cpython/Modules/_asynciomodule.c

#include "Python.h"
#include "pycore_pyerrors.h"      // _PyErr_ClearExcState()
#include <stddef.h>               // offsetof()


/*[clinic input]
module _asyncio
[clinic start generated code]*/
/*[clinic end generated code: output=da39a3ee5e6b4b0d input=8fd17862aa989c69]*/


/* identifiers used from some functions */
_Py_IDENTIFIER(__asyncio_running_event_loop__);
_Py_IDENTIFIER(_asyncio_future_blocking);
_Py_IDENTIFIER(add_done_callback);
_Py_IDENTIFIER(call_soon);
_Py_IDENTIFIER(cancel);
_Py_IDENTIFIER(get_event_loop);
_Py_IDENTIFIER(throw);


/* State of the _asyncio module */
static PyObject *asyncio_mod;
static PyObject *traceback_extract_stack;
static PyObject *asyncio_get_event_loop_policy;
static PyObject *asyncio_future_repr_info_func;
static PyObject *asyncio_iscoroutine_func;
static PyObject *asyncio_task_get_stack_func;
static PyObject *asyncio_task_print_stack_func;
static PyObject *asyncio_task_repr_info_func;
static PyObject *asyncio_InvalidStateError;
static PyObject *asyncio_CancelledError;
static PyObject *context_kwname;
static int module_initialized;

static PyObject *cached_running_holder;
static volatile uint64_t cached_running_holder_tsid;

/* Counter for autogenerated Task names */
static uint64_t task_name_counter = 0;

/* WeakSet containing all alive tasks. */
static PyObject *all_tasks;

/* Dictionary containing tasks that are currently active in
   all running event loops.  {EventLoop: Task} */
static PyObject *current_tasks;

/* An isinstance type cache for the 'is_coroutine()' function. */
static PyObject *iscoroutine_typecache;


typedef enum {
    STATE_PENDING,
    STATE_CANCELLED,
    STATE_FINISHED
} fut_state;

#define FutureObj_HEAD(prefix)                                              \
    PyObject_HEAD                                                           \
    PyObject *prefix##_loop;                                                \
    PyObject *prefix##_callback0;                                           \
    PyObject *prefix##_context0;                                            \
    PyObject *prefix##_callbacks;                                           \
    PyObject *prefix##_exception;                                           \
    PyObject *prefix##_result;                                              \
    PyObject *prefix##_source_tb;                                           \
    PyObject *prefix##_cancel_msg;                                          \
    fut_state prefix##_state;                                               \
    int prefix##_log_tb;                                                    \
    int prefix##_blocking;                                                  \
    PyObject *dict;                                                         \
    PyObject *prefix##_weakreflist;                                         \
    _PyErr_StackItem prefix##_cancelled_exc_state;

typedef struct {
    FutureObj_HEAD(fut)
} FutureObj;

typedef struct {
    FutureObj_HEAD(task)
    PyObject *task_fut_waiter;
    PyObject *task_coro;
    PyObject *task_name;
    PyObject *task_context;
    int task_must_cancel;
    int task_log_destroy_pending;
} TaskObj;

typedef struct {
    PyObject_HEAD
    TaskObj *sw_task;
    PyObject *sw_arg;
} TaskStepMethWrapper;

typedef struct {
    PyObject_HEAD
    PyObject *rl_loop;
#if defined(HAVE_GETPID) && !defined(MS_WINDOWS)
    pid_t rl_pid;
#endif
} PyRunningLoopHolder;


static PyTypeObject FutureType;
static PyTypeObject TaskType;
static PyTypeObject PyRunningLoopHolder_Type;


#define Future_CheckExact(obj) Py_IS_TYPE(obj, &FutureType)
#define Task_CheckExact(obj) Py_IS_TYPE(obj, &TaskType)

#define Future_Check(obj) PyObject_TypeCheck(obj, &FutureType)
#define Task_Check(obj) PyObject_TypeCheck(obj, &TaskType)

#include "clinic/_asynciomodule.c.h"


/*[clinic input]
class _asyncio.Future "FutureObj *" "&Future_Type"
[clinic start generated code]*/
/*[clinic end generated code: output=da39a3ee5e6b4b0d input=00d3e4abca711e0f]*/


/* Get FutureIter from Future */
static PyObject * future_new_iter(PyObject *);

static PyRunningLoopHolder * new_running_loop_holder(PyObject *);


static int
_is_coroutine(PyObject *coro)
{
    /* 'coro' is not a native coroutine, call asyncio.iscoroutine()
       to check if it's another coroutine flavour.

       Do this check after 'future_init()'; in case we need to raise
       an error, __del__ needs a properly initialized object.
    */
    PyObject *res = PyObject_CallOneArg(asyncio_iscoroutine_func, coro);
    if (res == NULL) {
        return -1;
    }

    int is_res_true = PyObject_IsTrue(res);
    Py_DECREF(res);
    if (is_res_true <= 0) {
        return is_res_true;
    }

    if (PySet_GET_SIZE(iscoroutine_typecache) < 100) {
        /* Just in case we don't want to cache more than 100
           positive types.  That shouldn't ever happen, unless
           someone stressing the system on purpose.
        */
        if (PySet_Add(iscoroutine_typecache, (PyObject*) Py_TYPE(coro))) {
            return -1;
        }
    }

    return 1;
}


static inline int
is_coroutine(PyObject *coro)
{
    if (PyCoro_CheckExact(coro)) {
        return 1;
    }

    /* Check if `type(coro)` is in the cache.
       Caching makes is_coroutine() function almost as fast as
       PyCoro_CheckExact() for non-native coroutine-like objects
       (like coroutines compiled with Cython).

       asyncio.iscoroutine() has its own type caching mechanism.
       This cache allows us to avoid the cost of even calling
       a pure-Python function in 99.9% cases.
    */
    int has_it = PySet_Contains(
        iscoroutine_typecache, (PyObject*) Py_TYPE(coro));
    if (has_it == 0) {
        /* type(coro) is not in iscoroutine_typecache */
        return _is_coroutine(coro);
    }

    /* either an error has occurred or
       type(coro) is in iscoroutine_typecache
    */
    return has_it;
}


static PyObject *
get_future_loop(PyObject *fut)
{
    /* Implementation of `asyncio.futures._get_loop` */

    _Py_IDENTIFIER(get_loop);
    _Py_IDENTIFIER(_loop);
    PyObject *getloop;

    if (Future_CheckExact(fut) || Task_CheckExact(fut)) {
        PyObject *loop = ((FutureObj *)fut)->fut_loop;
        Py_INCREF(loop);
        return loop;
    }

    if (_PyObject_LookupAttrId(fut, &PyId_get_loop, &getloop) < 0) {
        return NULL;
    }
    if (getloop != NULL) {
        PyObject *res = PyObject_CallNoArgs(getloop);
        Py_DECREF(getloop);
        return res;
    }

    return _PyObject_GetAttrId(fut, &PyId__loop);
}


static int
get_running_loop(PyObject **loop)
{
    PyObject *rl;

    PyThreadState *ts = PyThreadState_Get();
    uint64_t ts_id = PyThreadState_GetID(ts);
    if (ts_id == cached_running_holder_tsid && cached_running_holder != NULL) {
        // Fast path, check the cache.
        rl = cached_running_holder;  // borrowed
    }
    else {
        PyObject *ts_dict = _PyThreadState_GetDict(ts);  // borrowed
        if (ts_dict == NULL) {
            goto not_found;
        }

        rl = _PyDict_GetItemIdWithError(
            ts_dict, &PyId___asyncio_running_event_loop__);  // borrowed
        if (rl == NULL) {
            if (PyErr_Occurred()) {
                goto error;
            }
            else {
                goto not_found;
            }
        }

        cached_running_holder = rl;  // borrowed
        cached_running_holder_tsid = ts_id;
    }

    assert(Py_IS_TYPE(rl, &PyRunningLoopHolder_Type));
    PyObject *running_loop = ((PyRunningLoopHolder *)rl)->rl_loop;

    if (running_loop == Py_None) {
        goto not_found;
    }

#if defined(HAVE_GETPID) && !defined(MS_WINDOWS)
    /* On Windows there is no getpid, but there is also no os.fork(),
       so there is no need for this check.
    */
    if (getpid() != ((PyRunningLoopHolder *)rl)->rl_pid) {
        goto not_found;
    }
#endif

    Py_INCREF(running_loop);
    *loop = running_loop;
    return 0;

not_found:
    *loop = NULL;
    return 0;

error:
    *loop = NULL;
    return -1;
}


static int
set_running_loop(PyObject *loop)
{
    PyObject *ts_dict = NULL;

    PyThreadState *tstate = PyThreadState_Get();
    if (tstate != NULL) {
        ts_dict = _PyThreadState_GetDict(tstate);  // borrowed
    }

    if (ts_dict == NULL) {
        PyErr_SetString(
            PyExc_RuntimeError, "thread-local storage is not available");
        return -1;
    }

    PyRunningLoopHolder *rl = new_running_loop_holder(loop);
    if (rl == NULL) {
        return -1;
    }

    if (_PyDict_SetItemId(
            ts_dict, &PyId___asyncio_running_event_loop__, (PyObject *)rl) < 0)
    {
        Py_DECREF(rl);  // will cleanup loop & current_pid
        return -1;
    }
    Py_DECREF(rl);

    cached_running_holder = (PyObject *)rl;
    cached_running_holder_tsid = PyThreadState_GetID(tstate);

    return 0;
}


static PyObject *
get_event_loop(int stacklevel)
{
    PyObject *loop;
    PyObject *policy;

    if (get_running_loop(&loop)) {
        return NULL;
    }
    if (loop != NULL) {
        return loop;
    }

    if (PyErr_WarnEx(PyExc_DeprecationWarning,
                     "There is no current event loop",
                     stacklevel))
    {
        return NULL;
    }

    policy = PyObject_CallNoArgs(asyncio_get_event_loop_policy);
    if (policy == NULL) {
        return NULL;
    }

    loop = _PyObject_CallMethodIdNoArgs(policy, &PyId_get_event_loop);
    Py_DECREF(policy);
    return loop;
}


static int
call_soon(PyObject *loop, PyObject *func, PyObject *arg, PyObject *ctx)
{
    PyObject *handle;
    PyObject *stack[3];
    Py_ssize_t nargs;

    if (ctx == NULL) {
        handle = _PyObject_CallMethodIdObjArgs(
            loop, &PyId_call_soon, func, arg, NULL);
    }
    else {
        /* Use FASTCALL to pass a keyword-only argument to call_soon */

        PyObject *callable = _PyObject_GetAttrId(loop, &PyId_call_soon);
        if (callable == NULL) {
            return -1;
        }

        /* All refs in 'stack' are borrowed. */
        nargs = 1;
        stack[0] = func;
        if (arg != NULL) {
            stack[1] = arg;
            nargs++;
        }
        stack[nargs] = (PyObject *)ctx;

        handle = PyObject_Vectorcall(callable, stack, nargs, context_kwname);
        Py_DECREF(callable);
    }

    if (handle == NULL) {
        return -1;
    }
    Py_DECREF(handle);
    return 0;
}


static inline int
future_is_alive(FutureObj *fut)
{
    return fut->fut_loop != NULL;
}


static inline int
future_ensure_alive(FutureObj *fut)
{
    if (!future_is_alive(fut)) {
        PyErr_SetString(PyExc_RuntimeError,
                        "Future object is not initialized.");
        return -1;
    }
    return 0;
}


#define ENSURE_FUTURE_ALIVE(fut)                                \
    do {                                                        \
        assert(Future_Check(fut) || Task_Check(fut));           \
        if (future_ensure_alive((FutureObj*)fut)) {             \
            return NULL;                                        \
        }                                                       \
    } while(0);


static int
future_schedule_callbacks(FutureObj *fut)
{
    Py_ssize_t len;
    Py_ssize_t i;

    if (fut->fut_callback0 != NULL) {
        /* There's a 1st callback */

        int ret = call_soon(
            fut->fut_loop, fut->fut_callback0,
            (PyObject *)fut, fut->fut_context0);

        Py_CLEAR(fut->fut_callback0);
        Py_CLEAR(fut->fut_context0);
        if (ret) {
            /* If an error occurs in pure-Python implementation,
               all callbacks are cleared. */
            Py_CLEAR(fut->fut_callbacks);
            return ret;
        }

        /* we called the first callback, now try calling
           callbacks from the 'fut_callbacks' list. */
    }

    if (fut->fut_callbacks == NULL) {
        /* No more callbacks, return. */
        return 0;
    }

    len = PyList_GET_SIZE(fut->fut_callbacks);
    if (len == 0) {
        /* The list of callbacks was empty; clear it and return. */
        Py_CLEAR(fut->fut_callbacks);
        return 0;
    }

    for (i = 0; i < len; i++) {
        PyObject *cb_tup = PyList_GET_ITEM(fut->fut_callbacks, i);
        PyObject *cb = PyTuple_GET_ITEM(cb_tup, 0);
        PyObject *ctx = PyTuple_GET_ITEM(cb_tup, 1);

        if (call_soon(fut->fut_loop, cb, (PyObject *)fut, ctx)) {
            /* If an error occurs in pure-Python implementation,
               all callbacks are cleared. */
            Py_CLEAR(fut->fut_callbacks);
            return -1;
        }
    }

    Py_CLEAR(fut->fut_callbacks);
    return 0;
}


static int
future_init(FutureObj *fut, PyObject *loop)
{
    PyObject *res;
    int is_true;
    _Py_IDENTIFIER(get_debug);

    // Same to FutureObj_clear() but not clearing fut->dict
    Py_CLEAR(fut->fut_loop);
    Py_CLEAR(fut->fut_callback0);
    Py_CLEAR(fut->fut_context0);
    Py_CLEAR(fut->fut_callbacks);
    Py_CLEAR(fut->fut_result);
    Py_CLEAR(fut->fut_exception);
    Py_CLEAR(fut->fut_source_tb);
    Py_CLEAR(fut->fut_cancel_msg);
    _PyErr_ClearExcState(&fut->fut_cancelled_exc_state);

    fut->fut_state = STATE_PENDING;
    fut->fut_log_tb = 0;
    fut->fut_blocking = 0;

    if (loop == Py_None) {
        loop = get_event_loop(1);
        if (loop == NULL) {
            return -1;
        }
    }
    else {
        Py_INCREF(loop);
    }
    fut->fut_loop = loop;

    res = _PyObject_CallMethodIdNoArgs(fut->fut_loop, &PyId_get_debug);
    if (res == NULL) {
        return -1;
    }
    is_true = PyObject_IsTrue(res);
    Py_DECREF(res);
    if (is_true < 0) {
        return -1;
    }
    if (is_true && !_Py_IsFinalizing()) {
        /* Only try to capture the traceback if the interpreter is not being
           finalized.  The original motivation to add a `_Py_IsFinalizing()`
           call was to prevent SIGSEGV when a Future is created in a __del__
           method, which is called during the interpreter shutdown and the
           traceback module is already unloaded.
        */
        fut->fut_source_tb = PyObject_CallNoArgs(traceback_extract_stack);
        if (fut->fut_source_tb == NULL) {
            return -1;
        }
    }

    return 0;
}

static PyObject *
future_set_result(FutureObj *fut, PyObject *res)
{
    if (future_ensure_alive(fut)) {
        return NULL;
    }

    if (fut->fut_state != STATE_PENDING) {
        PyErr_SetString(asyncio_InvalidStateError, "invalid state");
        return NULL;
    }

    assert(!fut->fut_result);
    Py_INCREF(res);
    fut->fut_result = res;
    fut->fut_state = STATE_FINISHED;

    if (future_schedule_callbacks(fut) == -1) {
        return NULL;
    }
    Py_RETURN_NONE;
}

static PyObject *
future_set_exception(FutureObj *fut, PyObject *exc)
{
    PyObject *exc_val = NULL;

    if (fut->fut_state != STATE_PENDING) {
        PyErr_SetString(asyncio_InvalidStateError, "invalid state");
        return NULL;
    }

    if (PyExceptionClass_Check(exc)) {
        exc_val = PyObject_CallNoArgs(exc);
        if (exc_val == NULL) {
            return NULL;
        }
        if (fut->fut_state != STATE_PENDING) {
            Py_DECREF(exc_val);
            PyErr_SetString(asyncio_InvalidStateError, "invalid state");
            return NULL;
        }
    }
    else {
        exc_val = exc;
        Py_INCREF(exc_val);
    }
    if (!PyExceptionInstance_Check(exc_val)) {
        Py_DECREF(exc_val);
        PyErr_SetString(PyExc_TypeError, "invalid exception object");
        return NULL;
    }
    if (Py_IS_TYPE(exc_val, (PyTypeObject *)PyExc_StopIteration)) {
        Py_DECREF(exc_val);
        PyErr_SetString(PyExc_TypeError,
                        "StopIteration interacts badly with generators "
                        "and cannot be raised into a Future");
        return NULL;
    }

    assert(!fut->fut_exception);
    fut->fut_exception = exc_val;
    fut->fut_state = STATE_FINISHED;

    if (future_schedule_callbacks(fut) == -1) {
        return NULL;
    }

    fut->fut_log_tb = 1;
    Py_RETURN_NONE;
}

static PyObject *
create_cancelled_error(PyObject *msg)
{
    PyObject *exc;
    if (msg == NULL || msg == Py_None) {
        exc = PyObject_CallNoArgs(asyncio_CancelledError);
    } else {
        exc = PyObject_CallOneArg(asyncio_CancelledError, msg);
    }
    return exc;
}

static void
future_set_cancelled_error(FutureObj *fut)
{
    PyObject *exc = create_cancelled_error(fut->fut_cancel_msg);
    PyErr_SetObject(asyncio_CancelledError, exc);
    Py_DECREF(exc);

    _PyErr_ChainStackItem(&fut->fut_cancelled_exc_state);
}

static int
future_get_result(FutureObj *fut, PyObject **result)
{
    if (fut->fut_state == STATE_CANCELLED) {
        future_set_cancelled_error(fut);
        return -1;
    }

    if (fut->fut_state != STATE_FINISHED) {
        PyErr_SetString(asyncio_InvalidStateError, "Result is not set.");
        return -1;
    }

    fut->fut_log_tb = 0;
    if (fut->fut_exception != NULL) {
        Py_INCREF(fut->fut_exception);
        *result = fut->fut_exception;
        return 1;
    }

    Py_INCREF(fut->fut_result);
    *result = fut->fut_result;
    return 0;
}

static PyObject *
future_add_done_callback(FutureObj *fut, PyObject *arg, PyObject *ctx)
{
    if (!future_is_alive(fut)) {
        PyErr_SetString(PyExc_RuntimeError, "uninitialized Future object");
        return NULL;
    }

    if (fut->fut_state != STATE_PENDING) {
        /* The future is done/cancelled, so schedule the callback
           right away. */
        if (call_soon(fut->fut_loop, arg, (PyObject*) fut, ctx)) {
            return NULL;
        }
    }
    else {
        /* The future is pending, add a callback.

           Callbacks in the future object are stored as follows:

              callback0 -- a pointer to the first callback
              callbacks -- a list of 2nd, 3rd, ... callbacks

           Invariants:

            * callbacks != NULL:
                There are some callbacks in in the list.  Just
                add the new callback to it.

            * callbacks == NULL and callback0 == NULL:
                This is the first callback.  Set it to callback0.

            * callbacks == NULL and callback0 != NULL:
                This is a second callback.  Initialize callbacks
                with a new list and add the new callback to it.
        */

        if (fut->fut_callbacks == NULL && fut->fut_callback0 == NULL) {
            Py_INCREF(arg);
            fut->fut_callback0 = arg;
            Py_INCREF(ctx);
            fut->fut_context0 = ctx;
        }
        else {
            PyObject *tup = PyTuple_New(2);
            if (tup == NULL) {
                return NULL;
            }
            Py_INCREF(arg);
            PyTuple_SET_ITEM(tup, 0, arg);
            Py_INCREF(ctx);
            PyTuple_SET_ITEM(tup, 1, (PyObject *)ctx);

            if (fut->fut_callbacks != NULL) {
                int err = PyList_Append(fut->fut_callbacks, tup);
                if (err) {
                    Py_DECREF(tup);
                    return NULL;
                }
                Py_DECREF(tup);
            }
            else {
                fut->fut_callbacks = PyList_New(1);
                if (fut->fut_callbacks == NULL) {
                    Py_DECREF(tup);
                    return NULL;
                }

                PyList_SET_ITEM(fut->fut_callbacks, 0, tup);  /* borrow */
            }
        }
    }

    Py_RETURN_NONE;
}

static PyObject *
future_cancel(FutureObj *fut, PyObject *msg)
{
    fut->fut_log_tb = 0;

    if (fut->fut_state != STATE_PENDING) {
        Py_RETURN_FALSE;
    }
    fut->fut_state = STATE_CANCELLED;

    Py_XINCREF(msg);
    Py_XSETREF(fut->fut_cancel_msg, msg);

    if (future_schedule_callbacks(fut) == -1) {
        return NULL;
    }

    Py_RETURN_TRUE;
}

/*[clinic input]
_asyncio.Future.__init__

    *
    loop: object = None

This class is *almost* compatible with concurrent.futures.Future.

    Differences:

    - result() and exception() do not take a timeout argument and
      raise an exception when the future isn't done yet.

    - Callbacks registered with add_done_callback() are always called
      via the event loop's call_soon_threadsafe().

    - This class is not compatible with the wait() and as_completed()
      methods in the concurrent.futures package.
[clinic start generated code]*/

static int
_asyncio_Future___init___impl(FutureObj *self, PyObject *loop)
/*[clinic end generated code: output=9ed75799eaccb5d6 input=89af317082bc0bf8]*/

{
    return future_init(self, loop);
}

static int
FutureObj_clear(FutureObj *fut)
{
    Py_CLEAR(fut->fut_loop);
    Py_CLEAR(fut->fut_callback0);
    Py_CLEAR(fut->fut_context0);
    Py_CLEAR(fut->fut_callbacks);
    Py_CLEAR(fut->fut_result);
    Py_CLEAR(fut->fut_exception);
    Py_CLEAR(fut->fut_source_tb);
    Py_CLEAR(fut->fut_cancel_msg);
    _PyErr_ClearExcState(&fut->fut_cancelled_exc_state);
    Py_CLEAR(fut->dict);
    return 0;
}

static int
FutureObj_traverse(FutureObj *fut, visitproc visit, void *arg)
{
    Py_VISIT(fut->fut_loop);
    Py_VISIT(fut->fut_callback0);
    Py_VISIT(fut->fut_context0);
    Py_VISIT(fut->fut_callbacks);
    Py_VISIT(fut->fut_result);
    Py_VISIT(fut->fut_exception);
    Py_VISIT(fut->fut_source_tb);
    Py_VISIT(fut->fut_cancel_msg);
    Py_VISIT(fut->dict);

    _PyErr_StackItem *exc_state = &fut->fut_cancelled_exc_state;
    Py_VISIT(exc_state->exc_type);
    Py_VISIT(exc_state->exc_value);
    Py_VISIT(exc_state->exc_traceback);

    return 0;
}

/*[clinic input]
_asyncio.Future.result

Return the result this future represents.

If the future has been cancelled, raises CancelledError.  If the
future's result isn't yet available, raises InvalidStateError.  If
the future is done and has an exception set, this exception is raised.
[clinic start generated code]*/

static PyObject *
_asyncio_Future_result_impl(FutureObj *self)
/*[clinic end generated code: output=f35f940936a4b1e5 input=49ecf9cf5ec50dc5]*/
{
    PyObject *result;

    if (!future_is_alive(self)) {
        PyErr_SetString(asyncio_InvalidStateError,
                        "Future object is not initialized.");
        return NULL;
    }

    int res = future_get_result(self, &result);

    if (res == -1) {
        return NULL;
    }

    if (res == 0) {
        return result;
    }

    assert(res == 1);

    PyErr_SetObject(PyExceptionInstance_Class(result), result);
    Py_DECREF(result);
    return NULL;
}

/*[clinic input]
_asyncio.Future.exception

Return the exception that was set on this future.

The exception (or None if no exception was set) is returned only if
the future is done.  If the future has been cancelled, raises
CancelledError.  If the future isn't done yet, raises
InvalidStateError.
[clinic start generated code]*/

static PyObject *
_asyncio_Future_exception_impl(FutureObj *self)
/*[clinic end generated code: output=88b20d4f855e0710 input=733547a70c841c68]*/
{
    if (!future_is_alive(self)) {
        PyErr_SetString(asyncio_InvalidStateError,
                        "Future object is not initialized.");
        return NULL;
    }

    if (self->fut_state == STATE_CANCELLED) {
        future_set_cancelled_error(self);
        return NULL;
    }

    if (self->fut_state != STATE_FINISHED) {
        PyErr_SetString(asyncio_InvalidStateError, "Exception is not set.");
        return NULL;
    }

    if (self->fut_exception != NULL) {
        self->fut_log_tb = 0;
        Py_INCREF(self->fut_exception);
        return self->fut_exception;
    }

    Py_RETURN_NONE;
}

/*[clinic input]
_asyncio.Future.set_result

    result: object
    /

Mark the future done and set its result.

If the future is already done when this method is called, raises
InvalidStateError.
[clinic start generated code]*/

static PyObject *
_asyncio_Future_set_result(FutureObj *self, PyObject *result)
/*[clinic end generated code: output=1ec2e6bcccd6f2ce input=8b75172c2a7b05f1]*/
{
    ENSURE_FUTURE_ALIVE(self)
    return future_set_result(self, result);
}

/*[clinic input]
_asyncio.Future.set_exception

    exception: object
    /

Mark the future done and set an exception.

If the future is already done when this method is called, raises
InvalidStateError.
[clinic start generated code]*/

static PyObject *
_asyncio_Future_set_exception(FutureObj *self, PyObject *exception)
/*[clinic end generated code: output=f1c1b0cd321be360 input=e45b7d7aa71cc66d]*/
{
    ENSURE_FUTURE_ALIVE(self)
    return future_set_exception(self, exception);
}

/*[clinic input]
_asyncio.Future.add_done_callback

    fn: object
    /
    *
    context: object = NULL

Add a callback to be run when the future becomes done.

The callback is called with a single argument - the future object. If
the future is already done when this is called, the callback is
scheduled with call_soon.
[clinic start generated code]*/

static PyObject *
_asyncio_Future_add_done_callback_impl(FutureObj *self, PyObject *fn,
                                       PyObject *context)
/*[clinic end generated code: output=7ce635bbc9554c1e input=15ab0693a96e9533]*/
{
    if (context == NULL) {
        context = PyContext_CopyCurrent();
        if (context == NULL) {
            return NULL;
        }
        PyObject *res = future_add_done_callback(self, fn, context);
        Py_DECREF(context);
        return res;
    }
    return future_add_done_callback(self, fn, context);
}

/*[clinic input]
_asyncio.Future.remove_done_callback

    fn: object
    /

Remove all instances of a callback from the "call when done" list.

Returns the number of callbacks removed.
[clinic start generated code]*/

static PyObject *
_asyncio_Future_remove_done_callback(FutureObj *self, PyObject *fn)
/*[clinic end generated code: output=5ab1fb52b24ef31f input=0a43280a149d505b]*/
{
    PyObject *newlist;
    Py_ssize_t len, i, j=0;
    Py_ssize_t cleared_callback0 = 0;

    ENSURE_FUTURE_ALIVE(self)

    if (self->fut_callback0 != NULL) {
        int cmp = PyObject_RichCompareBool(self->fut_callback0, fn, Py_EQ);
        if (cmp == -1) {
            return NULL;
        }
        if (cmp == 1) {
            /* callback0 == fn */
            Py_CLEAR(self->fut_callback0);
            Py_CLEAR(self->fut_context0);
            cleared_callback0 = 1;
        }
    }

    if (self->fut_callbacks == NULL) {
        return PyLong_FromSsize_t(cleared_callback0);
    }

    len = PyList_GET_SIZE(self->fut_callbacks);
    if (len == 0) {
        Py_CLEAR(self->fut_callbacks);
        return PyLong_FromSsize_t(cleared_callback0);
    }

    if (len == 1) {
        PyObject *cb_tup = PyList_GET_ITEM(self->fut_callbacks, 0);
        int cmp = PyObject_RichCompareBool(
            PyTuple_GET_ITEM(cb_tup, 0), fn, Py_EQ);
        if (cmp == -1) {
            return NULL;
        }
        if (cmp == 1) {
            /* callbacks[0] == fn */
            Py_CLEAR(self->fut_callbacks);
            return PyLong_FromSsize_t(1 + cleared_callback0);
        }
        /* callbacks[0] != fn and len(callbacks) == 1 */
        return PyLong_FromSsize_t(cleared_callback0);
    }

    newlist = PyList_New(len);
    if (newlist == NULL) {
        return NULL;
    }

    for (i = 0; i < PyList_GET_SIZE(self->fut_callbacks); i++) {
        int ret;
        PyObject *item = PyList_GET_ITEM(self->fut_callbacks, i);
        Py_INCREF(item);
        ret = PyObject_RichCompareBool(PyTuple_GET_ITEM(item, 0), fn, Py_EQ);
        if (ret == 0) {
            if (j < len) {
                PyList_SET_ITEM(newlist, j, item);
                j++;
                continue;
            }
            ret = PyList_Append(newlist, item);
        }
        Py_DECREF(item);
        if (ret < 0) {
            goto fail;
        }
    }

    if (j == 0) {
        Py_CLEAR(self->fut_callbacks);
        Py_DECREF(newlist);
        return PyLong_FromSsize_t(len + cleared_callback0);
    }

    if (j < len) {
        Py_SET_SIZE(newlist, j);
    }
    j = PyList_GET_SIZE(newlist);
    len = PyList_GET_SIZE(self->fut_callbacks);
    if (j != len) {
        if (PyList_SetSlice(self->fut_callbacks, 0, len, newlist) < 0) {
            goto fail;
        }
    }
    Py_DECREF(newlist);
    return PyLong_FromSsize_t(len - j + cleared_callback0);

fail:
    Py_DECREF(newlist);
    return NULL;
}

/*[clinic input]
_asyncio.Future.cancel

    msg: object = None

Cancel the future and schedule callbacks.

If the future is already done or cancelled, return False.  Otherwise,
change the future's state to cancelled, schedule the callbacks and
return True.
[clinic start generated code]*/

static PyObject *
_asyncio_Future_cancel_impl(FutureObj *self, PyObject *msg)
/*[clinic end generated code: output=3edebbc668e5aba3 input=925eb545251f2c5a]*/
{
    ENSURE_FUTURE_ALIVE(self)
    return future_cancel(self, msg);
}

/*[clinic input]
_asyncio.Future.cancelled

Return True if the future was cancelled.
[clinic start generated code]*/

static PyObject *
_asyncio_Future_cancelled_impl(FutureObj *self)
/*[clinic end generated code: output=145197ced586357d input=943ab8b7b7b17e45]*/
{
    if (future_is_alive(self) && self->fut_state == STATE_CANCELLED) {
        Py_RETURN_TRUE;
    }
    else {
        Py_RETURN_FALSE;
    }
}

/*[clinic input]
_asyncio.Future.done

Return True if the future is done.

Done means either that a result / exception are available, or that the
future was cancelled.
[clinic start generated code]*/

static PyObject *
_asyncio_Future_done_impl(FutureObj *self)
/*[clinic end generated code: output=244c5ac351145096 input=28d7b23fdb65d2ac]*/
{
    if (!future_is_alive(self) || self->fut_state == STATE_PENDING) {
        Py_RETURN_FALSE;
    }
    else {
        Py_RETURN_TRUE;
    }
}

/*[clinic input]
_asyncio.Future.get_loop

Return the event loop the Future is bound to.
[clinic start generated code]*/

static PyObject *
_asyncio_Future_get_loop_impl(FutureObj *self)
/*[clinic end generated code: output=119b6ea0c9816c3f input=cba48c2136c79d1f]*/
{
    ENSURE_FUTURE_ALIVE(self)
    Py_INCREF(self->fut_loop);
    return self->fut_loop;
}

static PyObject *
FutureObj_get_blocking(FutureObj *fut, void *Py_UNUSED(ignored))
{
    if (future_is_alive(fut) && fut->fut_blocking) {
        Py_RETURN_TRUE;
    }
    else {
        Py_RETURN_FALSE;
    }
}

static int
FutureObj_set_blocking(FutureObj *fut, PyObject *val, void *Py_UNUSED(ignored))
{
    if (future_ensure_alive(fut)) {
        return -1;
    }
    if (val == NULL) {
        PyErr_SetString(PyExc_AttributeError, "cannot delete attribute");
        return -1;
    }

    int is_true = PyObject_IsTrue(val);
    if (is_true < 0) {
        return -1;
    }
    fut->fut_blocking = is_true;
    return 0;
}

static PyObject *
FutureObj_get_log_traceback(FutureObj *fut, void *Py_UNUSED(ignored))
{
    ENSURE_FUTURE_ALIVE(fut)
    if (fut->fut_log_tb) {
        Py_RETURN_TRUE;
    }
    else {
        Py_RETURN_FALSE;
    }
}

static int
FutureObj_set_log_traceback(FutureObj *fut, PyObject *val, void *Py_UNUSED(ignored))
{
    if (val == NULL) {
        PyErr_SetString(PyExc_AttributeError, "cannot delete attribute");
        return -1;
    }
    int is_true = PyObject_IsTrue(val);
    if (is_true < 0) {
        return -1;
    }
    if (is_true) {
        PyErr_SetString(PyExc_ValueError,
                        "_log_traceback can only be set to False");
        return -1;
    }
    fut->fut_log_tb = is_true;
    return 0;
}

static PyObject *
FutureObj_get_loop(FutureObj *fut, void *Py_UNUSED(ignored))
{
    if (!future_is_alive(fut)) {
        Py_RETURN_NONE;
    }
    Py_INCREF(fut->fut_loop);
    return fut->fut_loop;
}

static PyObject *
FutureObj_get_callbacks(FutureObj *fut, void *Py_UNUSED(ignored))
{
    Py_ssize_t i;

    ENSURE_FUTURE_ALIVE(fut)

    if (fut->fut_callback0 == NULL) {
        if (fut->fut_callbacks == NULL) {
            Py_RETURN_NONE;
        }

        Py_INCREF(fut->fut_callbacks);
        return fut->fut_callbacks;
    }

    Py_ssize_t len = 1;
    if (fut->fut_callbacks != NULL) {
        len += PyList_GET_SIZE(fut->fut_callbacks);
    }


    PyObject *new_list = PyList_New(len);
    if (new_list == NULL) {
        return NULL;
    }

    PyObject *tup0 = PyTuple_New(2);
    if (tup0 == NULL) {
        Py_DECREF(new_list);
        return NULL;
    }

    Py_INCREF(fut->fut_callback0);
    PyTuple_SET_ITEM(tup0, 0, fut->fut_callback0);
    assert(fut->fut_context0 != NULL);
    Py_INCREF(fut->fut_context0);
    PyTuple_SET_ITEM(tup0, 1, (PyObject *)fut->fut_context0);

    PyList_SET_ITEM(new_list, 0, tup0);

    if (fut->fut_callbacks != NULL) {
        for (i = 0; i < PyList_GET_SIZE(fut->fut_callbacks); i++) {
            PyObject *cb = PyList_GET_ITEM(fut->fut_callbacks, i);
            Py_INCREF(cb);
            PyList_SET_ITEM(new_list, i + 1, cb);
        }
    }

    return new_list;
}

static PyObject *
FutureObj_get_result(FutureObj *fut, void *Py_UNUSED(ignored))
{
    ENSURE_FUTURE_ALIVE(fut)
    if (fut->fut_result == NULL) {
        Py_RETURN_NONE;
    }
    Py_INCREF(fut->fut_result);
    return fut->fut_result;
}

static PyObject *
FutureObj_get_exception(FutureObj *fut, void *Py_UNUSED(ignored))
{
    ENSURE_FUTURE_ALIVE(fut)
    if (fut->fut_exception == NULL) {
        Py_RETURN_NONE;
    }
    Py_INCREF(fut->fut_exception);
    return fut->fut_exception;
}

static PyObject *
FutureObj_get_source_traceback(FutureObj *fut, void *Py_UNUSED(ignored))
{
    if (!future_is_alive(fut) || fut->fut_source_tb == NULL) {
        Py_RETURN_NONE;
    }
    Py_INCREF(fut->fut_source_tb);
    return fut->fut_source_tb;
}

static PyObject *
FutureObj_get_cancel_message(FutureObj *fut, void *Py_UNUSED(ignored))
{
    if (fut->fut_cancel_msg == NULL) {
        Py_RETURN_NONE;
    }
    Py_INCREF(fut->fut_cancel_msg);
    return fut->fut_cancel_msg;
}

static int
FutureObj_set_cancel_message(FutureObj *fut, PyObject *msg,
                             void *Py_UNUSED(ignored))
{
    if (msg == NULL) {
        PyErr_SetString(PyExc_AttributeError, "cannot delete attribute");
        return -1;
    }
    Py_INCREF(msg);
    Py_XSETREF(fut->fut_cancel_msg, msg);
    return 0;
}

static PyObject *
FutureObj_get_state(FutureObj *fut, void *Py_UNUSED(ignored))
{
    _Py_IDENTIFIER(PENDING);
    _Py_IDENTIFIER(CANCELLED);
    _Py_IDENTIFIER(FINISHED);
    PyObject *ret = NULL;

    ENSURE_FUTURE_ALIVE(fut)

    switch (fut->fut_state) {
    case STATE_PENDING:
        ret = _PyUnicode_FromId(&PyId_PENDING);
        break;
    case STATE_CANCELLED:
        ret = _PyUnicode_FromId(&PyId_CANCELLED);
        break;
    case STATE_FINISHED:
        ret = _PyUnicode_FromId(&PyId_FINISHED);
        break;
    default:
        assert (0);
    }
    Py_XINCREF(ret);
    return ret;
}

/*[clinic input]
_asyncio.Future._make_cancelled_error

Create the CancelledError to raise if the Future is cancelled.

This should only be called once when handling a cancellation since
it erases the context exception value.
[clinic start generated code]*/

static PyObject *
_asyncio_Future__make_cancelled_error_impl(FutureObj *self)
/*[clinic end generated code: output=a5df276f6c1213de input=ac6effe4ba795ecc]*/
{
    PyObject *exc = create_cancelled_error(self->fut_cancel_msg);
    _PyErr_StackItem *exc_state = &self->fut_cancelled_exc_state;
    /* Transfer ownership of exc_value from exc_state to exc since we are
       done with it. */
    PyException_SetContext(exc, exc_state->exc_value);
    exc_state->exc_value = NULL;

    return exc;
}

/*[clinic input]
_asyncio.Future._repr_info
[clinic start generated code]*/

static PyObject *
_asyncio_Future__repr_info_impl(FutureObj *self)
/*[clinic end generated code: output=fa69e901bd176cfb input=f21504d8e2ae1ca2]*/
{
    return PyObject_CallOneArg(asyncio_future_repr_info_func, (PyObject *)self);
}

static PyObject *
FutureObj_repr(FutureObj *fut)
{
    _Py_IDENTIFIER(_repr_info);

    ENSURE_FUTURE_ALIVE(fut)

    PyObject *rinfo = _PyObject_CallMethodIdNoArgs((PyObject*)fut,
                                                   &PyId__repr_info);
    if (rinfo == NULL) {
        return NULL;
    }

    PyObject *rinfo_s = PyUnicode_Join(NULL, rinfo);
    Py_DECREF(rinfo);
    if (rinfo_s == NULL) {
        return NULL;
    }

    PyObject *rstr = PyUnicode_FromFormat("<%s %U>",
                                          _PyType_Name(Py_TYPE(fut)), rinfo_s);
    Py_DECREF(rinfo_s);
    return rstr;
}

static void
FutureObj_finalize(FutureObj *fut)
{
    _Py_IDENTIFIER(call_exception_handler);
    _Py_IDENTIFIER(message);
    _Py_IDENTIFIER(exception);
    _Py_IDENTIFIER(future);
    _Py_IDENTIFIER(source_traceback);

    PyObject *error_type, *error_value, *error_traceback;
    PyObject *context;
    PyObject *message = NULL;
    PyObject *func;

    if (!fut->fut_log_tb) {
        return;
    }
    assert(fut->fut_exception != NULL);
    fut->fut_log_tb = 0;

    /* Save the current exception, if any. */
    PyErr_Fetch(&error_type, &error_value, &error_traceback);

    context = PyDict_New();
    if (context == NULL) {
        goto finally;
    }

    message = PyUnicode_FromFormat(
        "%s exception was never retrieved", _PyType_Name(Py_TYPE(fut)));
    if (message == NULL) {
        goto finally;
    }

    if (_PyDict_SetItemId(context, &PyId_message, message) < 0 ||
        _PyDict_SetItemId(context, &PyId_exception, fut->fut_exception) < 0 ||
        _PyDict_SetItemId(context, &PyId_future, (PyObject*)fut) < 0) {
        goto finally;
    }
    if (fut->fut_source_tb != NULL) {
        if (_PyDict_SetItemId(context, &PyId_source_traceback,
                              fut->fut_source_tb) < 0) {
            goto finally;
        }
    }

    func = _PyObject_GetAttrId(fut->fut_loop, &PyId_call_exception_handler);
    if (func != NULL) {
        PyObject *res = PyObject_CallOneArg(func, context);
        if (res == NULL) {
            PyErr_WriteUnraisable(func);
        }
        else {
            Py_DECREF(res);
        }
        Py_DECREF(func);
    }

finally:
    Py_XDECREF(context);
    Py_XDECREF(message);

    /* Restore the saved exception. */
    PyErr_Restore(error_type, error_value, error_traceback);
}

static PyObject *
future_cls_getitem(PyObject *cls, PyObject *type)
{
    Py_INCREF(cls);
    return cls;
}

static PyAsyncMethods FutureType_as_async = {
    (unaryfunc)future_new_iter,         /* am_await */
    0,                                  /* am_aiter */
    0,                                  /* am_anext */
    0,                                  /* am_send  */
};

static PyMethodDef FutureType_methods[] = {
    _ASYNCIO_FUTURE_RESULT_METHODDEF
    _ASYNCIO_FUTURE_EXCEPTION_METHODDEF
    _ASYNCIO_FUTURE_SET_RESULT_METHODDEF
    _ASYNCIO_FUTURE_SET_EXCEPTION_METHODDEF
    _ASYNCIO_FUTURE_ADD_DONE_CALLBACK_METHODDEF
    _ASYNCIO_FUTURE_REMOVE_DONE_CALLBACK_METHODDEF
    _ASYNCIO_FUTURE_CANCEL_METHODDEF
    _ASYNCIO_FUTURE_CANCELLED_METHODDEF
    _ASYNCIO_FUTURE_DONE_METHODDEF
    _ASYNCIO_FUTURE_GET_LOOP_METHODDEF
    _ASYNCIO_FUTURE__MAKE_CANCELLED_ERROR_METHODDEF
    _ASYNCIO_FUTURE__REPR_INFO_METHODDEF
    {"__class_getitem__", future_cls_getitem, METH_O|METH_CLASS, NULL},
    {NULL, NULL}        /* Sentinel */
};

#define FUTURE_COMMON_GETSETLIST                                              \
    {"_state", (getter)FutureObj_get_state, NULL, NULL},                      \
    {"_asyncio_future_blocking", (getter)FutureObj_get_blocking,              \
                                 (setter)FutureObj_set_blocking, NULL},       \
    {"_loop", (getter)FutureObj_get_loop, NULL, NULL},                        \
    {"_callbacks", (getter)FutureObj_get_callbacks, NULL, NULL},              \
    {"_result", (getter)FutureObj_get_result, NULL, NULL},                    \
    {"_exception", (getter)FutureObj_get_exception, NULL, NULL},              \
    {"_log_traceback", (getter)FutureObj_get_log_traceback,                   \
                       (setter)FutureObj_set_log_traceback, NULL},            \
    {"_source_traceback", (getter)FutureObj_get_source_traceback,             \
                          NULL, NULL},                                        \
    {"_cancel_message", (getter)FutureObj_get_cancel_message,                 \
                        (setter)FutureObj_set_cancel_message, NULL},

static PyGetSetDef FutureType_getsetlist[] = {
    FUTURE_COMMON_GETSETLIST
    {NULL} /* Sentinel */
};

static void FutureObj_dealloc(PyObject *self);

static PyTypeObject FutureType = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "_asyncio.Future",
    sizeof(FutureObj),                       /* tp_basicsize */
    .tp_dealloc = FutureObj_dealloc,
    .tp_as_async = &FutureType_as_async,
    .tp_repr = (reprfunc)FutureObj_repr,
    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
    .tp_doc = _asyncio_Future___init____doc__,
    .tp_traverse = (traverseproc)FutureObj_traverse,
    .tp_clear = (inquiry)FutureObj_clear,
    .tp_weaklistoffset = offsetof(FutureObj, fut_weakreflist),
    .tp_iter = (getiterfunc)future_new_iter,
    .tp_methods = FutureType_methods,
    .tp_getset = FutureType_getsetlist,
    .tp_dictoffset = offsetof(FutureObj, dict),
    .tp_init = (initproc)_asyncio_Future___init__,
    .tp_new = PyType_GenericNew,
    .tp_finalize = (destructor)FutureObj_finalize,
};

static void
FutureObj_dealloc(PyObject *self)
{
    FutureObj *fut = (FutureObj *)self;

    if (Future_CheckExact(fut)) {
        /* When fut is subclass of Future, finalizer is called from
         * subtype_dealloc.
         */
        if (PyObject_CallFinalizerFromDealloc(self) < 0) {
            // resurrected.
            return;
        }
    }

    PyObject_GC_UnTrack(self);

    if (fut->fut_weakreflist != NULL) {
        PyObject_ClearWeakRefs(self);
    }

    (void)FutureObj_clear(fut);
    Py_TYPE(fut)->tp_free(fut);
}


/*********************** Future Iterator **************************/

typedef struct {
    PyObject_HEAD
    FutureObj *future;
} futureiterobject;


#define FI_FREELIST_MAXLEN 255
static futureiterobject *fi_freelist = NULL;
static Py_ssize_t fi_freelist_len = 0;


static void
FutureIter_dealloc(futureiterobject *it)
{
    PyObject_GC_UnTrack(it);
    Py_CLEAR(it->future);

    if (fi_freelist_len < FI_FREELIST_MAXLEN) {
        fi_freelist_len++;
        it->future = (FutureObj*) fi_freelist;
        fi_freelist = it;
    }
    else {
        PyObject_GC_Del(it);
    }
}

static PySendResult
FutureIter_am_send(futureiterobject *it,
                   PyObject *Py_UNUSED(arg),
                   PyObject **result)
{
    /* arg is unused, see the comment on FutureIter_send for clarification */

    PyObject *res;
    FutureObj *fut = it->future;

    *result = NULL;
    if (fut == NULL) {
        return PYGEN_ERROR;
    }

    if (fut->fut_state == STATE_PENDING) {
        if (!fut->fut_blocking) {
            fut->fut_blocking = 1;
            Py_INCREF(fut);
            *result = (PyObject *)fut;
            return PYGEN_NEXT;
        }
        PyErr_SetString(PyExc_RuntimeError,
                        "await wasn't used with future");
        return PYGEN_ERROR;
    }

    it->future = NULL;
    res = _asyncio_Future_result_impl(fut);
    if (res != NULL) {
        Py_DECREF(fut);
        *result = res;
        return PYGEN_RETURN;
    }

    Py_DECREF(fut);
    return PYGEN_ERROR;
}

static PyObject *
FutureIter_iternext(futureiterobject *it)
{
    PyObject *result;
    switch (FutureIter_am_send(it, Py_None, &result)) {
        case PYGEN_RETURN:
            (void)_PyGen_SetStopIterationValue(result);
            Py_DECREF(result);
            return NULL;
        case PYGEN_NEXT:
            return result;
        case PYGEN_ERROR:
            return NULL;
        default:
            Py_UNREACHABLE();
    }
}

static PyObject *
FutureIter_send(futureiterobject *self, PyObject *unused)
{
    /* Future.__iter__ doesn't care about values that are pushed to the
     * generator, it just returns self.result().
     */
    return FutureIter_iternext(self);
}

static PyObject *
FutureIter_throw(futureiterobject *self, PyObject *args)
{
    PyObject *type, *val = NULL, *tb = NULL;
    if (!PyArg_ParseTuple(args, "O|OO", &type, &val, &tb))
        return NULL;

    if (val == Py_None) {
        val = NULL;
    }
    if (tb == Py_None) {
        tb = NULL;
    } else if (tb != NULL && !PyTraceBack_Check(tb)) {
        PyErr_SetString(PyExc_TypeError, "throw() third argument must be a traceback");
        return NULL;
    }

    Py_INCREF(type);
    Py_XINCREF(val);
    Py_XINCREF(tb);

    if (PyExceptionClass_Check(type)) {
        PyErr_NormalizeException(&type, &val, &tb);
        /* No need to call PyException_SetTraceback since we'll be calling
           PyErr_Restore for `type`, `val`, and `tb`. */
    } else if (PyExceptionInstance_Check(type)) {
        if (val) {
            PyErr_SetString(PyExc_TypeError,
                            "instance exception may not have a separate value");
            goto fail;
        }
        val = type;
        type = PyExceptionInstance_Class(type);
        Py_INCREF(type);
        if (tb == NULL)
            tb = PyException_GetTraceback(val);
    } else {
        PyErr_SetString(PyExc_TypeError,
                        "exceptions must be classes deriving BaseException or "
                        "instances of such a class");
        goto fail;
    }

    Py_CLEAR(self->future);

    PyErr_Restore(type, val, tb);

    return NULL;

  fail:
    Py_DECREF(type);
    Py_XDECREF(val);
    Py_XDECREF(tb);
    return NULL;
}

static PyObject *
FutureIter_close(futureiterobject *self, PyObject *arg)
{
    Py_CLEAR(self->future);
    Py_RETURN_NONE;
}

static int
FutureIter_traverse(futureiterobject *it, visitproc visit, void *arg)
{
    Py_VISIT(it->future);
    return 0;
}

static PyMethodDef FutureIter_methods[] = {
    {"send",  (PyCFunction)FutureIter_send, METH_O, NULL},
    {"throw", (PyCFunction)FutureIter_throw, METH_VARARGS, NULL},
    {"close", (PyCFunction)FutureIter_close, METH_NOARGS, NULL},
    {NULL, NULL}        /* Sentinel */
};

static PyAsyncMethods FutureIterType_as_async = {
    0,                                  /* am_await */
    0,                                  /* am_aiter */
    0,                                  /* am_anext */
    (sendfunc)FutureIter_am_send,       /* am_send  */
};


static PyTypeObject FutureIterType = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "_asyncio.FutureIter",
    .tp_basicsize = sizeof(futureiterobject),
    .tp_itemsize = 0,
    .tp_dealloc = (destructor)FutureIter_dealloc,
    .tp_as_async = &FutureIterType_as_async,
    .tp_getattro = PyObject_GenericGetAttr,
    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
    .tp_traverse = (traverseproc)FutureIter_traverse,
    .tp_iter = PyObject_SelfIter,
    .tp_iternext = (iternextfunc)FutureIter_iternext,
    .tp_methods = FutureIter_methods,
};

static PyObject *
future_new_iter(PyObject *fut)
{
    futureiterobject *it;

    if (!PyObject_TypeCheck(fut, &FutureType)) {
        PyErr_BadInternalCall();
        return NULL;
    }

    ENSURE_FUTURE_ALIVE(fut)

    if (fi_freelist_len) {
        fi_freelist_len--;
        it = fi_freelist;
        fi_freelist = (futureiterobject*) it->future;
        it->future = NULL;
        _Py_NewReference((PyObject*) it);
    }
    else {
        it = PyObject_GC_New(futureiterobject, &FutureIterType);
        if (it == NULL) {
            return NULL;
        }
    }

    Py_INCREF(fut);
    it->future = (FutureObj*)fut;
    PyObject_GC_Track(it);
    return (PyObject*)it;
}


/*********************** Task **************************/


/*[clinic input]
class _asyncio.Task "TaskObj *" "&Task_Type"
[clinic start generated code]*/
/*[clinic end generated code: output=da39a3ee5e6b4b0d input=719dcef0fcc03b37]*/

static int task_call_step_soon(TaskObj *, PyObject *);
static PyObject * task_wakeup(TaskObj *, PyObject *);
static PyObject * task_step(TaskObj *, PyObject *);

/* ----- Task._step wrapper */

static int
TaskStepMethWrapper_clear(TaskStepMethWrapper *o)
{
    Py_CLEAR(o->sw_task);
    Py_CLEAR(o->sw_arg);
    return 0;
}

static void
TaskStepMethWrapper_dealloc(TaskStepMethWrapper *o)
{
    PyObject_GC_UnTrack(o);
    (void)TaskStepMethWrapper_clear(o);
    Py_TYPE(o)->tp_free(o);
}

static PyObject *
TaskStepMethWrapper_call(TaskStepMethWrapper *o,
                         PyObject *args, PyObject *kwds)
{
    if (kwds != NULL && PyDict_GET_SIZE(kwds) != 0) {
        PyErr_SetString(PyExc_TypeError, "function takes no keyword arguments");
        return NULL;
    }
    if (args != NULL && PyTuple_GET_SIZE(args) != 0) {
        PyErr_SetString(PyExc_TypeError, "function takes no positional arguments");
        return NULL;
    }
    return task_step(o->sw_task, o->sw_arg);
}

static int
TaskStepMethWrapper_traverse(TaskStepMethWrapper *o,
                             visitproc visit, void *arg)
{
    Py_VISIT(o->sw_task);
    Py_VISIT(o->sw_arg);
    return 0;
}

static PyObject *
TaskStepMethWrapper_get___self__(TaskStepMethWrapper *o, void *Py_UNUSED(ignored))
{
    if (o->sw_task) {
        Py_INCREF(o->sw_task);
        return (PyObject*)o->sw_task;
    }
    Py_RETURN_NONE;
}

static PyGetSetDef TaskStepMethWrapper_getsetlist[] = {
    {"__self__", (getter)TaskStepMethWrapper_get___self__, NULL, NULL},
    {NULL} /* Sentinel */
};

static PyTypeObject TaskStepMethWrapper_Type = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "TaskStepMethWrapper",
    .tp_basicsize = sizeof(TaskStepMethWrapper),
    .tp_itemsize = 0,
    .tp_getset = TaskStepMethWrapper_getsetlist,
    .tp_dealloc = (destructor)TaskStepMethWrapper_dealloc,
    .tp_call = (ternaryfunc)TaskStepMethWrapper_call,
    .tp_getattro = PyObject_GenericGetAttr,
    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC,
    .tp_traverse = (traverseproc)TaskStepMethWrapper_traverse,
    .tp_clear = (inquiry)TaskStepMethWrapper_clear,
};

static PyObject *
TaskStepMethWrapper_new(TaskObj *task, PyObject *arg)
{
    TaskStepMethWrapper *o;
    o = PyObject_GC_New(TaskStepMethWrapper, &TaskStepMethWrapper_Type);
    if (o == NULL) {
        return NULL;
    }

    Py_INCREF(task);
    o->sw_task = task;

    Py_XINCREF(arg);
    o->sw_arg = arg;

    PyObject_GC_Track(o);
    return (PyObject*) o;
}

/* ----- Task._wakeup implementation */

static  PyMethodDef TaskWakeupDef = {
    "task_wakeup",
    (PyCFunction)task_wakeup,
    METH_O,
    NULL
};

/* ----- Task introspection helpers */

static int
register_task(PyObject *task)
{
    _Py_IDENTIFIER(add);

    PyObject *res = _PyObject_CallMethodIdOneArg(all_tasks,
                                                 &PyId_add, task);
    if (res == NULL) {
        return -1;
    }
    Py_DECREF(res);
    return 0;
}


static int
unregister_task(PyObject *task)
{
    _Py_IDENTIFIER(discard);

    PyObject *res = _PyObject_CallMethodIdOneArg(all_tasks,
                                                 &PyId_discard, task);
    if (res == NULL) {
        return -1;
    }
    Py_DECREF(res);
    return 0;
}


static int
enter_task(PyObject *loop, PyObject *task)
{
    PyObject *item;
    Py_hash_t hash;
    hash = PyObject_Hash(loop);
    if (hash == -1) {
        return -1;
    }
    item = _PyDict_GetItem_KnownHash(current_tasks, loop, hash);
    if (item != NULL) {
        Py_INCREF(item);
        PyErr_Format(
            PyExc_RuntimeError,
            "Cannot enter into task %R while another " \
            "task %R is being executed.",
            task, item, NULL);
        Py_DECREF(item);
        return -1;
    }
    if (PyErr_Occurred()) {
        return -1;
    }
    return _PyDict_SetItem_KnownHash(current_tasks, loop, task, hash);
}


static int
leave_task(PyObject *loop, PyObject *task)
/*[clinic end generated code: output=0ebf6db4b858fb41 input=51296a46313d1ad8]*/
{
    PyObject *item;
    Py_hash_t hash;
    hash = PyObject_Hash(loop);
    if (hash == -1) {
        return -1;
    }
    item = _PyDict_GetItem_KnownHash(current_tasks, loop, hash);
    if (item != task) {
        if (item == NULL) {
            /* Not entered, replace with None */
            item = Py_None;
        }
        PyErr_Format(
            PyExc_RuntimeError,
            "Leaving task %R does not match the current task %R.",
            task, item, NULL);
        return -1;
    }
    return _PyDict_DelItem_KnownHash(current_tasks, loop, hash);
}

/* ----- Task */

/*[clinic input]
_asyncio.Task.__init__

    coro: object
    *
    loop: object = None
    name: object = None

A coroutine wrapped in a Future.
[clinic start generated code]*/

static int
_asyncio_Task___init___impl(TaskObj *self, PyObject *coro, PyObject *loop,
                            PyObject *name)
/*[clinic end generated code: output=88b12b83d570df50 input=352a3137fe60091d]*/
{
    if (future_init((FutureObj*)self, loop)) {
        return -1;
    }

    int is_coro = is_coroutine(coro);
    if (is_coro == -1) {
        return -1;
    }
    if (is_coro == 0) {
        self->task_log_destroy_pending = 0;
        PyErr_Format(PyExc_TypeError,
                     "a coroutine was expected, got %R",
                     coro, NULL);
        return -1;
    }

    Py_XSETREF(self->task_context, PyContext_CopyCurrent());
    if (self->task_context == NULL) {
        return -1;
    }

    Py_CLEAR(self->task_fut_waiter);
    self->task_must_cancel = 0;
    self->task_log_destroy_pending = 1;
    Py_INCREF(coro);
    Py_XSETREF(self->task_coro, coro);

    if (name == Py_None) {
        name = PyUnicode_FromFormat("Task-%" PRIu64, ++task_name_counter);
    } else if (!PyUnicode_CheckExact(name)) {
        name = PyObject_Str(name);
    } else {
        Py_INCREF(name);
    }
    Py_XSETREF(self->task_name, name);
    if (self->task_name == NULL) {
        return -1;
    }

    if (task_call_step_soon(self, NULL)) {
        return -1;
    }
    return register_task((PyObject*)self);
}

static int
TaskObj_clear(TaskObj *task)
{
    (void)FutureObj_clear((FutureObj*) task);
    Py_CLEAR(task->task_context);
    Py_CLEAR(task->task_coro);
    Py_CLEAR(task->task_name);
    Py_CLEAR(task->task_fut_waiter);
    return 0;
}

static int
TaskObj_traverse(TaskObj *task, visitproc visit, void *arg)
{
    Py_VISIT(task->task_context);
    Py_VISIT(task->task_coro);
    Py_VISIT(task->task_name);
    Py_VISIT(task->task_fut_waiter);
    (void)FutureObj_traverse((FutureObj*) task, visit, arg);
    return 0;
}

static PyObject *
TaskObj_get_log_destroy_pending(TaskObj *task, void *Py_UNUSED(ignored))
{
    if (task->task_log_destroy_pending) {
        Py_RETURN_TRUE;
    }
    else {
        Py_RETURN_FALSE;
    }
}

static int
TaskObj_set_log_destroy_pending(TaskObj *task, PyObject *val, void *Py_UNUSED(ignored))
{
    if (val == NULL) {
        PyErr_SetString(PyExc_AttributeError, "cannot delete attribute");
        return -1;
    }
    int is_true = PyObject_IsTrue(val);
    if (is_true < 0) {
        return -1;
    }
    task->task_log_destroy_pending = is_true;
    return 0;
}

static PyObject *
TaskObj_get_must_cancel(TaskObj *task, void *Py_UNUSED(ignored))
{
    if (task->task_must_cancel) {
        Py_RETURN_TRUE;
    }
    else {
        Py_RETURN_FALSE;
    }
}

static PyObject *
TaskObj_get_coro(TaskObj *task, void *Py_UNUSED(ignored))
{
    if (task->task_coro) {
        Py_INCREF(task->task_coro);
        return task->task_coro;
    }

    Py_RETURN_NONE;
}

static PyObject *
TaskObj_get_fut_waiter(TaskObj *task, void *Py_UNUSED(ignored))
{
    if (task->task_fut_waiter) {
        Py_INCREF(task->task_fut_waiter);
        return task->task_fut_waiter;
    }

    Py_RETURN_NONE;
}

/*[clinic input]
_asyncio.Task._make_cancelled_error

Create the CancelledError to raise if the Task is cancelled.

This should only be called once when handling a cancellation since
it erases the context exception value.
[clinic start generated code]*/

static PyObject *
_asyncio_Task__make_cancelled_error_impl(TaskObj *self)
/*[clinic end generated code: output=55a819e8b4276fab input=52c0e32de8e2f840]*/
{
    FutureObj *fut = (FutureObj*)self;
    return _asyncio_Future__make_cancelled_error_impl(fut);
}


/*[clinic input]
_asyncio.Task._repr_info
[clinic start generated code]*/

static PyObject *
_asyncio_Task__repr_info_impl(TaskObj *self)
/*[clinic end generated code: output=6a490eb66d5ba34b input=3c6d051ed3ddec8b]*/
{
    return PyObject_CallOneArg(asyncio_task_repr_info_func, (PyObject *)self);
}

/*[clinic input]
_asyncio.Task.cancel

    msg: object = None

Request that this task cancel itself.

This arranges for a CancelledError to be thrown into the
wrapped coroutine on the next cycle through the event loop.
The coroutine then has a chance to clean up or even deny
the request using try/except/finally.

Unlike Future.cancel, this does not guarantee that the
task will be cancelled: the exception might be caught and
acted upon, delaying cancellation of the task or preventing
cancellation completely.  The task may also return a value or
raise a different exception.

Immediately after this method is called, Task.cancelled() will
not return True (unless the task was already cancelled).  A
task will be marked as cancelled when the wrapped coroutine
terminates with a CancelledError exception (even if cancel()
was not called).
[clinic start generated code]*/

static PyObject *
_asyncio_Task_cancel_impl(TaskObj *self, PyObject *msg)
/*[clinic end generated code: output=c66b60d41c74f9f1 input=f4ff8e8ffc5f1c00]*/
{
    self->task_log_tb = 0;

    if (self->task_state != STATE_PENDING) {
        Py_RETURN_FALSE;
    }

    if (self->task_fut_waiter) {
        PyObject *res;
        int is_true;

        res = _PyObject_CallMethodIdOneArg(self->task_fut_waiter,
                                           &PyId_cancel, msg);
        if (res == NULL) {
            return NULL;
        }

        is_true = PyObject_IsTrue(res);
        Py_DECREF(res);
        if (is_true < 0) {
            return NULL;
        }

        if (is_true) {
            Py_RETURN_TRUE;
        }
    }

    self->task_must_cancel = 1;
    Py_XINCREF(msg);
    Py_XSETREF(self->task_cancel_msg, msg);
    Py_RETURN_TRUE;
}

/*[clinic input]
_asyncio.Task.get_stack

    *
    limit: object = None

Return the list of stack frames for this task's coroutine.

If the coroutine is not done, this returns the stack where it is
suspended.  If the coroutine has completed successfully or was
cancelled, this returns an empty list.  If the coroutine was
terminated by an exception, this returns the list of traceback
frames.

The frames are always ordered from oldest to newest.

The optional limit gives the maximum number of frames to
return; by default all available frames are returned.  Its
meaning differs depending on whether a stack or a traceback is
returned: the newest frames of a stack are returned, but the
oldest frames of a traceback are returned.  (This matches the
behavior of the traceback module.)

For reasons beyond our control, only one stack frame is
returned for a suspended coroutine.
[clinic start generated code]*/

static PyObject *
_asyncio_Task_get_stack_impl(TaskObj *self, PyObject *limit)
/*[clinic end generated code: output=c9aeeeebd1e18118 input=05b323d42b809b90]*/
{
    return PyObject_CallFunctionObjArgs(
        asyncio_task_get_stack_func, self, limit, NULL);
}

/*[clinic input]
_asyncio.Task.print_stack

    *
    limit: object = None
    file: object = None

Print the stack or traceback for this task's coroutine.

This produces output similar to that of the traceback module,
for the frames retrieved by get_stack().  The limit argument
is passed to get_stack().  The file argument is an I/O stream
to which the output is written; by default output is written
to sys.stderr.
[clinic start generated code]*/

static PyObject *
_asyncio_Task_print_stack_impl(TaskObj *self, PyObject *limit,
                               PyObject *file)
/*[clinic end generated code: output=7339e10314cd3f4d input=1a0352913b7fcd92]*/
{
    return PyObject_CallFunctionObjArgs(
        asyncio_task_print_stack_func, self, limit, file, NULL);
}

/*[clinic input]
_asyncio.Task.set_result

    result: object
    /
[clinic start generated code]*/

static PyObject *
_asyncio_Task_set_result(TaskObj *self, PyObject *result)
/*[clinic end generated code: output=1dcae308bfcba318 input=9d1a00c07be41bab]*/
{
    PyErr_SetString(PyExc_RuntimeError,
                    "Task does not support set_result operation");
    return NULL;
}

/*[clinic input]
_asyncio.Task.set_exception

    exception: object
    /
[clinic start generated code]*/

static PyObject *
_asyncio_Task_set_exception(TaskObj *self, PyObject *exception)
/*[clinic end generated code: output=bc377fc28067303d input=9a8f65c83dcf893a]*/
{
    PyErr_SetString(PyExc_RuntimeError,
                    "Task does not support set_exception operation");
    return NULL;
}

/*[clinic input]
_asyncio.Task.get_coro
[clinic start generated code]*/

static PyObject *
_asyncio_Task_get_coro_impl(TaskObj *self)
/*[clinic end generated code: output=bcac27c8cc6c8073 input=d2e8606c42a7b403]*/
{
    Py_INCREF(self->task_coro);
    return self->task_coro;
}

/*[clinic input]
_asyncio.Task.get_name
[clinic start generated code]*/

static PyObject *
_asyncio_Task_get_name_impl(TaskObj *self)
/*[clinic end generated code: output=0ecf1570c3b37a8f input=a4a6595d12f4f0f8]*/
{
    if (self->task_name) {
        Py_INCREF(self->task_name);
        return self->task_name;
    }

    Py_RETURN_NONE;
}

/*[clinic input]
_asyncio.Task.set_name

    value: object
    /
[clinic start generated code]*/

static PyObject *
_asyncio_Task_set_name(TaskObj *self, PyObject *value)
/*[clinic end generated code: output=138a8d51e32057d6 input=a8359b6e65f8fd31]*/
{
    if (!PyUnicode_CheckExact(value)) {
        value = PyObject_Str(value);
        if (value == NULL) {
            return NULL;
        }
    } else {
        Py_INCREF(value);
    }

    Py_XSETREF(self->task_name, value);
    Py_RETURN_NONE;
}

static void
TaskObj_finalize(TaskObj *task)
{
    _Py_IDENTIFIER(call_exception_handler);
    _Py_IDENTIFIER(task);
    _Py_IDENTIFIER(message);
    _Py_IDENTIFIER(source_traceback);

    PyObject *context;
    PyObject *message = NULL;
    PyObject *func;
    PyObject *error_type, *error_value, *error_traceback;

    if (task->task_state != STATE_PENDING || !task->task_log_destroy_pending) {
        goto done;
    }

    /* Save the current exception, if any. */
    PyErr_Fetch(&error_type, &error_value, &error_traceback);

    context = PyDict_New();
    if (context == NULL) {
        goto finally;
    }

    message = PyUnicode_FromString("Task was destroyed but it is pending!");
    if (message == NULL) {
        goto finally;
    }

    if (_PyDict_SetItemId(context, &PyId_message, message) < 0 ||
        _PyDict_SetItemId(context, &PyId_task, (PyObject*)task) < 0)
    {
        goto finally;
    }

    if (task->task_source_tb != NULL) {
        if (_PyDict_SetItemId(context, &PyId_source_traceback,
                              task->task_source_tb) < 0)
        {
            goto finally;
        }
    }

    func = _PyObject_GetAttrId(task->task_loop, &PyId_call_exception_handler);
    if (func != NULL) {
        PyObject *res = PyObject_CallOneArg(func, context);
        if (res == NULL) {
            PyErr_WriteUnraisable(func);
        }
        else {
            Py_DECREF(res);
        }
        Py_DECREF(func);
    }

finally:
    Py_XDECREF(context);
    Py_XDECREF(message);

    /* Restore the saved exception. */
    PyErr_Restore(error_type, error_value, error_traceback);

done:
    FutureObj_finalize((FutureObj*)task);
}

static PyObject *
task_cls_getitem(PyObject *cls, PyObject *type)
{
    Py_INCREF(cls);
    return cls;
}

static void TaskObj_dealloc(PyObject *);  /* Needs Task_CheckExact */

static PyMethodDef TaskType_methods[] = {
    _ASYNCIO_FUTURE_RESULT_METHODDEF
    _ASYNCIO_FUTURE_EXCEPTION_METHODDEF
    _ASYNCIO_FUTURE_ADD_DONE_CALLBACK_METHODDEF
    _ASYNCIO_FUTURE_REMOVE_DONE_CALLBACK_METHODDEF
    _ASYNCIO_FUTURE_CANCELLED_METHODDEF
    _ASYNCIO_FUTURE_DONE_METHODDEF
    _ASYNCIO_TASK_SET_RESULT_METHODDEF
    _ASYNCIO_TASK_SET_EXCEPTION_METHODDEF
    _ASYNCIO_TASK_CANCEL_METHODDEF
    _ASYNCIO_TASK_GET_STACK_METHODDEF
    _ASYNCIO_TASK_PRINT_STACK_METHODDEF
    _ASYNCIO_TASK__MAKE_CANCELLED_ERROR_METHODDEF
    _ASYNCIO_TASK__REPR_INFO_METHODDEF
    _ASYNCIO_TASK_GET_NAME_METHODDEF
    _ASYNCIO_TASK_SET_NAME_METHODDEF
    _ASYNCIO_TASK_GET_CORO_METHODDEF
    {"__class_getitem__", task_cls_getitem, METH_O|METH_CLASS, NULL},
    {NULL, NULL}        /* Sentinel */
};

static PyGetSetDef TaskType_getsetlist[] = {
    FUTURE_COMMON_GETSETLIST
    {"_log_destroy_pending", (getter)TaskObj_get_log_destroy_pending,
                             (setter)TaskObj_set_log_destroy_pending, NULL},
    {"_must_cancel", (getter)TaskObj_get_must_cancel, NULL, NULL},
    {"_coro", (getter)TaskObj_get_coro, NULL, NULL},
    {"_fut_waiter", (getter)TaskObj_get_fut_waiter, NULL, NULL},
    {NULL} /* Sentinel */
};

static PyTypeObject TaskType = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "_asyncio.Task",
    sizeof(TaskObj),                       /* tp_basicsize */
    .tp_base = &FutureType,
    .tp_dealloc = TaskObj_dealloc,
    .tp_as_async = &FutureType_as_async,
    .tp_repr = (reprfunc)FutureObj_repr,
    .tp_flags = Py_TPFLAGS_DEFAULT | Py_TPFLAGS_HAVE_GC | Py_TPFLAGS_BASETYPE,
    .tp_doc = _asyncio_Task___init____doc__,
    .tp_traverse = (traverseproc)TaskObj_traverse,
    .tp_clear = (inquiry)TaskObj_clear,
    .tp_weaklistoffset = offsetof(TaskObj, task_weakreflist),
    .tp_iter = (getiterfunc)future_new_iter,
    .tp_methods = TaskType_methods,
    .tp_getset = TaskType_getsetlist,
    .tp_dictoffset = offsetof(TaskObj, dict),
    .tp_init = (initproc)_asyncio_Task___init__,
    .tp_new = PyType_GenericNew,
    .tp_finalize = (destructor)TaskObj_finalize,
};

static void
TaskObj_dealloc(PyObject *self)
{
    TaskObj *task = (TaskObj *)self;

    if (Task_CheckExact(self)) {
        /* When fut is subclass of Task, finalizer is called from
         * subtype_dealloc.
         */
        if (PyObject_CallFinalizerFromDealloc(self) < 0) {
            // resurrected.
            return;
        }
    }

    PyObject_GC_UnTrack(self);

    if (task->task_weakreflist != NULL) {
        PyObject_ClearWeakRefs(self);
    }

    (void)TaskObj_clear(task);
    Py_TYPE(task)->tp_free(task);
}

static int
task_call_step_soon(TaskObj *task, PyObject *arg)
{
    PyObject *cb = TaskStepMethWrapper_new(task, arg);
    if (cb == NULL) {
        return -1;
    }

    int ret = call_soon(task->task_loop, cb, NULL, task->task_context);
    Py_DECREF(cb);
    return ret;
}

static PyObject *
task_set_error_soon(TaskObj *task, PyObject *et, const char *format, ...)
{
    PyObject* msg;

    va_list vargs;
#ifdef HAVE_STDARG_PROTOTYPES
    va_start(vargs, format);
#else
    va_start(vargs);
#endif
    msg = PyUnicode_FromFormatV(format, vargs);
    va_end(vargs);

    if (msg == NULL) {
        return NULL;
    }

    PyObject *e = PyObject_CallOneArg(et, msg);
    Py_DECREF(msg);
    if (e == NULL) {
        return NULL;
    }

    if (task_call_step_soon(task, e) == -1) {
        Py_DECREF(e);
        return NULL;
    }

    Py_DECREF(e);
    Py_RETURN_NONE;
}

static inline int
gen_status_from_result(PyObject **result)
{
    if (*result != NULL) {
        return PYGEN_NEXT;
    }
    if (_PyGen_FetchStopIterationValue(result) == 0) {
        return PYGEN_RETURN;
    }

    assert(PyErr_Occurred());
    return PYGEN_ERROR;
}

static PyObject *
task_step_impl(TaskObj *task, PyObject *exc)
{
    int res;
    int clear_exc = 0;
    PyObject *result = NULL;
    PyObject *coro;
    PyObject *o;

    if (task->task_state != STATE_PENDING) {
        PyErr_Format(asyncio_InvalidStateError,
                     "_step(): already done: %R %R",
                     task,
                     exc ? exc : Py_None);
        goto fail;
    }

    if (task->task_must_cancel) {
        assert(exc != Py_None);

        if (exc) {
            /* Check if exc is a CancelledError */
            res = PyObject_IsInstance(exc, asyncio_CancelledError);
            if (res == -1) {
                /* An error occurred, abort */
                goto fail;
            }
            if (res == 0) {
                /* exc is not CancelledError; reset it to NULL */
                exc = NULL;
            }
        }

        if (!exc) {
            /* exc was not a CancelledError */
            exc = create_cancelled_error(task->task_cancel_msg);

            if (!exc) {
                goto fail;
            }
            clear_exc = 1;
        }

        task->task_must_cancel = 0;
    }

    Py_CLEAR(task->task_fut_waiter);

    coro = task->task_coro;
    if (coro == NULL) {
        PyErr_SetString(PyExc_RuntimeError, "uninitialized Task object");
        if (clear_exc) {
            /* We created 'exc' during this call */
            Py_DECREF(exc);
        }
        return NULL;
    }

    int gen_status = PYGEN_ERROR;
    if (exc == NULL) {
        gen_status = PyIter_Send(coro, Py_None, &result);
    }
    else {
        result = _PyObject_CallMethodIdOneArg(coro, &PyId_throw, exc);
        gen_status = gen_status_from_result(&result);
        if (clear_exc) {
            /* We created 'exc' during this call */
            Py_DECREF(exc);
        }
    }

    if (gen_status == PYGEN_RETURN || gen_status == PYGEN_ERROR) {
        PyObject *et, *ev, *tb;

        if (result != NULL) {
            /* The error is StopIteration and that means that
               the underlying coroutine has resolved */

            PyObject *res;
            if (task->task_must_cancel) {
                // Task is cancelled right before coro stops.
                task->task_must_cancel = 0;
                res = future_cancel((FutureObj*)task, task->task_cancel_msg);
            }
            else {
                res = future_set_result((FutureObj*)task, result);
            }

            Py_DECREF(result);

            if (res == NULL) {
                return NULL;
            }
            Py_DECREF(res);
            Py_RETURN_NONE;
        }

        if (PyErr_ExceptionMatches(asyncio_CancelledError)) {
            /* CancelledError */
            PyErr_Fetch(&et, &ev, &tb);

            FutureObj *fut = (FutureObj*)task;
            _PyErr_StackItem *exc_state = &fut->fut_cancelled_exc_state;
            exc_state->exc_type = et;
            exc_state->exc_value = ev;
            exc_state->exc_traceback = tb;

            return future_cancel(fut, NULL);
        }

        /* Some other exception; pop it and call Task.set_exception() */
        PyErr_Fetch(&et, &ev, &tb);

        assert(et);
        if (!ev || !PyObject_TypeCheck(ev, (PyTypeObject *) et)) {
            PyErr_NormalizeException(&et, &ev, &tb);
        }
        if (tb != NULL) {
            PyException_SetTraceback(ev, tb);
        }
        o = future_set_exception((FutureObj*)task, ev);
        if (!o) {
            /* An exception in Task.set_exception() */
            Py_DECREF(et);
            Py_XDECREF(tb);
            Py_XDECREF(ev);
            goto fail;
        }
        assert(o == Py_None);
        Py_DECREF(o);

        if (PyErr_GivenExceptionMatches(et, PyExc_KeyboardInterrupt) ||
            PyErr_GivenExceptionMatches(et, PyExc_SystemExit))
        {
            /* We've got a KeyboardInterrupt or a SystemError; re-raise it */
            PyErr_Restore(et, ev, tb);
            goto fail;
        }

        Py_DECREF(et);
        Py_XDECREF(tb);
        Py_XDECREF(ev);

        Py_RETURN_NONE;
    }

    if (result == (PyObject*)task) {
        /* We have a task that wants to await on itself */
        goto self_await;
    }

    /* Check if `result` is FutureObj or TaskObj (and not a subclass) */
    if (Future_CheckExact(result) || Task_CheckExact(result)) {
        PyObject *wrapper;
        PyObject *res;
        FutureObj *fut = (FutureObj*)result;

        /* Check if `result` future is attached to a different loop */
        if (fut->fut_loop != task->task_loop) {
            goto different_loop;
        }

        if (!fut->fut_blocking) {
            goto yield_insteadof_yf;
        }

        fut->fut_blocking = 0;

        /* result.add_done_callback(task._wakeup) */
        wrapper = PyCFunction_New(&TaskWakeupDef, (PyObject *)task);
        if (wrapper == NULL) {
            goto fail;
        }
        res = future_add_done_callback(
            (FutureObj*)result, wrapper, task->task_context);
        Py_DECREF(wrapper);
        if (res == NULL) {
            goto fail;
        }
        Py_DECREF(res);

        /* task._fut_waiter = result */
        task->task_fut_waiter = result;  /* no incref is necessary */

        if (task->task_must_cancel) {
            PyObject *r;
            int is_true;
            r = _PyObject_CallMethodIdOneArg(result, &PyId_cancel,
                                             task->task_cancel_msg);
            if (r == NULL) {
                return NULL;
            }
            is_true = PyObject_IsTrue(r);
            Py_DECREF(r);
            if (is_true < 0) {
                return NULL;
            }
            else if (is_true) {
                task->task_must_cancel = 0;
            }
        }

        Py_RETURN_NONE;
    }

    /* Check if `result` is None */
    if (result == Py_None) {
        /* Bare yield relinquishes control for one event loop iteration. */
        if (task_call_step_soon(task, NULL)) {
            goto fail;
        }
        return result;
    }

    /* Check if `result` is a Future-compatible object */
    if (_PyObject_LookupAttrId(result, &PyId__asyncio_future_blocking, &o) < 0) {
        goto fail;
    }
    if (o != NULL && o != Py_None) {
        /* `result` is a Future-compatible object */
        PyObject *wrapper;
        PyObject *res;

        int blocking = PyObject_IsTrue(o);
        Py_DECREF(o);
        if (blocking < 0) {
            goto fail;
        }

        /* Check if `result` future is attached to a different loop */
        PyObject *oloop = get_future_loop(result);
        if (oloop == NULL) {
            goto fail;
        }
        if (oloop != task->task_loop) {
            Py_DECREF(oloop);
            goto different_loop;
        }
        Py_DECREF(oloop);

        if (!blocking) {
            goto yield_insteadof_yf;
        }

        /* result._asyncio_future_blocking = False */
        if (_PyObject_SetAttrId(
                result, &PyId__asyncio_future_blocking, Py_False) == -1) {
            goto fail;
        }

        wrapper = PyCFunction_New(&TaskWakeupDef, (PyObject *)task);
        if (wrapper == NULL) {
            goto fail;
        }

        /* result.add_done_callback(task._wakeup) */
        PyObject *add_cb = _PyObject_GetAttrId(
            result, &PyId_add_done_callback);
        if (add_cb == NULL) {
            Py_DECREF(wrapper);
            goto fail;
        }
        PyObject *stack[2];
        stack[0] = wrapper;
        stack[1] = (PyObject *)task->task_context;
        res = PyObject_Vectorcall(add_cb, stack, 1, context_kwname);
        Py_DECREF(add_cb);
        Py_DECREF(wrapper);
        if (res == NULL) {
            goto fail;
        }
        Py_DECREF(res);

        /* task._fut_waiter = result */
        task->task_fut_waiter = result;  /* no incref is necessary */

        if (task->task_must_cancel) {
            PyObject *r;
            int is_true;
            r = _PyObject_CallMethodIdOneArg(result, &PyId_cancel,
                                             task->task_cancel_msg);
            if (r == NULL) {
                return NULL;
            }
            is_true = PyObject_IsTrue(r);
            Py_DECREF(r);
            if (is_true < 0) {
                return NULL;
            }
            else if (is_true) {
                task->task_must_cancel = 0;
            }
        }

        Py_RETURN_NONE;
    }

    Py_XDECREF(o);
    /* Check if `result` is a generator */
    res = PyObject_IsInstance(result, (PyObject*)&PyGen_Type);
    if (res < 0) {
        goto fail;
    }
    if (res) {
        /* `result` is a generator */
        o = task_set_error_soon(
            task, PyExc_RuntimeError,
            "yield was used instead of yield from for "
            "generator in task %R with %R", task, result);
        Py_DECREF(result);
        return o;
    }

    /* The `result` is none of the above */
    o = task_set_error_soon(
        task, PyExc_RuntimeError, "Task got bad yield: %R", result);
    Py_DECREF(result);
    return o;

self_await:
    o = task_set_error_soon(
        task, PyExc_RuntimeError,
        "Task cannot await on itself: %R", task);
    Py_DECREF(result);
    return o;

yield_insteadof_yf:
    o = task_set_error_soon(
        task, PyExc_RuntimeError,
        "yield was used instead of yield from "
        "in task %R with %R",
        task, result);
    Py_DECREF(result);
    return o;

different_loop:
    o = task_set_error_soon(
        task, PyExc_RuntimeError,
        "Task %R got Future %R attached to a different loop",
        task, result);
    Py_DECREF(result);
    return o;

fail:
    Py_XDECREF(result);
    return NULL;
}

static PyObject *
task_step(TaskObj *task, PyObject *exc)
{
    PyObject *res;

    if (enter_task(task->task_loop, (PyObject*)task) < 0) {
        return NULL;
    }

    res = task_step_impl(task, exc);

    if (res == NULL) {
        PyObject *et, *ev, *tb;
        PyErr_Fetch(&et, &ev, &tb);
        leave_task(task->task_loop, (PyObject*)task);
        _PyErr_ChainExceptions(et, ev, tb);
        return NULL;
    }
    else {
        if (leave_task(task->task_loop, (PyObject*)task) < 0) {
            Py_DECREF(res);
            return NULL;
        }
        else {
            return res;
        }
    }
}

static PyObject *
task_wakeup(TaskObj *task, PyObject *o)
{
    PyObject *et, *ev, *tb;
    PyObject *result;
    assert(o);

    if (Future_CheckExact(o) || Task_CheckExact(o)) {
        PyObject *fut_result = NULL;
        int res = future_get_result((FutureObj*)o, &fut_result);

        switch(res) {
        case -1:
            assert(fut_result == NULL);
            break; /* exception raised */
        case 0:
            Py_DECREF(fut_result);
            return task_step(task, NULL);
        default:
            assert(res == 1);
            result = task_step(task, fut_result);
            Py_DECREF(fut_result);
            return result;
        }
    }
    else {
        PyObject *fut_result = PyObject_CallMethod(o, "result", NULL);
        if (fut_result != NULL) {
            Py_DECREF(fut_result);
            return task_step(task, NULL);
        }
        /* exception raised */
    }

    PyErr_Fetch(&et, &ev, &tb);
    if (!ev || !PyObject_TypeCheck(ev, (PyTypeObject *) et)) {
        PyErr_NormalizeException(&et, &ev, &tb);
    }

    result = task_step(task, ev);

    Py_DECREF(et);
    Py_XDECREF(tb);
    Py_XDECREF(ev);

    return result;
}


/*********************** Functions **************************/


/*[clinic input]
_asyncio._get_running_loop

Return the running event loop or None.

This is a low-level function intended to be used by event loops.
This function is thread-specific.

[clinic start generated code]*/

static PyObject *
_asyncio__get_running_loop_impl(PyObject *module)
/*[clinic end generated code: output=b4390af721411a0a input=0a21627e25a4bd43]*/
{
    PyObject *loop;
    if (get_running_loop(&loop)) {
        return NULL;
    }
    if (loop == NULL) {
        /* There's no currently running event loop */
        Py_RETURN_NONE;
    }
    return loop;
}

/*[clinic input]
_asyncio._set_running_loop
    loop: 'O'
    /

Set the running event loop.

This is a low-level function intended to be used by event loops.
This function is thread-specific.
[clinic start generated code]*/

static PyObject *
_asyncio__set_running_loop(PyObject *module, PyObject *loop)
/*[clinic end generated code: output=ae56bf7a28ca189a input=4c9720233d606604]*/
{
    if (set_running_loop(loop)) {
        return NULL;
    }
    Py_RETURN_NONE;
}

/*[clinic input]
_asyncio.get_event_loop

Return an asyncio event loop.

When called from a coroutine or a callback (e.g. scheduled with
call_soon or similar API), this function will always return the
running event loop.

If there is no running event loop set, the function will return
the result of `get_event_loop_policy().get_event_loop()` call.
[clinic start generated code]*/

static PyObject *
_asyncio_get_event_loop_impl(PyObject *module)
/*[clinic end generated code: output=2a2d8b2f824c648b input=9364bf2916c8655d]*/
{
    return get_event_loop(1);
}

/*[clinic input]
_asyncio._get_event_loop
    stacklevel: int = 3
[clinic start generated code]*/

static PyObject *
_asyncio__get_event_loop_impl(PyObject *module, int stacklevel)
/*[clinic end generated code: output=9c1d6d3c802e67c9 input=d17aebbd686f711d]*/
{
    return get_event_loop(stacklevel-1);
}

/*[clinic input]
_asyncio.get_running_loop

Return the running event loop.  Raise a RuntimeError if there is none.

This function is thread-specific.
[clinic start generated code]*/

static PyObject *
_asyncio_get_running_loop_impl(PyObject *module)
/*[clinic end generated code: output=c247b5f9e529530e input=2a3bf02ba39f173d]*/
{
    PyObject *loop;
    if (get_running_loop(&loop)) {
        return NULL;
    }
    if (loop == NULL) {
        /* There's no currently running event loop */
        PyErr_SetString(
            PyExc_RuntimeError, "no running event loop");
    }
    return loop;
}

/*[clinic input]
_asyncio._register_task

    task: object

Register a new task in asyncio as executed by loop.

Returns None.
[clinic start generated code]*/

static PyObject *
_asyncio__register_task_impl(PyObject *module, PyObject *task)
/*[clinic end generated code: output=8672dadd69a7d4e2 input=21075aaea14dfbad]*/
{
    if (register_task(task) < 0) {
        return NULL;
    }
    Py_RETURN_NONE;
}


/*[clinic input]
_asyncio._unregister_task

    task: object

Unregister a task.

Returns None.
[clinic start generated code]*/

static PyObject *
_asyncio__unregister_task_impl(PyObject *module, PyObject *task)
/*[clinic end generated code: output=6e5585706d568a46 input=28fb98c3975f7bdc]*/
{
    if (unregister_task(task) < 0) {
        return NULL;
    }
    Py_RETURN_NONE;
}


/*[clinic input]
_asyncio._enter_task

    loop: object
    task: object

Enter into task execution or resume suspended task.

Task belongs to loop.

Returns None.
[clinic start generated code]*/

static PyObject *
_asyncio__enter_task_impl(PyObject *module, PyObject *loop, PyObject *task)
/*[clinic end generated code: output=a22611c858035b73 input=de1b06dca70d8737]*/
{
    if (enter_task(loop, task) < 0) {
        return NULL;
    }
    Py_RETURN_NONE;
}


/*[clinic input]
_asyncio._leave_task

    loop: object
    task: object

Leave task execution or suspend a task.

Task belongs to loop.

Returns None.
[clinic start generated code]*/

static PyObject *
_asyncio__leave_task_impl(PyObject *module, PyObject *loop, PyObject *task)
/*[clinic end generated code: output=0ebf6db4b858fb41 input=51296a46313d1ad8]*/
{
    if (leave_task(loop, task) < 0) {
        return NULL;
    }
    Py_RETURN_NONE;
}


/*********************** PyRunningLoopHolder ********************/


static PyRunningLoopHolder *
new_running_loop_holder(PyObject *loop)
{
    PyRunningLoopHolder *rl = PyObject_New(
        PyRunningLoopHolder, &PyRunningLoopHolder_Type);
    if (rl == NULL) {
        return NULL;
    }

#if defined(HAVE_GETPID) && !defined(MS_WINDOWS)
    rl->rl_pid = getpid();
#endif

    Py_INCREF(loop);
    rl->rl_loop = loop;

    return rl;
}


static void
PyRunningLoopHolder_tp_dealloc(PyRunningLoopHolder *rl)
{
    if (cached_running_holder == (PyObject *)rl) {
        cached_running_holder = NULL;
    }
    Py_CLEAR(rl->rl_loop);
    PyObject_Free(rl);
}


static PyTypeObject PyRunningLoopHolder_Type = {
    PyVarObject_HEAD_INIT(NULL, 0)
    "_RunningLoopHolder",
    sizeof(PyRunningLoopHolder),
    .tp_getattro = PyObject_GenericGetAttr,
    .tp_flags = Py_TPFLAGS_DEFAULT,
    .tp_dealloc = (destructor)PyRunningLoopHolder_tp_dealloc,
};


/*********************** Module **************************/


static void
module_free_freelists(void)
{
    PyObject *next;
    PyObject *current;

    next = (PyObject*) fi_freelist;
    while (next != NULL) {
        assert(fi_freelist_len > 0);
        fi_freelist_len--;

        current = next;
        next = (PyObject*) ((futureiterobject*) current)->future;
        PyObject_GC_Del(current);
    }
    assert(fi_freelist_len == 0);
    fi_freelist = NULL;
}


static void
module_free(void *m)
{
    Py_CLEAR(asyncio_mod);
    Py_CLEAR(traceback_extract_stack);
    Py_CLEAR(asyncio_future_repr_info_func);
    Py_CLEAR(asyncio_get_event_loop_policy);
    Py_CLEAR(asyncio_iscoroutine_func);
    Py_CLEAR(asyncio_task_get_stack_func);
    Py_CLEAR(asyncio_task_print_stack_func);
    Py_CLEAR(asyncio_task_repr_info_func);
    Py_CLEAR(asyncio_InvalidStateError);
    Py_CLEAR(asyncio_CancelledError);

    Py_CLEAR(all_tasks);
    Py_CLEAR(current_tasks);
    Py_CLEAR(iscoroutine_typecache);

    Py_CLEAR(context_kwname);

    module_free_freelists();

    module_initialized = 0;
}

static int
module_init(void)
{
    PyObject *module = NULL;
    if (module_initialized) {
        return 0;
    }

    asyncio_mod = PyImport_ImportModule("asyncio");
    if (asyncio_mod == NULL) {
        goto fail;
    }

    current_tasks = PyDict_New();
    if (current_tasks == NULL) {
        goto fail;
    }

    iscoroutine_typecache = PySet_New(NULL);
    if (iscoroutine_typecache == NULL) {
        goto fail;
    }


    context_kwname = Py_BuildValue("(s)", "context");
    if (context_kwname == NULL) {
        goto fail;
    }

#define WITH_MOD(NAME) \
    Py_CLEAR(module); \
    module = PyImport_ImportModule(NAME); \
    if (module == NULL) { \
        goto fail; \
    }

#define GET_MOD_ATTR(VAR, NAME) \
    VAR = PyObject_GetAttrString(module, NAME); \
    if (VAR == NULL) { \
        goto fail; \
    }

    WITH_MOD("asyncio.events")
    GET_MOD_ATTR(asyncio_get_event_loop_policy, "get_event_loop_policy")

    WITH_MOD("asyncio.base_futures")
    GET_MOD_ATTR(asyncio_future_repr_info_func, "_future_repr_info")

    WITH_MOD("asyncio.exceptions")
    GET_MOD_ATTR(asyncio_InvalidStateError, "InvalidStateError")
    GET_MOD_ATTR(asyncio_CancelledError, "CancelledError")

    WITH_MOD("asyncio.base_tasks")
    GET_MOD_ATTR(asyncio_task_repr_info_func, "_task_repr_info")
    GET_MOD_ATTR(asyncio_task_get_stack_func, "_task_get_stack")
    GET_MOD_ATTR(asyncio_task_print_stack_func, "_task_print_stack")

    WITH_MOD("asyncio.coroutines")
    GET_MOD_ATTR(asyncio_iscoroutine_func, "iscoroutine")

    WITH_MOD("traceback")
    GET_MOD_ATTR(traceback_extract_stack, "extract_stack")

    PyObject *weak_set;
    WITH_MOD("weakref")
    GET_MOD_ATTR(weak_set, "WeakSet");
    all_tasks = PyObject_CallNoArgs(weak_set);
    Py_CLEAR(weak_set);
    if (all_tasks == NULL) {
        goto fail;
    }

    module_initialized = 1;
    Py_DECREF(module);
    return 0;

fail:
    Py_CLEAR(module);
    module_free(NULL);
    return -1;

#undef WITH_MOD
#undef GET_MOD_ATTR
}

PyDoc_STRVAR(module_doc, "Accelerator module for asyncio");

static PyMethodDef asyncio_methods[] = {
    _ASYNCIO_GET_EVENT_LOOP_METHODDEF
    _ASYNCIO__GET_EVENT_LOOP_METHODDEF
    _ASYNCIO_GET_RUNNING_LOOP_METHODDEF
    _ASYNCIO__GET_RUNNING_LOOP_METHODDEF
    _ASYNCIO__SET_RUNNING_LOOP_METHODDEF
    _ASYNCIO__REGISTER_TASK_METHODDEF
    _ASYNCIO__UNREGISTER_TASK_METHODDEF
    _ASYNCIO__ENTER_TASK_METHODDEF
    _ASYNCIO__LEAVE_TASK_METHODDEF
    {NULL, NULL}
};

static struct PyModuleDef _asynciomodule = {
    PyModuleDef_HEAD_INIT,      /* m_base */
    "_asyncio",                 /* m_name */
    module_doc,                 /* m_doc */
    -1,                         /* m_size */
    asyncio_methods,            /* m_methods */
    NULL,                       /* m_slots */
    NULL,                       /* m_traverse */
    NULL,                       /* m_clear */
    (freefunc)module_free       /* m_free */
};


PyMODINIT_FUNC
PyInit__asyncio(void)
{
    if (module_init() < 0) {
        return NULL;
    }
    if (PyType_Ready(&FutureIterType) < 0) {
        return NULL;
    }
    if (PyType_Ready(&TaskStepMethWrapper_Type) < 0) {
        return NULL;
    }
    if (PyType_Ready(&PyRunningLoopHolder_Type) < 0) {
        return NULL;
    }

    PyObject *m = PyModule_Create(&_asynciomodule);
    if (m == NULL) {
        return NULL;
    }

    /* FutureType and TaskType are made ready by PyModule_AddType() calls below. */
    if (PyModule_AddType(m, &FutureType) < 0) {
        Py_DECREF(m);
        return NULL;
    }

    if (PyModule_AddType(m, &TaskType) < 0) {
        Py_DECREF(m);
        return NULL;
    }

    Py_INCREF(all_tasks);
    if (PyModule_AddObject(m, "_all_tasks", all_tasks) < 0) {
        Py_DECREF(all_tasks);
        Py_DECREF(m);
        return NULL;
    }

    Py_INCREF(current_tasks);
    if (PyModule_AddObject(m, "_current_tasks", current_tasks) < 0) {
        Py_DECREF(current_tasks);
        Py_DECREF(m);
        return NULL;
    }

    return m;
}