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bpo-42972: Fully implement GC protocol for functools keywrapper and partial types #26363

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Merged
vstinner merged 11 commits into from
May 28, 2021
Merged

bpo-42972: Fully implement GC protocol for functools keywrapper and partial types #26363

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8ef6e98
bpo-42972: Fully implement GC protocol for functools types
May 25, 2021
6d7cdaf
Address review
May 26, 2021
a15b49f
Address review: visit node type for each strong ref
May 27, 2021
0c8df1f
Address review: visit members in correct order
May 27, 2021
f9ac4c5
Address review: revert Py_CLEAR change in lru_list_elem_dealloc
May 27, 2021
fc39917
Revert LRU cache element optimisation
May 27, 2021
07f2182
Address review: revert redundant slot removal
May 27, 2021
0861788
Address review: use tp_free
May 27, 2021
7c2f750
Revert lru list elem clear fn
May 27, 2021
6313247
Revert "Revert LRU cache element optimisation"
May 28, 2021
1539195
Restore lru_cache_tp_traverse
May 28, 2021
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Revert LRU cache element optimisation 
This reverts commit 6d7cdaf.
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Erlend E. Aasland committed May 27, 2021
commit fc39917b8808c70553ac4606154dfb628b559134
32 changes: 26 additions & 6 deletions Modules/_functoolsmodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -762,10 +762,28 @@ typedef struct lru_list_elem {
PyObject *key, *result;
} lru_list_elem;

static int
lru_list_elem_clear(lru_list_elem *link)
{
Py_CLEAR(link->key);
Py_CLEAR(link->result);
return 0;
}

static int
lru_list_elem_traverse(lru_list_elem *link, visitproc visit, void *arg)
{
Py_VISIT(link->key);
Py_VISIT(link->result);
Py_VISIT(Py_TYPE(link));
return 0;
}
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@vstinner vstinner May 27, 2021

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A lru_list_elem_clear() function would also be needed to implement the GC protocol, no?

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@erlend-aasland erlend-aasland May 27, 2021

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A lru_list_elem_clear() function would also be needed to implement the GC protocol, no?

I was under the impression traverse was enough. I may be wrong.

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@vstinner vstinner May 27, 2021

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Without clear, the ref count cannot each zero, and the GC cannot break cycles: https://devguide.python.org/garbage_collector/#destroying-unreachable-objects

cc @pablogsal

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@methane methane May 28, 2021

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_lru_list_elem is not a standalone type. It is an internal of _lru_cache_wrapper.

  • lru_cache_tp_traverse() visits lru_list_elem.
  • lru_cache_tp_clear() clears lru_list_elem.

That's why we can keep _lru_list_elem non-GC type.

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@shihai1991 shihai1991 May 28, 2021

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IMO, Inada's #26416 is acceptable.


static void
lru_list_elem_dealloc(lru_list_elem *link)
{
PyTypeObject *tp = Py_TYPE(link);
PyObject_GC_UnTrack(link);
Py_XDECREF(link->key);
Py_XDECREF(link->result);
tp->tp_free(link);
Expand All @@ -774,13 +792,16 @@ lru_list_elem_dealloc(lru_list_elem *link)

static PyType_Slot lru_list_elem_type_slots[] = {
{Py_tp_dealloc, lru_list_elem_dealloc},
{Py_tp_traverse, lru_list_elem_traverse},
{Py_tp_clear, lru_list_elem_clear},
{0, 0}
};

static PyType_Spec lru_list_elem_type_spec = {
.name = "functools._lru_list_elem",
.basicsize = sizeof(lru_list_elem),
.flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISALLOW_INSTANTIATION,
.flags = (Py_TPFLAGS_DEFAULT | Py_TPFLAGS_DISALLOW_INSTANTIATION |
Py_TPFLAGS_HAVE_GC),
.slots = lru_list_elem_type_slots
};

Expand Down Expand Up @@ -1045,8 +1066,8 @@ bounded_lru_cache_wrapper(lru_cache_object *self, PyObject *args, PyObject *kwds
self->root.next == &self->root)
{
/* Cache is not full, so put the result in a new link */
link = (lru_list_elem *)PyObject_New(lru_list_elem,
self->lru_list_elem_type);
link = (lru_list_elem *)PyObject_GC_New(lru_list_elem,
self->lru_list_elem_type);
if (link == NULL) {
Py_DECREF(key);
Py_DECREF(result);
Expand All @@ -1056,6 +1077,7 @@ bounded_lru_cache_wrapper(lru_cache_object *self, PyObject *args, PyObject *kwds
link->hash = hash;
link->key = key;
link->result = result;
PyObject_GC_Track(link);
/* What is really needed here is a SetItem variant with a "no clobber"
option. If the __eq__ call triggers a reentrant call that adds
this same key, then this setitem call will update the cache dict
Expand Down Expand Up @@ -1345,9 +1367,7 @@ lru_cache_tp_traverse(lru_cache_object *self, visitproc visit, void *arg)
lru_list_elem *link = self->root.next;
while (link != &self->root) {
lru_list_elem *next = link->next;
Py_VISIT(link->key);
Py_VISIT(link->result);
Py_VISIT(Py_TYPE(link));
Py_VISIT(link);
link = next;
}
Py_VISIT(self->cache);
Expand Down