bpo-44531: Add _PyType_AllocNoTrack() function (GH-26947)

Add an internal _PyType_AllocNoTrack() function to allocate an object
without tracking it in the GC.

Modify dict_new() to use _PyType_AllocNoTrack(): dict subclasses are
now only tracked once all PyDictObject members are initialized.
Calling _PyObject_GC_UNTRACK() is no longer needed for the dict type.

Similar change in tuple_subtype_new() for tuple subclasses.

Replace tuple_gc_track() with _PyObject_GC_TRACK().

Objects/tupleobject.c

@@ -25,13 +25,6 @@ get_tuple_state(void)
 #endif
 
 
-static inline void
-tuple_gc_track(PyTupleObject *op)
-{
-    _PyObject_GC_TRACK(op);
-}
-
-
 /* Print summary info about the state of the optimized allocator */
 void
 _PyTuple_DebugMallocStats(FILE *out)
@@ -48,10 +41,12 @@ _PyTuple_DebugMallocStats(FILE *out)
 #endif
 }
 
-/* Allocate an uninitialized tuple object. Before making it public following
+/* Allocate an uninitialized tuple object. Before making it public, following
    steps must be done:
-   - initialize its items
-   - call tuple_gc_track() on it
+
+   - Initialize its items.
+   - Call _PyObject_GC_TRACK() on it.
+
    Because the empty tuple is always reused and it's already tracked by GC,
    this function must not be called with size == 0 (unless from PyTuple_New()
    which wraps this function).
@@ -161,7 +156,7 @@ PyTuple_New(Py_ssize_t size)
     for (Py_ssize_t i = 0; i < size; i++) {
         op->ob_item[i] = NULL;
     }
-    tuple_gc_track(op);
+    _PyObject_GC_TRACK(op);
     return (PyObject *) op;
 }
 
@@ -257,7 +252,7 @@ PyTuple_Pack(Py_ssize_t n, ...)
         items[i] = o;
     }
     va_end(vargs);
-    tuple_gc_track(result);
+    _PyObject_GC_TRACK(result);
     return (PyObject *)result;
 }
 
@@ -473,7 +468,7 @@ _PyTuple_FromArray(PyObject *const *src, Py_ssize_t n)
         Py_INCREF(item);
         dst[i] = item;
     }
-    tuple_gc_track(tuple);
+    _PyObject_GC_TRACK(tuple);
     return (PyObject *)tuple;
 }
 
@@ -551,7 +546,7 @@ tupleconcat(PyTupleObject *a, PyObject *bb)
         Py_INCREF(v);
         dest[i] = v;
     }
-    tuple_gc_track(np);
+    _PyObject_GC_TRACK(np);
     return (PyObject *)np;
 }
 
@@ -588,7 +583,7 @@ tuplerepeat(PyTupleObject *a, Py_ssize_t n)
             p++;
         }
     }
-    tuple_gc_track(np);
+    _PyObject_GC_TRACK(np);
     return (PyObject *) np;
 }
 
@@ -783,6 +778,9 @@ tuple_subtype_new(PyTypeObject *type, PyObject *iterable)
     Py_ssize_t i, n;
 
     assert(PyType_IsSubtype(type, &PyTuple_Type));
+    // tuple subclasses must implement the GC protocol
+    assert(_PyType_IS_GC(type));
+
     tmp = tuple_new_impl(&PyTuple_Type, iterable);
     if (tmp == NULL)
         return NULL;
@@ -798,6 +796,11 @@ tuple_subtype_new(PyTypeObject *type, PyObject *iterable)
         PyTuple_SET_ITEM(newobj, i, item);
     }
     Py_DECREF(tmp);
+
+    // Don't track if a subclass tp_alloc is PyType_GenericAlloc()
+    if (!_PyObject_GC_IS_TRACKED(newobj)) {
+        _PyObject_GC_TRACK(newobj);
+    }
     return newobj;
 }
 
@@ -857,7 +860,7 @@ tuplesubscript(PyTupleObject* self, PyObject* item)
                 dest[i] = it;
             }
 
-            tuple_gc_track(result);
+            _PyObject_GC_TRACK(result);
             return (PyObject *)result;
         }
     }