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bpo-37879: Suppress subtype_dealloc decref when base type is a C heap type (GH-15323)

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The instance destructor for a type is responsible for preparing
an instance for deallocation by decrementing the reference counts
of its referents.

If an instance belongs to a heap type, the type object of an instance
has its reference count decremented while for static types, which
are permanently allocated, the type object is unaffected by the
instance destructor.

Previously, the default instance destructor searched the class
hierarchy for an inherited instance destructor and, if present,
would invoke it.

Then, if the instance type is a heap type, it would decrement the
reference count of that heap type.  However, this could result in the
premature destruction of a type because the inherited instance
destructor should have already decremented the reference count
of the type object.

This change avoids the premature destruction of the type object
by suppressing the decrement of its reference count when an
inherited, non-default instance destructor has been invoked.

Finally, an assertion on the Py_SIZE of a type was deleted.  Heap
types have a non zero size, making this into an incorrect assertion.

https://github.com/python/cpython/pull/15323
This commit is contained in:
Eddie Elizondo 2019-09-11 05:17:13 -04:00 • committed by Petr Viktorin
parent 7264e92b71
commit ff023ed36e
4 changed files with 318 additions and 10 deletions
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20
Objects/typeobject.c
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@ -1157,11 +1157,9 @@ subtype_dealloc(PyObject *self)
/* Test whether the type has GC exactly once */
if (!PyType_IS_GC(type)) {
/* It's really rare to find a dynamic type that doesn't have
GC; it can only happen when deriving from 'object' and not
adding any slots or instance variables. This allows
certain simplifications: there's no need to call
clear_slots(), or DECREF the dict, or clear weakrefs. */
/* A non GC dynamic type allows certain simplifications:
there's no need to call clear_slots(), or DECREF the dict,
or clear weakrefs. */
/* Maybe call finalizer; exit early if resurrected */
if (type->tp_finalize) {
@ -1177,7 +1175,6 @@ subtype_dealloc(PyObject *self)
/* Find the nearest base with a different tp_dealloc */
base = type;
while ((basedealloc = base->tp_dealloc) == subtype_dealloc) {
assert(Py_SIZE(base) == 0);
base = base->tp_base;
assert(base);
}
@ -1189,8 +1186,10 @@ subtype_dealloc(PyObject *self)
assert(basedealloc);
basedealloc(self);
/* Can't reference self beyond this point */
Py_DECREF(type);
/* Only decref if the base type is not already a heap allocated type.
Otherwise, basedealloc should have decref'd it already */
if (type->tp_flags & Py_TPFLAGS_HEAPTYPE && !(base->tp_flags & Py_TPFLAGS_HEAPTYPE))
Py_DECREF(type);
/* Done */
return;
@ -1289,8 +1288,9 @@ subtype_dealloc(PyObject *self)
/* Can't reference self beyond this point. It's possible tp_del switched
our type from a HEAPTYPE to a non-HEAPTYPE, so be careful about
reference counting. */
if (type->tp_flags & Py_TPFLAGS_HEAPTYPE)
reference counting. Only decref if the base type is not already a heap
allocated type. Otherwise, basedealloc should have decref'd it already */
if (type->tp_flags & Py_TPFLAGS_HEAPTYPE && !(base->tp_flags & Py_TPFLAGS_HEAPTYPE))
Py_DECREF(type);
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