Modules/_threadmodule.c

/* Thread module */
/* Interface to Sjoerd's portable C thread library */

#include "Python.h"
#include "structmember.h" /* offsetof */

#ifndef WITH_THREAD
#error "Error!  The rest of Python is not compiled with thread support."
#error "Rerun configure, adding a --with-threads option."
#error "Then run `make clean' followed by `make'."
#endif

#include "pythread.h"

static PyObject *ThreadError;
static long nb_threads = 0;
static PyObject *str_dict;

_Py_IDENTIFIER(stderr);

/* Lock objects */

typedef struct {
    PyObject_HEAD
    PyThread_type_lock lock_lock;
    PyObject *in_weakreflist;
    char locked; /* for sanity checking */
} lockobject;

static void
lock_dealloc(lockobject *self)
{
    if (self->in_weakreflist != NULL)
        PyObject_ClearWeakRefs((PyObject *) self);
    if (self->lock_lock != NULL) {
        /* Unlock the lock so it's safe to free it */
        if (self->locked)
            PyThread_release_lock(self->lock_lock);
        PyThread_free_lock(self->lock_lock);
    }
    PyObject_Del(self);
}

/* Helper to acquire an interruptible lock with a timeout.  If the lock acquire
 * is interrupted, signal handlers are run, and if they raise an exception,
 * PY_LOCK_INTR is returned.  Otherwise, PY_LOCK_ACQUIRED or PY_LOCK_FAILURE
 * are returned, depending on whether the lock can be acquired withing the
 * timeout.
 */
static PyLockStatus
acquire_timed(PyThread_type_lock lock, _PyTime_t timeout)
{
    PyLockStatus r;
    _PyTime_t endtime = 0;
    _PyTime_t microseconds;

    if (timeout > 0)
        endtime = _PyTime_GetMonotonicClock() + timeout;

    do {
        microseconds = _PyTime_AsMicroseconds(timeout, _PyTime_ROUND_CEILING);

        /* first a simple non-blocking try without releasing the GIL */
        r = PyThread_acquire_lock_timed(lock, 0, 0);
        if (r == PY_LOCK_FAILURE && microseconds != 0) {
            Py_BEGIN_ALLOW_THREADS
            r = PyThread_acquire_lock_timed(lock, microseconds, 1);
            Py_END_ALLOW_THREADS
        }

        if (r == PY_LOCK_INTR) {
            /* Run signal handlers if we were interrupted.  Propagate
             * exceptions from signal handlers, such as KeyboardInterrupt, by
             * passing up PY_LOCK_INTR.  */
            if (Py_MakePendingCalls() < 0) {
                return PY_LOCK_INTR;
            }

            /* If we're using a timeout, recompute the timeout after processing
             * signals, since those can take time.  */
            if (timeout > 0) {
                timeout = endtime - _PyTime_GetMonotonicClock();

                /* Check for negative values, since those mean block forever.
                 */
                if (timeout < 0) {
                    r = PY_LOCK_FAILURE;
                }
            }
        }
    } while (r == PY_LOCK_INTR);  /* Retry if we were interrupted. */

    return r;
}

static int
lock_acquire_parse_args(PyObject *args, PyObject *kwds,
                        _PyTime_t *timeout)
{
    char *kwlist[] = {"blocking", "timeout", NULL};
    int blocking = 1;
    PyObject *timeout_obj = NULL;
    const _PyTime_t unset_timeout = _PyTime_FromSeconds(-1);

    *timeout = unset_timeout ;

    if (!PyArg_ParseTupleAndKeywords(args, kwds, "|iO:acquire", kwlist,
                                     &blocking, &timeout_obj))
        return -1;

    if (timeout_obj
        && _PyTime_FromSecondsObject(timeout,
                                     timeout_obj, _PyTime_ROUND_CEILING) < 0)
        return -1;

    if (!blocking && *timeout != unset_timeout ) {
        PyErr_SetString(PyExc_ValueError,
                        "can't specify a timeout for a non-blocking call");
        return -1;
    }
    if (*timeout < 0 && *timeout != unset_timeout) {
        PyErr_SetString(PyExc_ValueError,
                        "timeout value must be positive");
        return -1;
    }
    if (!blocking)
        *timeout = 0;
    else if (*timeout != unset_timeout) {
        _PyTime_t microseconds;

        microseconds = _PyTime_AsMicroseconds(*timeout, _PyTime_ROUND_CEILING);
        if (microseconds >= PY_TIMEOUT_MAX) {
            PyErr_SetString(PyExc_OverflowError,
                            "timeout value is too large");
            return -1;
        }
    }
    return 0;
}

static PyObject *
lock_PyThread_acquire_lock(lockobject *self, PyObject *args, PyObject *kwds)
{
    _PyTime_t timeout;
    PyLockStatus r;

    if (lock_acquire_parse_args(args, kwds, &timeout) < 0)
        return NULL;

    r = acquire_timed(self->lock_lock, timeout);
    if (r == PY_LOCK_INTR) {
        return NULL;
    }

    if (r == PY_LOCK_ACQUIRED)
        self->locked = 1;
    return PyBool_FromLong(r == PY_LOCK_ACQUIRED);
}

PyDoc_STRVAR(acquire_doc,
"acquire([wait]) -> bool\n\
(acquire_lock() is an obsolete synonym)\n\
\n\
Lock the lock.  Without argument, this blocks if the lock is already\n\
locked (even by the same thread), waiting for another thread to release\n\
the lock, and return True once the lock is acquired.\n\
With an argument, this will only block if the argument is true,\n\
and the return value reflects whether the lock is acquired.\n\
The blocking operation is interruptible.");

static PyObject *
lock_PyThread_release_lock(lockobject *self)
{
    /* Sanity check: the lock must be locked */
    if (!self->locked) {
        PyErr_SetString(ThreadError, "release unlocked lock");
        return NULL;
    }

    PyThread_release_lock(self->lock_lock);
    self->locked = 0;
    Py_INCREF(Py_None);
    return Py_None;
}

PyDoc_STRVAR(release_doc,
"release()\n\
(release_lock() is an obsolete synonym)\n\
\n\
Release the lock, allowing another thread that is blocked waiting for\n\
the lock to acquire the lock.  The lock must be in the locked state,\n\
but it needn't be locked by the same thread that unlocks it.");

static PyObject *
lock_locked_lock(lockobject *self)
{
    return PyBool_FromLong((long)self->locked);
}

PyDoc_STRVAR(locked_doc,
"locked() -> bool\n\
(locked_lock() is an obsolete synonym)\n\
\n\
Return whether the lock is in the locked state.");

static PyObject *
lock_repr(lockobject *self)
{
    return PyUnicode_FromFormat("<%s %s object at %p>",
        self->locked ? "locked" : "unlocked", Py_TYPE(self)->tp_name, self);
}

static PyMethodDef lock_methods[] = {
    {"acquire_lock", (PyCFunction)lock_PyThread_acquire_lock,
     METH_VARARGS | METH_KEYWORDS, acquire_doc},
    {"acquire",      (PyCFunction)lock_PyThread_acquire_lock,
     METH_VARARGS | METH_KEYWORDS, acquire_doc},
    {"release_lock", (PyCFunction)lock_PyThread_release_lock,
     METH_NOARGS, release_doc},
    {"release",      (PyCFunction)lock_PyThread_release_lock,
     METH_NOARGS, release_doc},
    {"locked_lock",  (PyCFunction)lock_locked_lock,
     METH_NOARGS, locked_doc},
    {"locked",       (PyCFunction)lock_locked_lock,
     METH_NOARGS, locked_doc},
    {"__enter__",    (PyCFunction)lock_PyThread_acquire_lock,
     METH_VARARGS | METH_KEYWORDS, acquire_doc},
    {"__exit__",    (PyCFunction)lock_PyThread_release_lock,
     METH_VARARGS, release_doc},
    {NULL,           NULL}              /* sentinel */
};

static PyTypeObject Locktype = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "_thread.lock",                     /*tp_name*/
    sizeof(lockobject),                 /*tp_size*/
    0,                                  /*tp_itemsize*/
    /* methods */
    (destructor)lock_dealloc,           /*tp_dealloc*/
    0,                                  /*tp_print*/
    0,                                  /*tp_getattr*/
    0,                                  /*tp_setattr*/
    0,                                  /*tp_reserved*/
    (reprfunc)lock_repr,                /*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*/
    Py_TPFLAGS_DEFAULT,                 /*tp_flags*/
    0,                                  /*tp_doc*/
    0,                                  /*tp_traverse*/
    0,                                  /*tp_clear*/
    0,                                  /*tp_richcompare*/
    offsetof(lockobject, in_weakreflist), /*tp_weaklistoffset*/
    0,                                  /*tp_iter*/
    0,                                  /*tp_iternext*/
    lock_methods,                       /*tp_methods*/
};

/* Recursive lock objects */

typedef struct {
    PyObject_HEAD
    PyThread_type_lock rlock_lock;
    long rlock_owner;
    unsigned long rlock_count;
    PyObject *in_weakreflist;
} rlockobject;

static void
rlock_dealloc(rlockobject *self)
{
    if (self->in_weakreflist != NULL)
        PyObject_ClearWeakRefs((PyObject *) self);
    /* self->rlock_lock can be NULL if PyThread_allocate_lock() failed
       in rlock_new() */
    if (self->rlock_lock != NULL) {
        /* Unlock the lock so it's safe to free it */
        if (self->rlock_count > 0)
            PyThread_release_lock(self->rlock_lock);

        PyThread_free_lock(self->rlock_lock);
    }
    Py_TYPE(self)->tp_free(self);
}

static PyObject *
rlock_acquire(rlockobject *self, PyObject *args, PyObject *kwds)
{
    _PyTime_t timeout;
    long tid;
    PyLockStatus r = PY_LOCK_ACQUIRED;

    if (lock_acquire_parse_args(args, kwds, &timeout) < 0)
        return NULL;

    tid = PyThread_get_thread_ident();
    if (self->rlock_count > 0 && tid == self->rlock_owner) {
        unsigned long count = self->rlock_count + 1;
        if (count <= self->rlock_count) {
            PyErr_SetString(PyExc_OverflowError,
                            "Internal lock count overflowed");
            return NULL;
        }
        self->rlock_count = count;
        Py_RETURN_TRUE;
    }
    r = acquire_timed(self->rlock_lock, timeout);
    if (r == PY_LOCK_ACQUIRED) {
        assert(self->rlock_count == 0);
        self->rlock_owner = tid;
        self->rlock_count = 1;
    }
    else if (r == PY_LOCK_INTR) {
        return NULL;
    }

    return PyBool_FromLong(r == PY_LOCK_ACQUIRED);
}

PyDoc_STRVAR(rlock_acquire_doc,
"acquire(blocking=True) -> bool\n\
\n\
Lock the lock.  `blocking` indicates whether we should wait\n\
for the lock to be available or not.  If `blocking` is False\n\
and another thread holds the lock, the method will return False\n\
immediately.  If `blocking` is True and another thread holds\n\
the lock, the method will wait for the lock to be released,\n\
take it and then return True.\n\
(note: the blocking operation is interruptible.)\n\
\n\
In all other cases, the method will return True immediately.\n\
Precisely, if the current thread already holds the lock, its\n\
internal counter is simply incremented. If nobody holds the lock,\n\
the lock is taken and its internal counter initialized to 1.");

static PyObject *
rlock_release(rlockobject *self)
{
    long tid = PyThread_get_thread_ident();

    if (self->rlock_count == 0 || self->rlock_owner != tid) {
        PyErr_SetString(PyExc_RuntimeError,
                        "cannot release un-acquired lock");
        return NULL;
    }
    if (--self->rlock_count == 0) {
        self->rlock_owner = 0;
        PyThread_release_lock(self->rlock_lock);
    }
    Py_RETURN_NONE;
}

PyDoc_STRVAR(rlock_release_doc,
"release()\n\
\n\
Release the lock, allowing another thread that is blocked waiting for\n\
the lock to acquire the lock.  The lock must be in the locked state,\n\
and must be locked by the same thread that unlocks it; otherwise a\n\
`RuntimeError` is raised.\n\
\n\
Do note that if the lock was acquire()d several times in a row by the\n\
current thread, release() needs to be called as many times for the lock\n\
to be available for other threads.");

static PyObject *
rlock_acquire_restore(rlockobject *self, PyObject *args)
{
    long owner;
    unsigned long count;
    int r = 1;

    if (!PyArg_ParseTuple(args, "(kl):_acquire_restore", &count, &owner))
        return NULL;

    if (!PyThread_acquire_lock(self->rlock_lock, 0)) {
        Py_BEGIN_ALLOW_THREADS
        r = PyThread_acquire_lock(self->rlock_lock, 1);
        Py_END_ALLOW_THREADS
    }
    if (!r) {
        PyErr_SetString(ThreadError, "couldn't acquire lock");
        return NULL;
    }
    assert(self->rlock_count == 0);
    self->rlock_owner = owner;
    self->rlock_count = count;
    Py_RETURN_NONE;
}

PyDoc_STRVAR(rlock_acquire_restore_doc,
"_acquire_restore(state) -> None\n\
\n\
For internal use by `threading.Condition`.");

static PyObject *
rlock_release_save(rlockobject *self)
{
    long owner;
    unsigned long count;

    if (self->rlock_count == 0) {
        PyErr_SetString(PyExc_RuntimeError,
                        "cannot release un-acquired lock");
        return NULL;
    }

    owner = self->rlock_owner;
    count = self->rlock_count;
    self->rlock_count = 0;
    self->rlock_owner = 0;
    PyThread_release_lock(self->rlock_lock);
    return Py_BuildValue("kl", count, owner);
}

PyDoc_STRVAR(rlock_release_save_doc,
"_release_save() -> tuple\n\
\n\
For internal use by `threading.Condition`.");


static PyObject *
rlock_is_owned(rlockobject *self)
{
    long tid = PyThread_get_thread_ident();

    if (self->rlock_count > 0 && self->rlock_owner == tid) {
        Py_RETURN_TRUE;
    }
    Py_RETURN_FALSE;
}

PyDoc_STRVAR(rlock_is_owned_doc,
"_is_owned() -> bool\n\
\n\
For internal use by `threading.Condition`.");

static PyObject *
rlock_new(PyTypeObject *type, PyObject *args, PyObject *kwds)
{
    rlockobject *self;

    self = (rlockobject *) type->tp_alloc(type, 0);
    if (self != NULL) {
        self->in_weakreflist = NULL;
        self->rlock_owner = 0;
        self->rlock_count = 0;

        self->rlock_lock = PyThread_allocate_lock();
        if (self->rlock_lock == NULL) {
            Py_DECREF(self);
            PyErr_SetString(ThreadError, "can't allocate lock");
            return NULL;
        }
    }

    return (PyObject *) self;
}

static PyObject *
rlock_repr(rlockobject *self)
{
    return PyUnicode_FromFormat("<%s %s object owner=%ld count=%lu at %p>",
        self->rlock_count ? "locked" : "unlocked",
        Py_TYPE(self)->tp_name, self->rlock_owner,
        self->rlock_count, self);
}


static PyMethodDef rlock_methods[] = {
    {"acquire",      (PyCFunction)rlock_acquire,
     METH_VARARGS | METH_KEYWORDS, rlock_acquire_doc},
    {"release",      (PyCFunction)rlock_release,
     METH_NOARGS, rlock_release_doc},
    {"_is_owned",     (PyCFunction)rlock_is_owned,
     METH_NOARGS, rlock_is_owned_doc},
    {"_acquire_restore", (PyCFunction)rlock_acquire_restore,
     METH_VARARGS, rlock_acquire_restore_doc},
    {"_release_save", (PyCFunction)rlock_release_save,
     METH_NOARGS, rlock_release_save_doc},
    {"__enter__",    (PyCFunction)rlock_acquire,
     METH_VARARGS | METH_KEYWORDS, rlock_acquire_doc},
    {"__exit__",    (PyCFunction)rlock_release,
     METH_VARARGS, rlock_release_doc},
    {NULL,           NULL}              /* sentinel */
};


static PyTypeObject RLocktype = {
    PyVarObject_HEAD_INIT(&PyType_Type, 0)
    "_thread.RLock",                    /*tp_name*/
    sizeof(rlockobject),                /*tp_size*/
    0,                                  /*tp_itemsize*/
    /* methods */
    (destructor)rlock_dealloc,          /*tp_dealloc*/
    0,                                  /*tp_print*/
    0,                                  /*tp_getattr*/
    0,                                  /*tp_setattr*/
    0,                                  /*tp_reserved*/
    (reprfunc)rlock_repr,               /*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*/
    Py_TPFLAGS_DEFAULT | Py_TPFLAGS_BASETYPE, /* tp_flags */
    0,                                  /*tp_doc*/
    0,                                  /*tp_traverse*/
    0,                                  /*tp_clear*/
    0,                                  /*tp_richcompare*/
    offsetof(rlockobject, in_weakreflist), /*tp_weaklistoffset*/
    0,                                  /*tp_iter*/
    0,                                  /*tp_iternext*/
    rlock_methods,                      /*tp_methods*/
    0,                                  /* tp_members */
    0,                                  /* tp_getset */
    0,                                  /* tp_base */
    0,                                  /* tp_dict */
    0,                                  /* tp_descr_get */
    0,                                  /* tp_descr_set */
    0,                                  /* tp_dictoffset */
    0,                                  /* tp_init */
    PyType_GenericAlloc,                /* tp_alloc */
    rlock_new                           /* tp_new */
};

static lockobject *
newlockobject(void)
{
    lockobject *self;
    self = PyObject_New(lockobject, &Locktype);
    if (self == NULL)
        return NULL;
    self->lock_lock = PyThread_allocate_lock();
    self->locked = 0;
    self->in_weakreflist = NULL;
    if (self->lock_lock == NULL) {
        Py_DECREF(self);
        PyErr_SetString(ThreadError, "can't allocate lock");
        return NULL;
    }
    return self;
}

/* Thread-local objects */

#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;
    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.
   Returns a borrowed reference of the corresponding local dict */
static PyObject *
_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_DECREF(ldict);
    return ldict;

err:
    Py_XDECREF(ldict);
    Py_XDECREF(wr);
    Py_XDECREF(dummy);
    return NULL;
}

static PyObject *
local_new(PyTypeObject *type, PyObject *args, PyObject *kw)
{
    localobject *self;
    PyObject *wr;
    static PyMethodDef wr_callback_def = {
        "_localdummy_destroyed", (PyCFunction) _localdummy_destroyed, METH_O
    };

    if (type->tp_init == PyBaseObject_Type.tp_init) {
        int rc = 0;
        if (args != NULL)
            rc = PyObject_IsTrue(args);
        if (rc == 0 && kw != NULL)
            rc = PyObject_IsTrue(kw);
        if (rc != 0) {
            if (rc > 0)
                PyErr_SetString(PyExc_TypeError,
                          "Initialization arguments are not supported");
            return NULL;
        }
    }

    self = (localobject *)type->tp_alloc(type, 0);
    if (self == NULL)
        return NULL;

    Py_XINCREF(args);
    self->args = args;
    Py_XINCREF(kw);
    self->kw = kw;
    self->key = PyUnicode_FromFormat("thread.local.%p", self);
    if (self->key == NULL)
        goto err;

    self->dummies = PyDict_New();
    if (self->dummies == NULL)
        goto err;

    /* 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_NewEx(&wr_callback_def, wr, NULL);
    Py_DECREF(wr);
    if (self->wr_callback == NULL)
        goto err;

    if (_local_create_dummy(self) == NULL)
        goto err;

    return (PyObject *)self;

  err:
    Py_DECREF(self);
    return NULL;
}

static int
local_traverse(localobject *self, visitproc visit, void *arg)
{
    Py_VISIT(self->args);
    Py_VISIT(self->kw);
    Py_VISIT(self->dummies);
    return 0;
}

static int
local_clear(localobject *self)
{
    PyThreadState *tstate;
    Py_CLEAR(self->args);
    Py_CLEAR(self->kw);
    Py_CLEAR(self->dummies);
    Py_CLEAR(self->wr_callback);
    /* Remove all strong references to dummies from the thread states */
    if (self->key
        && (tstate = PyThreadState_Get())
        && tstate->interp) {
        for(tstate = PyInterpreterState_ThreadHead(tstate->interp);
            tstate;
            tstate = PyThreadState_Next(tstate))
            if (tstate->dict &&
                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);

    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, *dummy;

    tdict = PyThreadState_GetDict();
    if (tdict == NULL) {
        PyErr_SetString(PyExc_SystemError,
                        "Couldn't get thread-state dictionary");
        return NULL;
    }

    dummy = PyDict_GetItem(tdict, self->key);
    if (dummy == NULL) {
        ldict = _local_create_dummy(self);
        if (ldict == NULL)
            return NULL;

        if (Py_TYPE(self)->tp_init != PyBaseObject_Type.tp_init &&
            Py_TYPE(self)->tp_init((PyObject*)self,
                                   self->args, self->kw) < 0) {
            /* we need to get rid of ldict from thread so
               we create a new one the next time we do an attr
               access */
            PyDict_DelItem(tdict, self->key);
            return NULL;
        }
    }
    else {
        assert(Py_TYPE(dummy) == &localdummytype);
        ldict = ((localdummyobject *) dummy)->localdict;
    }

    return ldict;
}

static int
local_setattro(localobject *self, PyObject *name, PyObject *v)
{
    PyObject *ldict;
    int r;

    ldict = _ldict(self);
    if (ldict == NULL)
        return -1;

    r = PyObject_RichCompareBool(name, str_dict, Py_EQ);
    if (r == 1) {
        PyErr_Format(PyExc_AttributeError,
                     "'%.50s' object attribute '%U' is read-only",
                     Py_TYPE(self)->tp_name, name);
        return -1;
    }
    if (r == -1)
        return -1;

    return _PyObject_GenericSetAttrWithDict((PyObject *)self, name, v, ldict);
}

static PyObject *local_getattro(localobject *, PyObject *);

static PyTypeObject localtype = {
    PyVarObject_HEAD_INIT(NULL, 0)
    /* tp_name           */ "_thread._local",
    /* tp_basicsize      */ sizeof(localobject),
    /* tp_itemsize       */ 0,
    /* tp_dealloc        */ (destructor)local_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       */ (getattrofunc)local_getattro,
    /* tp_setattro       */ (setattrofunc)local_setattro,
    /* tp_as_buffer      */ 0,
    /* 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 */ offsetof(localobject, weakreflist),
    /* tp_iter           */ 0,
    /* tp_iternext       */ 0,
    /* tp_methods        */ 0,
    /* tp_members        */ 0,
    /* tp_getset         */ 0,
    /* tp_base           */ 0,
    /* tp_dict           */ 0, /* internal use */
    /* tp_descr_get      */ 0,
    /* tp_descr_set      */ 0,
    /* tp_dictoffset     */ 0,
    /* tp_init           */ 0,
    /* tp_alloc          */ 0,
    /* tp_new            */ local_new,
    /* tp_free           */ 0, /* Low-level free-mem routine */
    /* tp_is_gc          */ 0, /* For PyObject_IS_GC */
};

static PyObject *
local_getattro(localobject *self, PyObject *name)
{
    PyObject *ldict, *value;
    int r;

    ldict = _ldict(self);
    if (ldict == NULL)
        return NULL;

    r = PyObject_RichCompareBool(name, str_dict, Py_EQ);
    if (r == 1) {
        Py_INCREF(ldict);
        return ldict;
    }
    if (r == -1)
        return NULL;

    if (Py_TYPE(self) != &localtype)
        /* use generic lookup for subtypes */
        return _PyObject_GenericGetAttrWithDict((PyObject *)self, name, ldict);

    /* Optimization: just look in dict ourselves */
    value = PyDict_GetItem(ldict, name);
    if (value == NULL)
        /* Fall back on generic to get __class__ and __dict__ */
        return _PyObject_GenericGetAttrWithDict((PyObject *)self, name, ldict);

    Py_INCREF(value);
    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) {
            PyDict_DelItem(self->dummies, dummyweakref);
        }
        if (PyErr_Occurred())
            PyErr_WriteUnraisable(obj);
    }
    Py_DECREF(obj);
    Py_RETURN_NONE;
}

/* Module functions */

struct bootstate {
    PyInterpreterState *interp;
    PyObject *func;
    PyObject *args;
    PyObject *keyw;
    PyThreadState *tstate;
};

static void
t_bootstrap(void *boot_raw)
{
    struct bootstate *boot = (struct bootstate *) boot_raw;
    PyThreadState *tstate;
    PyObject *res;

    tstate = boot->tstate;
    tstate->thread_id = PyThread_get_thread_ident();
    _PyThreadState_Init(tstate);
    PyEval_AcquireThread(tstate);
    nb_threads++;
    res = PyEval_CallObjectWithKeywords(
        boot->func, boot->args, boot->keyw);
    if (res == NULL) {
        if (PyErr_ExceptionMatches(PyExc_SystemExit))
            PyErr_Clear();
        else {
            PyObject *file;
            PyObject *exc, *value, *tb;
            PySys_WriteStderr(
                "Unhandled exception in thread started by ");
            PyErr_Fetch(&exc, &value, &tb);
            file = _PySys_GetObjectId(&PyId_stderr);
            if (file != NULL && file != Py_None)
                PyFile_WriteObject(boot->func, file, 0);
            else
                PyObject_Print(boot->func, stderr, 0);
            PySys_WriteStderr("\n");
            PyErr_Restore(exc, value, tb);
            PyErr_PrintEx(0);
        }
    }
    else
        Py_DECREF(res);
    Py_DECREF(boot->func);
    Py_DECREF(boot->args);
    Py_XDECREF(boot->keyw);
    PyMem_DEL(boot_raw);
    nb_threads--;
    PyThreadState_Clear(tstate);
    PyThreadState_DeleteCurrent();
    PyThread_exit_thread();
}

static PyObject *
thread_PyThread_start_new_thread(PyObject *self, PyObject *fargs)
{
    PyObject *func, *args, *keyw = NULL;
    struct bootstate *boot;
    long ident;

    if (!PyArg_UnpackTuple(fargs, "start_new_thread", 2, 3,
                           &func, &args, &keyw))
        return NULL;
    if (!PyCallable_Check(func)) {
        PyErr_SetString(PyExc_TypeError,
                        "first arg must be callable");
        return NULL;
    }
    if (!PyTuple_Check(args)) {
        PyErr_SetString(PyExc_TypeError,
                        "2nd arg must be a tuple");
        return NULL;
    }
    if (keyw != NULL && !PyDict_Check(keyw)) {
        PyErr_SetString(PyExc_TypeError,
                        "optional 3rd arg must be a dictionary");
        return NULL;
    }
    boot = PyMem_NEW(struct bootstate, 1);
    if (boot == NULL)
        return PyErr_NoMemory();
    boot->interp = PyThreadState_GET()->interp;
    boot->func = func;
    boot->args = args;
    boot->keyw = keyw;
    boot->tstate = _PyThreadState_Prealloc(boot->interp);
    if (boot->tstate == NULL) {
        PyMem_DEL(boot);
        return PyErr_NoMemory();
    }
    Py_INCREF(func);
    Py_INCREF(args);
    Py_XINCREF(keyw);
    PyEval_InitThreads(); /* Start the interpreter's thread-awareness */
    ident = PyThread_start_new_thread(t_bootstrap, (void*) boot);
    if (ident == -1) {
        PyErr_SetString(ThreadError, "can't start new thread");
        Py_DECREF(func);
        Py_DECREF(args);
        Py_XDECREF(keyw);
        PyThreadState_Clear(boot->tstate);
        PyMem_DEL(boot);
        return NULL;
    }
    return PyLong_FromLong(ident);
}

PyDoc_STRVAR(start_new_doc,
"start_new_thread(function, args[, kwargs])\n\
(start_new() is an obsolete synonym)\n\
\n\
Start a new thread and return its identifier.  The thread will call the\n\
function with positional arguments from the tuple args and keyword arguments\n\
taken from the optional dictionary kwargs.  The thread exits when the\n\
function returns; the return value is ignored.  The thread will also exit\n\
when the function raises an unhandled exception; a stack trace will be\n\
printed unless the exception is SystemExit.\n");

static PyObject *
thread_PyThread_exit_thread(PyObject *self)
{
    PyErr_SetNone(PyExc_SystemExit);
    return NULL;
}

PyDoc_STRVAR(exit_doc,
"exit()\n\
(exit_thread() is an obsolete synonym)\n\
\n\
This is synonymous to ``raise SystemExit''.  It will cause the current\n\
thread to exit silently unless the exception is caught.");

static PyObject *
thread_PyThread_interrupt_main(PyObject * self)
{
    PyErr_SetInterrupt();
    Py_INCREF(Py_None);
    return Py_None;
}

PyDoc_STRVAR(interrupt_doc,
"interrupt_main()\n\
\n\
Raise a KeyboardInterrupt in the main thread.\n\
A subthread can use this function to interrupt the main thread."
);

static lockobject *newlockobject(void);

static PyObject *
thread_PyThread_allocate_lock(PyObject *self)
{
    return (PyObject *) newlockobject();
}

PyDoc_STRVAR(allocate_doc,
"allocate_lock() -> lock object\n\
(allocate() is an obsolete synonym)\n\
\n\
Create a new lock object.  See help(LockType) for information about locks.");

static PyObject *
thread_get_ident(PyObject *self)
{
    long ident;
    ident = PyThread_get_thread_ident();
    if (ident == -1) {
        PyErr_SetString(ThreadError, "no current thread ident");
        return NULL;
    }
    return PyLong_FromLong(ident);
}

PyDoc_STRVAR(get_ident_doc,
"get_ident() -> integer\n\
\n\
Return a non-zero integer that uniquely identifies the current thread\n\
amongst other threads that exist simultaneously.\n\
This may be used to identify per-thread resources.\n\
Even though on some platforms threads identities may appear to be\n\
allocated consecutive numbers starting at 1, this behavior should not\n\
be relied upon, and the number should be seen purely as a magic cookie.\n\
A thread's identity may be reused for another thread after it exits.");

static PyObject *
thread__count(PyObject *self)
{
    return PyLong_FromLong(nb_threads);
}

PyDoc_STRVAR(_count_doc,
"_count() -> integer\n\
\n\
\
Return the number of currently running Python threads, excluding \n\
the main thread. The returned number comprises all threads created\n\
through `start_new_thread()` as well as `threading.Thread`, and not\n\
yet finished.\n\
\n\
This function is meant for internal and specialized purposes only.\n\
In most applications `threading.enumerate()` should be used instead.");

static void
release_sentinel(void *wr)
{
    /* Tricky: this function is called when the current thread state
       is being deleted.  Therefore, only simple C code can safely
       execute here. */
    PyObject *obj = PyWeakref_GET_OBJECT(wr);
    lockobject *lock;
    if (obj != Py_None) {
        assert(Py_TYPE(obj) == &Locktype);
        lock = (lockobject *) obj;
        if (lock->locked) {
            PyThread_release_lock(lock->lock_lock);
            lock->locked = 0;
        }
    }
    /* Deallocating a weakref with a NULL callback only calls
       PyObject_GC_Del(), which can't call any Python code. */
    Py_DECREF(wr);
}

static PyObject *
thread__set_sentinel(PyObject *self)
{
    PyObject *wr;
    PyThreadState *tstate = PyThreadState_Get();
    lockobject *lock;

    if (tstate->on_delete_data != NULL) {
        /* We must support the re-creation of the lock from a
           fork()ed child. */
        assert(tstate->on_delete == &release_sentinel);
        wr = (PyObject *) tstate->on_delete_data;
        tstate->on_delete = NULL;
        tstate->on_delete_data = NULL;
        Py_DECREF(wr);
    }
    lock = newlockobject();
    if (lock == NULL)
        return NULL;
    /* The lock is owned by whoever called _set_sentinel(), but the weakref
       hangs to the thread state. */
    wr = PyWeakref_NewRef((PyObject *) lock, NULL);
    if (wr == NULL) {
        Py_DECREF(lock);
        return NULL;
    }
    tstate->on_delete_data = (void *) wr;
    tstate->on_delete = &release_sentinel;
    return (PyObject *) lock;
}

PyDoc_STRVAR(_set_sentinel_doc,
"_set_sentinel() -> lock\n\
\n\
Set a sentinel lock that will be released when the current thread\n\
state is finalized (after it is untied from the interpreter).\n\
\n\
This is a private API for the threading module.");

static PyObject *
thread_stack_size(PyObject *self, PyObject *args)
{
    size_t old_size;
    Py_ssize_t new_size = 0;
    int rc;

    if (!PyArg_ParseTuple(args, "|n:stack_size", &new_size))
        return NULL;

    if (new_size < 0) {
        PyErr_SetString(PyExc_ValueError,
                        "size must be 0 or a positive value");
        return NULL;
    }

    old_size = PyThread_get_stacksize();

    rc = PyThread_set_stacksize((size_t) new_size);
    if (rc == -1) {
        PyErr_Format(PyExc_ValueError,
                     "size not valid: %zd bytes",
                     new_size);
        return NULL;
    }
    if (rc == -2) {
        PyErr_SetString(ThreadError,
                        "setting stack size not supported");
        return NULL;
    }

    return PyLong_FromSsize_t((Py_ssize_t) old_size);
}

PyDoc_STRVAR(stack_size_doc,
"stack_size([size]) -> size\n\
\n\
Return the thread stack size used when creating new threads.  The\n\
optional size argument specifies the stack size (in bytes) to be used\n\
for subsequently created threads, and must be 0 (use platform or\n\
configured default) or a positive integer value of at least 32,768 (32k).\n\
If changing the thread stack size is unsupported, a ThreadError\n\
exception is raised.  If the specified size is invalid, a ValueError\n\
exception is raised, and the stack size is unmodified.  32k bytes\n\
 currently the minimum supported stack size value to guarantee\n\
sufficient stack space for the interpreter itself.\n\
\n\
Note that some platforms may have particular restrictions on values for\n\
the stack size, such as requiring a minimum stack size larger than 32kB or\n\
requiring allocation in multiples of the system memory page size\n\
- platform documentation should be referred to for more information\n\
(4kB pages are common; using multiples of 4096 for the stack size is\n\
the suggested approach in the absence of more specific information).");

static PyMethodDef thread_methods[] = {
    {"start_new_thread",        (PyCFunction)thread_PyThread_start_new_thread,
     METH_VARARGS, start_new_doc},
    {"start_new",               (PyCFunction)thread_PyThread_start_new_thread,
     METH_VARARGS, start_new_doc},
    {"allocate_lock",           (PyCFunction)thread_PyThread_allocate_lock,
     METH_NOARGS, allocate_doc},
    {"allocate",                (PyCFunction)thread_PyThread_allocate_lock,
     METH_NOARGS, allocate_doc},
    {"exit_thread",             (PyCFunction)thread_PyThread_exit_thread,
     METH_NOARGS, exit_doc},
    {"exit",                    (PyCFunction)thread_PyThread_exit_thread,
     METH_NOARGS, exit_doc},
    {"interrupt_main",          (PyCFunction)thread_PyThread_interrupt_main,
     METH_NOARGS, interrupt_doc},
    {"get_ident",               (PyCFunction)thread_get_ident,
     METH_NOARGS, get_ident_doc},
    {"_count",                  (PyCFunction)thread__count,
     METH_NOARGS, _count_doc},
    {"stack_size",              (PyCFunction)thread_stack_size,
     METH_VARARGS, stack_size_doc},
    {"_set_sentinel",           (PyCFunction)thread__set_sentinel,
     METH_NOARGS, _set_sentinel_doc},
    {NULL,                      NULL}           /* sentinel */
};


/* Initialization function */

PyDoc_STRVAR(thread_doc,
"This module provides primitive operations to write multi-threaded programs.\n\
The 'threading' module provides a more convenient interface.");

PyDoc_STRVAR(lock_doc,
"A lock object is a synchronization primitive.  To create a lock,\n\
call the PyThread_allocate_lock() function.  Methods are:\n\
\n\
acquire() -- lock the lock, possibly blocking until it can be obtained\n\
release() -- unlock of the lock\n\
locked() -- test whether the lock is currently locked\n\
\n\
A lock is not owned by the thread that locked it; another thread may\n\
unlock it.  A thread attempting to lock a lock that it has already locked\n\
will block until another thread unlocks it.  Deadlocks may ensue.");

static struct PyModuleDef threadmodule = {
    PyModuleDef_HEAD_INIT,
    "_thread",
    thread_doc,
    -1,
    thread_methods,
    NULL,
    NULL,
    NULL,
    NULL
};


PyMODINIT_FUNC
PyInit__thread(void)
{
    PyObject *m, *d, *v;
    double time_max;
    double timeout_max;

    /* Initialize types: */
    if (PyType_Ready(&localdummytype) < 0)
        return NULL;
    if (PyType_Ready(&localtype) < 0)
        return NULL;
    if (PyType_Ready(&Locktype) < 0)
        return NULL;
    if (PyType_Ready(&RLocktype) < 0)
        return NULL;

    /* Create the module and add the functions */
    m = PyModule_Create(&threadmodule);
    if (m == NULL)
        return NULL;

    timeout_max = PY_TIMEOUT_MAX / 1000000;
    time_max = floor(_PyTime_AsSecondsDouble(_PyTime_MAX));
    timeout_max = Py_MIN(timeout_max, time_max);

    v = PyFloat_FromDouble(timeout_max);
    if (!v)
        return NULL;
    if (PyModule_AddObject(m, "TIMEOUT_MAX", v) < 0)
        return NULL;

    /* Add a symbolic constant */
    d = PyModule_GetDict(m);
    ThreadError = PyExc_RuntimeError;
    Py_INCREF(ThreadError);

    PyDict_SetItemString(d, "error", ThreadError);
    Locktype.tp_doc = lock_doc;
    Py_INCREF(&Locktype);
    PyDict_SetItemString(d, "LockType", (PyObject *)&Locktype);

    Py_INCREF(&RLocktype);
    if (PyModule_AddObject(m, "RLock", (PyObject *)&RLocktype) < 0)
        return NULL;

    Py_INCREF(&localtype);
    if (PyModule_AddObject(m, "_local", (PyObject *)&localtype) < 0)
        return NULL;

    nb_threads = 0;

    str_dict = PyUnicode_InternFromString("__dict__");
    if (str_dict == NULL)
        return NULL;

    /* Initialize the C thread library */
    PyThread_init_thread();
    return m;
}