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Issue #3757: thread-local objects now support cyclic garbage collection. 

Thread-local objects involved in reference cycles will be deallocated
timely by the cyclic GC, even if the underlying thread is still running.
This commit is contained in:
Antoine Pitrou 2010-08-09 22:38:19 +00:00
parent 6e451df800
commit 5af4f4b983
3 changed files with 244 additions and 47 deletions
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18
Lib/test/test_threading_local.py
View File

@ -38,9 +38,9 @@ class BaseLocalTest:
gc.collect()
self.assertEqual(len(weaklist), n)
# XXX threading.local keeps the local of the last stopped thread alive.
# XXX _threading_local keeps the local of the last stopped thread alive.
deadlist = [weak for weak in weaklist if weak() is None]
self.assertEqual(len(deadlist), n-1)
self.assertIn(len(deadlist), (n-1, n))
# Assignment to the same thread local frees it sometimes (!)
local.someothervar = None
@ -127,6 +127,20 @@ class BaseLocalTest:
class ThreadLocalTest(unittest.TestCase, BaseLocalTest):
_local = _thread._local
# Fails for the pure Python implementation
def test_cycle_collection(self):
class X:
pass
x = X()
x.local = self._local()
x.local.x = x
wr = weakref.ref(x)
del x
gc.collect()
self.assertIs(wr(), None)
class PyThreadingLocalTest(unittest.TestCase, BaseLocalTest):
_local = _threading_local.local

4
Misc/NEWS
View File

@ -67,6 +67,10 @@ Extensions
Library
-------
- Issue #3757: thread-local objects now support cyclic garbage collection.
Thread-local objects involved in reference cycles will be deallocated
timely by the cyclic GC, even if the underlying thread is still running.
- Issue #9452: Add read_file, read_string, and read_dict to the configparser
API; new source attribute to exceptions.

269
Modules/_threadmodule.c
View File

@ -499,19 +499,163 @@ newlockobject(void)
#include "structmember.h"
/* Quick overview:
We need to be able to reclaim reference cycles as soon as possible
(both when a thread is being terminated, or a thread-local object
becomes unreachable from user data). Constraints:
- it must not be possible for thread-state dicts to be involved in
reference cycles (otherwise the cyclic GC will refuse to consider
objects referenced from a reachable thread-state dict, even though
local_dealloc would clear them)
- the death of a thread-state dict must still imply destruction of the
corresponding local dicts in all thread-local objects.
Our implementation uses small "localdummy" objects in order to break
the reference chain. These trivial objects are hashable (using the
default scheme of identity hashing) and weakrefable.
Each thread-state holds a separate localdummy for each local object
(as a /strong reference/),
and each thread-local object holds a dict mapping /weak references/
of localdummies to local dicts.
Therefore:
- only the thread-state dict holds a strong reference to the dummies
- only the thread-local object holds a strong reference to the local dicts
- only outside objects (application- or library-level) hold strong
references to the thread-local objects
- as soon as a thread-state dict is destroyed, the weakref callbacks of all
dummies attached to that thread are called, and destroy the corresponding
local dicts from thread-local objects
- as soon as a thread-local object is destroyed, its local dicts are
destroyed and its dummies are manually removed from all thread states
- the GC can do its work correctly when a thread-local object is dangling,
without any interference from the thread-state dicts
As an additional optimization, each localdummy holds a borrowed reference
to the corresponding localdict. This borrowed reference is only used
by the thread-local object which has created the localdummy, which should
guarantee that the localdict still exists when accessed.
*/
typedef struct {
PyObject_HEAD
PyObject *localdict; /* Borrowed reference! */
PyObject *weakreflist; /* List of weak references to self */
} localdummyobject;
static void
localdummy_dealloc(localdummyobject *self)
{
if (self->weakreflist != NULL)
PyObject_ClearWeakRefs((PyObject *) self);
Py_TYPE(self)->tp_free((PyObject*)self);
}
static PyTypeObject localdummytype = {
PyVarObject_HEAD_INIT(NULL, 0)
/* tp_name */ "_thread._localdummy",
/* tp_basicsize */ sizeof(localdummyobject),
/* tp_itemsize */ 0,
/* tp_dealloc */ (destructor)localdummy_dealloc,
/* tp_print */ 0,
/* tp_getattr */ 0,
/* tp_setattr */ 0,
/* tp_reserved */ 0,
/* tp_repr */ 0,
/* tp_as_number */ 0,
/* tp_as_sequence */ 0,
/* tp_as_mapping */ 0,
/* tp_hash */ 0,
/* tp_call */ 0,
/* tp_str */ 0,
/* tp_getattro */ 0,
/* tp_setattro */ 0,
/* tp_as_buffer */ 0,
/* tp_flags */ Py_TPFLAGS_DEFAULT,
/* tp_doc */ "Thread-local dummy",
/* tp_traverse */ 0,
/* tp_clear */ 0,
/* tp_richcompare */ 0,
/* tp_weaklistoffset */ offsetof(localdummyobject, weakreflist)
};
typedef struct {
PyObject_HEAD
PyObject *key;
PyObject *args;
PyObject *kw;
/* The current thread's local dict (necessary for tp_dictoffset) */
PyObject *dict;
PyObject *weakreflist; /* List of weak references to self */
/* A {localdummy weakref -> localdict} dict */
PyObject *dummies;
/* The callback for weakrefs to localdummies */
PyObject *wr_callback;
} localobject;
/* Forward declaration */
static PyObject *_ldict(localobject *self);
static PyObject *_localdummy_destroyed(PyObject *meth_self, PyObject *dummyweakref);
/* Create and register the dummy for the current thread, as well as the
corresponding local dict */
static int
_local_create_dummy(localobject *self)
{
PyObject *tdict, *ldict = NULL, *wr = NULL;
localdummyobject *dummy = NULL;
int r;
tdict = PyThreadState_GetDict();
if (tdict == NULL) {
PyErr_SetString(PyExc_SystemError,
"Couldn't get thread-state dictionary");
goto err;
}
ldict = PyDict_New();
if (ldict == NULL)
goto err;
dummy = (localdummyobject *) localdummytype.tp_alloc(&localdummytype, 0);
if (dummy == NULL)
goto err;
dummy->localdict = ldict;
wr = PyWeakref_NewRef((PyObject *) dummy, self->wr_callback);
if (wr == NULL)
goto err;
/* As a side-effect, this will cache the weakref's hash before the
dummy gets deleted */
r = PyDict_SetItem(self->dummies, wr, ldict);
if (r < 0)
goto err;
Py_CLEAR(wr);
r = PyDict_SetItem(tdict, self->key, (PyObject *) dummy);
if (r < 0)
goto err;
Py_CLEAR(dummy);
Py_CLEAR(self->dict);
self->dict = ldict;
return 0;
err:
Py_XDECREF(ldict);
Py_XDECREF(wr);
Py_XDECREF(dummy);
return -1;
}
static PyObject *
local_new(PyTypeObject *type, PyObject *args, PyObject *kw)
{
localobject *self;
PyObject *tdict;
PyObject *wr;
static PyMethodDef wr_callback_def = {
"_localdummy_destroyed", (PyCFunction) _localdummy_destroyed, METH_O
};
if (type->tp_init == PyBaseObject_Type.tp_init
&& ((args && PyObject_IsTrue(args))
@ -529,23 +673,25 @@ local_new(PyTypeObject *type, PyObject *args, PyObject *kw)
self->args = args;
Py_XINCREF(kw);
self->kw = kw;
self->dict = NULL; /* making sure */
self->key = PyUnicode_FromFormat("thread.local.%p", self);
if (self->key == NULL)
goto err;
self->dict = PyDict_New();
if (self->dict == NULL)
self->dummies = PyDict_New();
if (self->dummies == NULL)
goto err;
tdict = PyThreadState_GetDict();
if (tdict == NULL) {
PyErr_SetString(PyExc_SystemError,
"Couldn't get thread-state dictionary");
/* We use a weak reference to self in the callback closure
in order to avoid spurious reference cycles */
wr = PyWeakref_NewRef((PyObject *) self, NULL);
if (wr == NULL)
goto err;
self->wr_callback = PyCFunction_New(&wr_callback_def, wr);
Py_DECREF(wr);
if (self->wr_callback == NULL)
goto err;
}
if (PyDict_SetItem(tdict, self->key, self->dict) < 0)
if (_local_create_dummy(self) < 0)
goto err;
return (PyObject *)self;
@ -560,6 +706,7 @@ local_traverse(localobject *self, visitproc visit, void *arg)
{
Py_VISIT(self->args);
Py_VISIT(self->kw);
Py_VISIT(self->dummies);
Py_VISIT(self->dict);
return 0;
}
@ -567,16 +714,13 @@ local_traverse(localobject *self, visitproc visit, void *arg)
static int
local_clear(localobject *self)
{
PyThreadState *tstate;
Py_CLEAR(self->args);
Py_CLEAR(self->kw);
Py_CLEAR(self->dummies);
Py_CLEAR(self->dict);
return 0;
}
static void
local_dealloc(localobject *self)
{
PyThreadState *tstate;
Py_CLEAR(self->wr_callback);
/* Remove all strong references to dummies from the thread states */
if (self->key
&& (tstate = PyThreadState_Get())
&& tstate->interp) {
@ -587,16 +731,29 @@ local_dealloc(localobject *self)
PyDict_GetItem(tstate->dict, self->key))
PyDict_DelItem(tstate->dict, self->key);
}
return 0;
}
static void
local_dealloc(localobject *self)
{
/* Weakrefs must be invalidated right now, otherwise they can be used
from code called below, which is very dangerous since Py_REFCNT(self) == 0 */
if (self->weakreflist != NULL)
PyObject_ClearWeakRefs((PyObject *) self);
PyObject_GC_UnTrack(self);
Py_XDECREF(self->key);
local_clear(self);
Py_XDECREF(self->key);
Py_TYPE(self)->tp_free((PyObject*)self);
}
/* Returns a borrowed reference to the local dict, creating it if necessary */
static PyObject *
_ldict(localobject *self)
{
PyObject *tdict, *ldict;
PyObject *tdict, *ldict, *dummy;
tdict = PyThreadState_GetDict();
if (tdict == NULL) {
@ -605,22 +762,11 @@ _ldict(localobject *self)
return NULL;
}
ldict = PyDict_GetItem(tdict, self->key);
if (ldict == NULL) {
ldict = PyDict_New(); /* we own ldict */
if (ldict == NULL)
dummy = PyDict_GetItem(tdict, self->key);
if (dummy == NULL) {
if (_local_create_dummy(self) < 0)
return NULL;
else {
int i = PyDict_SetItem(tdict, self->key, ldict);
Py_DECREF(ldict); /* now ldict is borrowed */
if (i < 0)
return NULL;
}
Py_CLEAR(self->dict);
Py_INCREF(ldict);
self->dict = ldict; /* still borrowed */
ldict = self->dict;
if (Py_TYPE(self)->tp_init != PyBaseObject_Type.tp_init &&
Py_TYPE(self)->tp_init((PyObject*)self,
@ -631,14 +777,17 @@ _ldict(localobject *self)
PyDict_DelItem(tdict, self->key);
return NULL;
}
}
else {
assert(Py_TYPE(dummy) == &localdummytype);
ldict = ((localdummyobject *) dummy)->localdict;
}
/* The call to tp_init above may have caused another thread to run.
Install our ldict again. */
if (self->dict != ldict) {
Py_CLEAR(self->dict);
Py_INCREF(ldict);
Py_CLEAR(self->dict);
self->dict = ldict;
}
@ -660,13 +809,10 @@ local_setattro(localobject *self, PyObject *name, PyObject *v)
static PyObject *
local_getdict(localobject *self, void *closure)
{
if (self->dict == NULL) {
PyErr_SetString(PyExc_AttributeError, "__dict__");
return NULL;
}
Py_INCREF(self->dict);
return self->dict;
PyObject *ldict;
ldict = _ldict(self);
Py_XINCREF(ldict);
return ldict;
}
static PyGetSetDef local_getset[] = {
@ -697,12 +843,13 @@ static PyTypeObject localtype = {
/* tp_getattro */ (getattrofunc)local_getattro,
/* tp_setattro */ (setattrofunc)local_setattro,
/* tp_as_buffer */ 0,
/* tp_flags */ Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE,
/* tp_flags */ Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE
| Py_TPFLAGS_HAVE_GC,
/* tp_doc */ "Thread-local data",
/* tp_traverse */ (traverseproc)local_traverse,
/* tp_clear */ (inquiry)local_clear,
/* tp_richcompare */ 0,
/* tp_weaklistoffset */ 0,
/* tp_weaklistoffset */ offsetof(localobject, weakreflist),
/* tp_iter */ 0,
/* tp_iternext */ 0,
/* tp_methods */ 0,
@ -743,6 +890,36 @@ local_getattro(localobject *self, PyObject *name)
return value;
}
/* Called when a dummy is destroyed. */
static PyObject *
_localdummy_destroyed(PyObject *localweakref, PyObject *dummyweakref)
{
PyObject *obj;
localobject *self;
assert(PyWeakref_CheckRef(localweakref));
obj = PyWeakref_GET_OBJECT(localweakref);
if (obj == Py_None)
Py_RETURN_NONE;
Py_INCREF(obj);
assert(PyObject_TypeCheck(obj, &localtype));
/* If the thread-local object is still alive and not being cleared,
remove the corresponding local dict */
self = (localobject *) obj;
if (self->dummies != NULL) {
PyObject *ldict;
ldict = PyDict_GetItem(self->dummies, dummyweakref);
if (ldict != NULL) {
if (ldict == self->dict)
Py_CLEAR(self->dict);
PyDict_DelItem(self->dummies, dummyweakref);
}
if (PyErr_Occurred())
PyErr_WriteUnraisable(obj);
}
Py_DECREF(obj);
Py_RETURN_NONE;
}
/* Module functions */
struct bootstate {
@ -1063,6 +1240,8 @@ PyInit__thread(void)
PyObject *m, *d, *timeout_max;
/* Initialize types: */
if (PyType_Ready(&localdummytype) < 0)
return NULL;
if (PyType_Ready(&localtype) < 0)
return NULL;
if (PyType_Ready(&Locktype) < 0)