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gh-128923: Use zero to indicate unassigned unique id (#128925)

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In the free threading build, the per thread reference counting uses a
unique id for some objects to index into the local reference count
table. Use 0 instead of -1 to indicate that the id is not assigned. This
avoids bugs where zero-initialized heap type objects look like they have
a unique id assigned.
This commit is contained in:
Sam Gross 2025-01-17 10:42:27 -05:00 • committed by GitHub
parent 767c89ba7c
commit d66c08aa75
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9 changed files with 110 additions and 33 deletions
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24
Include/internal/pycore_object.h
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@ -336,20 +336,20 @@ _Py_THREAD_INCREF_OBJECT(PyObject *obj, Py_ssize_t unique_id)
{
_PyThreadStateImpl *tstate = (_PyThreadStateImpl *)_PyThreadState_GET();
// Unsigned comparison so that `unique_id=-1`, which indicates that
// per-thread refcounting has been disabled on this object, is handled by
// the "else".
if ((size_t)unique_id < (size_t)tstate->refcounts.size) {
// The table index is `unique_id - 1` because 0 is not a valid unique id.
// Unsigned comparison so that `idx=-1` is handled by the "else".
size_t idx = (size_t)(unique_id - 1);
if (idx < (size_t)tstate->refcounts.size) {
# ifdef Py_REF_DEBUG
_Py_INCREF_IncRefTotal();
# endif
_Py_INCREF_STAT_INC();
tstate->refcounts.values[unique_id]++;
tstate->refcounts.values[idx]++;
}
else {
// The slow path resizes the per-thread refcount array if necessary.
// It handles the unique_id=-1 case to keep the inlinable function smaller.
_PyObject_ThreadIncrefSlow(obj, unique_id);
// It handles the unique_id=0 case to keep the inlinable function smaller.
_PyObject_ThreadIncrefSlow(obj, idx);
}
}
@ -386,15 +386,15 @@ _Py_THREAD_DECREF_OBJECT(PyObject *obj, Py_ssize_t unique_id)
{
_PyThreadStateImpl *tstate = (_PyThreadStateImpl *)_PyThreadState_GET();
// Unsigned comparison so that `unique_id=-1`, which indicates that
// per-thread refcounting has been disabled on this object, is handled by
// the "else".
if ((size_t)unique_id < (size_t)tstate->refcounts.size) {
// The table index is `unique_id - 1` because 0 is not a valid unique id.
// Unsigned comparison so that `idx=-1` is handled by the "else".
size_t idx = (size_t)(unique_id - 1);
if (idx < (size_t)tstate->refcounts.size) {
# ifdef Py_REF_DEBUG
_Py_DECREF_DecRefTotal();
# endif
_Py_DECREF_STAT_INC();
tstate->refcounts.values[unique_id]--;
tstate->refcounts.values[idx]--;
}
else {
// Directly decref the object if the id is not assigned or if