta);
+  // Py_SETREF replaces strong references without an intermediate invalid
+  // object state, which code executed by base->message_meta's destructor
+  // might see, but it's Python 3.8+.
+  PyObject* tmp = base->message_meta;
+  base->message_meta = meta;
+  Py_XDECREF(tmp);
 }
 
 // The LookupNested*() functions provide name lookup for entities nested inside

python/descriptor_pool.c

@@ -99,6 +99,7 @@ PyObject* PyUpb_DescriptorPool_Get(const upb_DefPool* symtab) {
 }
 
 static void PyUpb_DescriptorPool_Dealloc(PyUpb_DescriptorPool* self) {
+  PyObject_GC_UnTrack(self);
   PyUpb_DescriptorPool_Clear(self);
   upb_DefPool_Free(self->symtab);
   PyUpb_ObjCache_Delete(self->symtab);

python/message.c

@@ -1850,7 +1850,15 @@ static PyObject* PyUpb_MessageMeta_New(PyTypeObject* type, PyObject* args,
 static void PyUpb_MessageMeta_Dealloc(PyObject* self) {
   PyUpb_MessageMeta* meta = PyUpb_GetMessageMeta(self);
   PyUpb_ObjCache_Delete(meta->layout);
-  Py_DECREF(meta->py_message_descriptor);
+  // The MessageMeta type is a GC type, which means we should untrack the
+  // object before invalidating internal state (so that code executed by the
+  // GC doesn't see the invalid state). Unfortunately since we're calling
+  // cpython_bits.type_dealloc, which also untracks the object, we can't.
+  // Instead just make sure the internal state remains reasonable by using
+  // Py_CLEAR(), which sets the struct member to NULL. The tp_traverse and
+  // tp_clear methods, which are called by Python's GC, already allow for it
+  // to be NULL.
+  Py_CLEAR(meta->py_message_descriptor);
   PyTypeObject* tp = Py_TYPE(self);
   cpython_bits.type_dealloc(self);
   Py_DECREF(tp);
@@ -1948,6 +1956,8 @@ static int PyUpb_MessageMeta_Traverse(PyObject* self, visitproc visit,
 }
 
 static int PyUpb_MessageMeta_Clear(PyObject* self, visitproc visit, void* arg) {
+  PyUpb_MessageMeta* meta = PyUpb_GetMessageMeta(self);
+  Py_CLEAR(meta->py_message_descriptor);
   return cpython_bits.type_clear(self);
 }