pystate.c

/* Thread and interpreter state structures and their interfaces */

#include "Python.h"

/* --------------------------------------------------------------------------
CAUTION

Always use malloc() and free() directly in this file.  A number of these
functions are advertised as safe to call when the GIL isn't held, and in
a debug build Python redirects (e.g.) PyMem_NEW (etc) to Python's debugging
obmalloc functions.  Those aren't thread-safe (they rely on the GIL to avoid
the expense of doing their own locking).
-------------------------------------------------------------------------- */

#ifdef HAVE_DLOPEN
#ifdef HAVE_DLFCN_H
#include <dlfcn.h>
#endif
#ifndef RTLD_LAZY
#define RTLD_LAZY 1
#endif
#endif

#ifdef __cplusplus
extern "C" {
#endif

#ifdef WITH_THREAD
#include "pythread.h"
static PyThread_type_lock head_mutex = NULL; /* Protects interp->tstate_head */
#define HEAD_INIT() (void)(head_mutex || (head_mutex = PyThread_allocate_lock()))
#define HEAD_LOCK() PyThread_acquire_lock(head_mutex, WAIT_LOCK)
#define HEAD_UNLOCK() PyThread_release_lock(head_mutex)

/* The single PyInterpreterState used by this process'
   GILState implementation
*/
static PyInterpreterState *autoInterpreterState = NULL;
static int autoTLSkey = 0;
#else
#define HEAD_INIT() /* Nothing */
#define HEAD_LOCK() /* Nothing */
#define HEAD_UNLOCK() /* Nothing */
#endif

static PyInterpreterState *interp_head = NULL;

/* Assuming the current thread holds the GIL, this is the
   PyThreadState for the current thread. */
_Py_atomic_address _PyThreadState_Current = {NULL};
PyThreadFrameGetter _PyThreadState_GetFrame = NULL;

#ifdef WITH_THREAD
static void _PyGILState_NoteThreadState(PyThreadState* tstate);
#endif


PyInterpreterState *
PyInterpreterState_New(void)
{
    PyInterpreterState *interp = (PyInterpreterState *)
                                 malloc(sizeof(PyInterpreterState));

    if (interp != NULL) {
        HEAD_INIT();
#ifdef WITH_THREAD
        if (head_mutex == NULL)
            Py_FatalError("Can't initialize threads for interpreter");
#endif
        interp->modules = NULL;
        interp->modules_by_index = NULL;
        interp->sysdict = NULL;
        interp->builtins = NULL;
        interp->tstate_head = NULL;
        interp->codec_search_path = NULL;
        interp->codec_search_cache = NULL;
        interp->codec_error_registry = NULL;
        interp->codecs_initialized = 0;
        interp->fscodec_initialized = 0;
        interp->importlib = NULL;
#ifdef HAVE_DLOPEN
#ifdef RTLD_NOW
        interp->dlopenflags = RTLD_NOW;
#else
        interp->dlopenflags = RTLD_LAZY;
#endif
#endif
#ifdef WITH_TSC
        interp->tscdump = 0;
#endif

        HEAD_LOCK();
        interp->next = interp_head;
        interp_head = interp;
        HEAD_UNLOCK();
    }

    return interp;
}


void
PyInterpreterState_Clear(PyInterpreterState *interp)
{
    PyThreadState *p;
    HEAD_LOCK();
    for (p = interp->tstate_head; p != NULL; p = p->next)
        PyThreadState_Clear(p);
    HEAD_UNLOCK();
    Py_CLEAR(interp->codec_search_path);
    Py_CLEAR(interp->codec_search_cache);
    Py_CLEAR(interp->codec_error_registry);
    Py_CLEAR(interp->modules);
    Py_CLEAR(interp->modules_by_index);
    Py_CLEAR(interp->sysdict);
    Py_CLEAR(interp->builtins);
    Py_CLEAR(interp->importlib);
}


static void
zapthreads(PyInterpreterState *interp)
{
    PyThreadState *p;
    /* No need to lock the mutex here because this should only happen
       when the threads are all really dead (XXX famous last words). */
    while ((p = interp->tstate_head) != NULL) {
        PyThreadState_Delete(p);
    }
}


void
PyInterpreterState_Delete(PyInterpreterState *interp)
{
    PyInterpreterState **p;
    zapthreads(interp);
    HEAD_LOCK();
    for (p = &interp_head; ; p = &(*p)->next) {
        if (*p == NULL)
            Py_FatalError(
                "PyInterpreterState_Delete: invalid interp");
        if (*p == interp)
            break;
    }
    if (interp->tstate_head != NULL)
        Py_FatalError("PyInterpreterState_Delete: remaining threads");
    *p = interp->next;
    HEAD_UNLOCK();
    free(interp);
#ifdef WITH_THREAD
    if (interp_head == NULL && head_mutex != NULL) {
        PyThread_free_lock(head_mutex);
        head_mutex = NULL;
    }
#endif
}


/* Default implementation for _PyThreadState_GetFrame */
static struct _frame *
threadstate_getframe(PyThreadState *self)
{
    return self->frame;
}

static PyThreadState *
new_threadstate(PyInterpreterState *interp, int init)
{
    PyThreadState *tstate = (PyThreadState *)malloc(sizeof(PyThreadState));

    if (_PyThreadState_GetFrame == NULL)
        _PyThreadState_GetFrame = threadstate_getframe;

    if (tstate != NULL) {
        tstate->interp = interp;

        tstate->frame = NULL;
        tstate->recursion_depth = 0;
        tstate->overflowed = 0;
        tstate->recursion_critical = 0;
        tstate->tracing = 0;
        tstate->use_tracing = 0;
        tstate->tick_counter = 0;
        tstate->gilstate_counter = 0;
        tstate->async_exc = NULL;
#ifdef WITH_THREAD
        tstate->thread_id = PyThread_get_thread_ident();
#else
        tstate->thread_id = 0;
#endif

        tstate->dict = NULL;

        tstate->curexc_type = NULL;
        tstate->curexc_value = NULL;
        tstate->curexc_traceback = NULL;

        tstate->exc_type = NULL;
        tstate->exc_value = NULL;
        tstate->exc_traceback = NULL;

        tstate->c_profilefunc = NULL;
        tstate->c_tracefunc = NULL;
        tstate->c_profileobj = NULL;
        tstate->c_traceobj = NULL;

        tstate->trash_delete_nesting = 0;
        tstate->trash_delete_later = NULL;

        if (init)
            _PyThreadState_Init(tstate);

        HEAD_LOCK();
        tstate->next = interp->tstate_head;
        interp->tstate_head = tstate;
        HEAD_UNLOCK();
    }

    return tstate;
}

PyThreadState *
PyThreadState_New(PyInterpreterState *interp)
{
    return new_threadstate(interp, 1);
}

PyThreadState *
_PyThreadState_Prealloc(PyInterpreterState *interp)
{
    return new_threadstate(interp, 0);
}

void
_PyThreadState_Init(PyThreadState *tstate)
{
#ifdef WITH_THREAD
    _PyGILState_NoteThreadState(tstate);
#endif
}

PyObject*
PyState_FindModule(struct PyModuleDef* module)
{
    Py_ssize_t index = module->m_base.m_index;
    PyInterpreterState *state = PyThreadState_GET()->interp;
    PyObject *res;
    if (index == 0)
        return NULL;
    if (state->modules_by_index == NULL)
        return NULL;
    if (index >= PyList_GET_SIZE(state->modules_by_index))
        return NULL;
    res = PyList_GET_ITEM(state->modules_by_index, index);
    return res==Py_None ? NULL : res;
}

int
_PyState_AddModule(PyObject* module, struct PyModuleDef* def)
{
    PyInterpreterState *state = PyThreadState_GET()->interp;
    if (!def)
        return -1;
    if (!state->modules_by_index) {
        state->modules_by_index = PyList_New(0);
        if (!state->modules_by_index)
            return -1;
    }
    while(PyList_GET_SIZE(state->modules_by_index) <= def->m_base.m_index)
        if (PyList_Append(state->modules_by_index, Py_None) < 0)
            return -1;
    Py_INCREF(module);
    return PyList_SetItem(state->modules_by_index,
                          def->m_base.m_index, module);
}

int
PyState_AddModule(PyObject* module, struct PyModuleDef* def)
{
    Py_ssize_t index;
    PyInterpreterState *state = PyThreadState_GET()->interp;
    if (!def) {
        Py_FatalError("PyState_AddModule: Module Definition is NULL");
        return -1;
    }
    index = def->m_base.m_index;
    if (state->modules_by_index) {
        if(PyList_GET_SIZE(state->modules_by_index) >= index) {
            if(module == PyList_GET_ITEM(state->modules_by_index, index)) {
                Py_FatalError("PyState_AddModule: Module already added!");
                return -1;
            }
        }
    }
    return _PyState_AddModule(module, def);
}

int
PyState_RemoveModule(struct PyModuleDef* def)
{
    Py_ssize_t index = def->m_base.m_index;
    PyInterpreterState *state = PyThreadState_GET()->interp;
    if (index == 0) {
        Py_FatalError("PyState_RemoveModule: Module index invalid.");
        return -1;
    }
    if (state->modules_by_index == NULL) {
        Py_FatalError("PyState_RemoveModule: Interpreters module-list not acessible.");
        return -1;
    }
    if (index > PyList_GET_SIZE(state->modules_by_index)) {
        Py_FatalError("PyState_RemoveModule: Module index out of bounds.");
        return -1;
    }
    return PyList_SetItem(state->modules_by_index, index, Py_None);
}

void
PyThreadState_Clear(PyThreadState *tstate)
{
    if (Py_VerboseFlag && tstate->frame != NULL)
        fprintf(stderr,
          "PyThreadState_Clear: warning: thread still has a frame\n");

    Py_CLEAR(tstate->frame);

    Py_CLEAR(tstate->dict);
    Py_CLEAR(tstate->async_exc);

    Py_CLEAR(tstate->curexc_type);
    Py_CLEAR(tstate->curexc_value);
    Py_CLEAR(tstate->curexc_traceback);

    Py_CLEAR(tstate->exc_type);
    Py_CLEAR(tstate->exc_value);
    Py_CLEAR(tstate->exc_traceback);

    tstate->c_profilefunc = NULL;
    tstate->c_tracefunc = NULL;
    Py_CLEAR(tstate->c_profileobj);
    Py_CLEAR(tstate->c_traceobj);
}


/* Common code for PyThreadState_Delete() and PyThreadState_DeleteCurrent() */
static void
tstate_delete_common(PyThreadState *tstate)
{
    PyInterpreterState *interp;
    PyThreadState **p;
    PyThreadState *prev_p = NULL;
    if (tstate == NULL)
        Py_FatalError("PyThreadState_Delete: NULL tstate");
    interp = tstate->interp;
    if (interp == NULL)
        Py_FatalError("PyThreadState_Delete: NULL interp");
    HEAD_LOCK();
    for (p = &interp->tstate_head; ; p = &(*p)->next) {
        if (*p == NULL)
            Py_FatalError(
                "PyThreadState_Delete: invalid tstate");
        if (*p == tstate)
            break;
        /* Sanity check.  These states should never happen but if
         * they do we must abort.  Otherwise we'll end up spinning in
         * in a tight loop with the lock held.  A similar check is done
         * in thread.c find_key().  */
        if (*p == prev_p)
            Py_FatalError(
                "PyThreadState_Delete: small circular list(!)"
                " and tstate not found.");
        prev_p = *p;
        if ((*p)->next == interp->tstate_head)
            Py_FatalError(
                "PyThreadState_Delete: circular list(!) and"
                " tstate not found.");
    }
    *p = tstate->next;
    HEAD_UNLOCK();
    free(tstate);
}


void
PyThreadState_Delete(PyThreadState *tstate)
{
    if (tstate == _Py_atomic_load_relaxed(&_PyThreadState_Current))
        Py_FatalError("PyThreadState_Delete: tstate is still current");
    tstate_delete_common(tstate);
#ifdef WITH_THREAD
    if (autoInterpreterState && PyThread_get_key_value(autoTLSkey) == tstate)
        PyThread_delete_key_value(autoTLSkey);
#endif /* WITH_THREAD */
}


#ifdef WITH_THREAD
void
PyThreadState_DeleteCurrent()
{
    PyThreadState *tstate = (PyThreadState*)_Py_atomic_load_relaxed(
        &_PyThreadState_Current);
    if (tstate == NULL)
        Py_FatalError(
            "PyThreadState_DeleteCurrent: no current tstate");
    _Py_atomic_store_relaxed(&_PyThreadState_Current, NULL);
    tstate_delete_common(tstate);
    if (autoInterpreterState && PyThread_get_key_value(autoTLSkey) == tstate)
        PyThread_delete_key_value(autoTLSkey);
    PyEval_ReleaseLock();
}
#endif /* WITH_THREAD */


/*
 * Delete all thread states except the one passed as argument.
 * Note that, if there is a current thread state, it *must* be the one
 * passed as argument.  Also, this won't touch any other interpreters
 * than the current one, since we don't know which thread state should
 * be kept in those other interpreteres.
 */
void
_PyThreadState_DeleteExcept(PyThreadState *tstate)
{
    PyInterpreterState *interp = tstate->interp;
    PyThreadState *p, *next, *garbage;
    HEAD_LOCK();
    /* Remove all thread states, except tstate, from the linked list of
       thread states.  This will allow calling PyThreadState_Clear()
       without holding the lock.
       XXX This would be simpler with a doubly-linked list. */
    garbage = interp->tstate_head;
    interp->tstate_head = tstate;
    if (garbage == tstate) {
        garbage = garbage->next;
        tstate->next = NULL;
    }
    else {
        for (p = garbage; p; p = p->next) {
            if (p->next == tstate) {
                p->next = tstate->next;
                tstate->next = NULL;
                break;
            }
        }
    }
    if (tstate->next != NULL)
        Py_FatalError("_PyThreadState_DeleteExcept: tstate not found "
                      "in interpreter thread states");
    HEAD_UNLOCK();
    /* Clear and deallocate all stale thread states.  Even if this
       executes Python code, we should be safe since it executes
       in the current thread, not one of the stale threads. */
    for (p = garbage; p; p = next) {
        next = p->next;
        PyThreadState_Clear(p);
        free(p);
    }
}


PyThreadState *
PyThreadState_Get(void)
{
    PyThreadState *tstate = (PyThreadState*)_Py_atomic_load_relaxed(
        &_PyThreadState_Current);
    if (tstate == NULL)
        Py_FatalError("PyThreadState_Get: no current thread");

    return tstate;
}


PyThreadState *
PyThreadState_Swap(PyThreadState *newts)
{
    PyThreadState *oldts = (PyThreadState*)_Py_atomic_load_relaxed(
        &_PyThreadState_Current);

    _Py_atomic_store_relaxed(&_PyThreadState_Current, newts);
    /* It should not be possible for more than one thread state
       to be used for a thread.  Check this the best we can in debug
       builds.
    */
#if defined(Py_DEBUG) && defined(WITH_THREAD)
    if (newts) {
        /* This can be called from PyEval_RestoreThread(). Similar
           to it, we need to ensure errno doesn't change.
        */
        int err = errno;
        PyThreadState *check = PyGILState_GetThisThreadState();
        if (check && check->interp == newts->interp && check != newts)
            Py_FatalError("Invalid thread state for this thread");
        errno = err;
    }
#endif
    return oldts;
}

/* An extension mechanism to store arbitrary additional per-thread state.
   PyThreadState_GetDict() returns a dictionary that can be used to hold such
   state; the caller should pick a unique key and store its state there.  If
   PyThreadState_GetDict() returns NULL, an exception has *not* been raised
   and the caller should assume no per-thread state is available. */

PyObject *
PyThreadState_GetDict(void)
{
    PyThreadState *tstate = (PyThreadState*)_Py_atomic_load_relaxed(
        &_PyThreadState_Current);
    if (tstate == NULL)
        return NULL;

    if (tstate->dict == NULL) {
        PyObject *d;
        tstate->dict = d = PyDict_New();
        if (d == NULL)
            PyErr_Clear();
    }
    return tstate->dict;
}


/* Asynchronously raise an exception in a thread.
   Requested by Just van Rossum and Alex Martelli.
   To prevent naive misuse, you must write your own extension
   to call this, or use ctypes.  Must be called with the GIL held.
   Returns the number of tstates modified (normally 1, but 0 if `id` didn't
   match any known thread id).  Can be called with exc=NULL to clear an
   existing async exception.  This raises no exceptions. */

int
PyThreadState_SetAsyncExc(long id, PyObject *exc) {
    PyThreadState *tstate = PyThreadState_GET();
    PyInterpreterState *interp = tstate->interp;
    PyThreadState *p;

    /* Although the GIL is held, a few C API functions can be called
     * without the GIL held, and in particular some that create and
     * destroy thread and interpreter states.  Those can mutate the
     * list of thread states we're traversing, so to prevent that we lock
     * head_mutex for the duration.
     */
    HEAD_LOCK();
    for (p = interp->tstate_head; p != NULL; p = p->next) {
        if (p->thread_id == id) {
            /* Tricky:  we need to decref the current value
             * (if any) in p->async_exc, but that can in turn
             * allow arbitrary Python code to run, including
             * perhaps calls to this function.  To prevent
             * deadlock, we need to release head_mutex before
             * the decref.
             */
            PyObject *old_exc = p->async_exc;
            Py_XINCREF(exc);
            p->async_exc = exc;
            HEAD_UNLOCK();
            Py_XDECREF(old_exc);
            _PyEval_SignalAsyncExc();
            return 1;
        }
    }
    HEAD_UNLOCK();
    return 0;
}


/* Routines for advanced debuggers, requested by David Beazley.
   Don't use unless you know what you are doing! */

PyInterpreterState *
PyInterpreterState_Head(void)
{
    return interp_head;
}

PyInterpreterState *
PyInterpreterState_Next(PyInterpreterState *interp) {
    return interp->next;
}

PyThreadState *
PyInterpreterState_ThreadHead(PyInterpreterState *interp) {
    return interp->tstate_head;
}

PyThreadState *
PyThreadState_Next(PyThreadState *tstate) {
    return tstate->next;
}

/* The implementation of sys._current_frames().  This is intended to be
   called with the GIL held, as it will be when called via
   sys._current_frames().  It's possible it would work fine even without
   the GIL held, but haven't thought enough about that.
*/
PyObject *
_PyThread_CurrentFrames(void)
{
    PyObject *result;
    PyInterpreterState *i;

    result = PyDict_New();
    if (result == NULL)
        return NULL;

    /* for i in all interpreters:
     *     for t in all of i's thread states:
     *          if t's frame isn't NULL, map t's id to its frame
     * Because these lists can mutate even when the GIL is held, we
     * need to grab head_mutex for the duration.
     */
    HEAD_LOCK();
    for (i = interp_head; i != NULL; i = i->next) {
        PyThreadState *t;
        for (t = i->tstate_head; t != NULL; t = t->next) {
            PyObject *id;
            int stat;
            struct _frame *frame = t->frame;
            if (frame == NULL)
                continue;
            id = PyLong_FromLong(t->thread_id);
            if (id == NULL)
                goto Fail;
            stat = PyDict_SetItem(result, id, (PyObject *)frame);
            Py_DECREF(id);
            if (stat < 0)
                goto Fail;
        }
    }
    HEAD_UNLOCK();
    return result;

 Fail:
    HEAD_UNLOCK();
    Py_DECREF(result);
    return NULL;
}

/* Python "auto thread state" API. */
#ifdef WITH_THREAD

/* Keep this as a static, as it is not reliable!  It can only
   ever be compared to the state for the *current* thread.
   * If not equal, then it doesn't matter that the actual
     value may change immediately after comparison, as it can't
     possibly change to the current thread's state.
   * If equal, then the current thread holds the lock, so the value can't
     change until we yield the lock.
*/
static int
PyThreadState_IsCurrent(PyThreadState *tstate)
{
    /* Must be the tstate for this thread */
    assert(PyGILState_GetThisThreadState()==tstate);
    return tstate == _Py_atomic_load_relaxed(&_PyThreadState_Current);
}

/* Internal initialization/finalization functions called by
   Py_Initialize/Py_Finalize
*/
void
_PyGILState_Init(PyInterpreterState *i, PyThreadState *t)
{
    assert(i && t); /* must init with valid states */
    autoTLSkey = PyThread_create_key();
    if (autoTLSkey == -1)
        Py_FatalError("Could not allocate TLS entry");
    autoInterpreterState = i;
    assert(PyThread_get_key_value(autoTLSkey) == NULL);
    assert(t->gilstate_counter == 0);

    _PyGILState_NoteThreadState(t);
}

void
_PyGILState_Fini(void)
{
    PyThread_delete_key(autoTLSkey);
    autoInterpreterState = NULL;
}

/* Reset the TLS key - called by PyOS_AfterFork().
 * This should not be necessary, but some - buggy - pthread implementations
 * don't reset TLS upon fork(), see issue #10517.
 */
void
_PyGILState_Reinit(void)
{
    PyThreadState *tstate = PyGILState_GetThisThreadState();
    PyThread_delete_key(autoTLSkey);
    if ((autoTLSkey = PyThread_create_key()) == -1)
        Py_FatalError("Could not allocate TLS entry");

    /* If the thread had an associated auto thread state, reassociate it with
     * the new key. */
    if (tstate && PyThread_set_key_value(autoTLSkey, (void *)tstate) < 0)
        Py_FatalError("Couldn't create autoTLSkey mapping");
}

/* When a thread state is created for a thread by some mechanism other than
   PyGILState_Ensure, it's important that the GILState machinery knows about
   it so it doesn't try to create another thread state for the thread (this is
   a better fix for SF bug #1010677 than the first one attempted).
*/
static void
_PyGILState_NoteThreadState(PyThreadState* tstate)
{
    /* If autoTLSkey isn't initialized, this must be the very first
       threadstate created in Py_Initialize().  Don't do anything for now
       (we'll be back here when _PyGILState_Init is called). */
    if (!autoInterpreterState)
        return;

    /* Stick the thread state for this thread in thread local storage.

       The only situation where you can legitimately have more than one
       thread state for an OS level thread is when there are multiple
       interpreters, when:

           a) You shouldn't really be using the PyGILState_ APIs anyway,
          and:

           b) The slightly odd way PyThread_set_key_value works (see
          comments by its implementation) means that the first thread
          state created for that given OS level thread will "win",
          which seems reasonable behaviour.
    */
    if (PyThread_set_key_value(autoTLSkey, (void *)tstate) < 0)
        Py_FatalError("Couldn't create autoTLSkey mapping");

    /* PyGILState_Release must not try to delete this thread state. */
    tstate->gilstate_counter = 1;
}

/* The public functions */
PyThreadState *
PyGILState_GetThisThreadState(void)
{
    if (autoInterpreterState == NULL)
        return NULL;
    return (PyThreadState *)PyThread_get_key_value(autoTLSkey);
}

int
PyGILState_Check(void)
{
    /* can't use PyThreadState_Get() since it will assert that it has the GIL */
    PyThreadState *tstate = (PyThreadState*)_Py_atomic_load_relaxed(
        &_PyThreadState_Current);
    return tstate && (tstate == PyGILState_GetThisThreadState());
}

PyGILState_STATE
PyGILState_Ensure(void)
{
    int current;
    PyThreadState *tcur;
    /* Note that we do not auto-init Python here - apart from
       potential races with 2 threads auto-initializing, pep-311
       spells out other issues.  Embedders are expected to have
       called Py_Initialize() and usually PyEval_InitThreads().
    */
    assert(autoInterpreterState); /* Py_Initialize() hasn't been called! */
    tcur = (PyThreadState *)PyThread_get_key_value(autoTLSkey);
    if (tcur == NULL) {
        /* Create a new thread state for this thread */
        tcur = PyThreadState_New(autoInterpreterState);
        if (tcur == NULL)
            Py_FatalError("Couldn't create thread-state for new thread");
        /* This is our thread state!  We'll need to delete it in the
           matching call to PyGILState_Release(). */
        tcur->gilstate_counter = 0;
        current = 0; /* new thread state is never current */
    }
    else
        current = PyThreadState_IsCurrent(tcur);
    if (current == 0)
        PyEval_RestoreThread(tcur);
    /* Update our counter in the thread-state - no need for locks:
       - tcur will remain valid as we hold the GIL.
       - the counter is safe as we are the only thread "allowed"
         to modify this value
    */
    ++tcur->gilstate_counter;
    return current ? PyGILState_LOCKED : PyGILState_UNLOCKED;
}

void
PyGILState_Release(PyGILState_STATE oldstate)
{
    PyThreadState *tcur = (PyThreadState *)PyThread_get_key_value(
                                                            autoTLSkey);
    if (tcur == NULL)
        Py_FatalError("auto-releasing thread-state, "
                      "but no thread-state for this thread");
    /* We must hold the GIL and have our thread state current */
    /* XXX - remove the check - the assert should be fine,
       but while this is very new (April 2003), the extra check
       by release-only users can't hurt.
    */
    if (! PyThreadState_IsCurrent(tcur))
        Py_FatalError("This thread state must be current when releasing");
    assert(PyThreadState_IsCurrent(tcur));
    --tcur->gilstate_counter;
    assert(tcur->gilstate_counter >= 0); /* illegal counter value */

    /* If we're going to destroy this thread-state, we must
     * clear it while the GIL is held, as destructors may run.
     */
    if (tcur->gilstate_counter == 0) {
        /* can't have been locked when we created it */
        assert(oldstate == PyGILState_UNLOCKED);
        PyThreadState_Clear(tcur);
        /* Delete the thread-state.  Note this releases the GIL too!
         * It's vital that the GIL be held here, to avoid shutdown
         * races; see bugs 225673 and 1061968 (that nasty bug has a
         * habit of coming back).
         */
        PyThreadState_DeleteCurrent();
    }
    /* Release the lock if necessary */
    else if (oldstate == PyGILState_UNLOCKED)
        PyEval_SaveThread();
}

#endif /* WITH_THREAD */

#ifdef __cplusplus
}
#endif