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bpo-46771: Implement task cancel requests by Tinche · Pull Request #31513 · python/cpython · GitHub

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bpo-46771: Implement task cancel requests #31513

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Merged
gvanrossum merged 3 commits into from
Feb 24, 2022
Conversation 16 Commits 3 Checks 0 Files changed
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bpo-46771: Implement task cancel requests #31513

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a231718
Implement task cancel requests
Tinche Feb 22, 2022
ab5bce9
Tweak task docstrings
Tinche Feb 24, 2022
532a5be
Run `make regen-all`
Tinche Feb 24, 2022
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30 changes: 21 additions & 9 deletions Lib/asyncio/tasks.py
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Original file line number Diff line number Diff line change
Expand Up @@ -105,7 +105,7 @@ def __init__(self, coro, *, loop=None, name=None):
else:
self._name = str(name)

self._cancel_requested = False
self._num_cancels_requested = 0
self._must_cancel = False
self._fut_waiter = None
self._coro = coro
Expand Down Expand Up @@ -198,13 +198,15 @@ def cancel(self, msg=None):
task will be marked as cancelled when the wrapped coroutine
terminates with a CancelledError exception (even if cancel()
was not called).

This also increases the task's count of cancellation requests.
"""
self._log_traceback = False
if self.done():
return False
if self._cancel_requested:
self._num_cancels_requested += 1
if self._num_cancels_requested > 1:
return False
self._cancel_requested = True
if self._fut_waiter is not None:
if self._fut_waiter.cancel(msg=msg):
# Leave self._fut_waiter; it may be a Task that
Expand All @@ -217,14 +219,24 @@ def cancel(self, msg=None):
return True

def cancelling(self):

asvetlov Feb 23, 2022

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Is the cancelling() the best name for returning the number or cancel requests?
It was good for bool flag, a counter needs better name IMHO.

gvanrossum Feb 23, 2022

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Hm, do you have a suggestion? I was thinking that it's still usable as a "truthy" value. Usually when you're interested in the cancel count you should call uncancel() and use its return value.

asvetlov Feb 23, 2022

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

I have no good answer.
cancel_requests_count() describes exact meaning but the name is too long.
cancelling() can return bool but it looks like procrastination: why return less info than we really have.
The third option is dropping cancelling() method entirely, it is not required by the current asyncio code.
We can consider adding the method later on-demand.

asvetlov Feb 23, 2022

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Note: .cancelling() doesn't exist in Python 3.10

gvanrossum Feb 23, 2022

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

It's used in TaskGroup currently, so if we were to delete it we'd have to rewrite the code there in this same PR. It's also used by Task.__repr__() (via _task_repr_info()). But of those will work even though we upgraded it from a bool to an int. I'd say let's keep it for now, if we regret it we can rename or remove it before beta 1.

asvetlov Feb 23, 2022

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Choose a reason Spam Abuse Off Topic Outdated Duplicate Resolved Low Quality

Deal!
return self._cancel_requested
"""Return the count of the task's cancellation requests.

This count is incremented when .cancel() is called
and may be decremented using .uncancel().
"""
return self._num_cancels_requested

def uncancel(self):
if self._cancel_requested:
self._cancel_requested = False
return True
else:
return False
"""Decrement the task's count of cancellation requests.

This should be used by tasks that catch CancelledError
and wish to continue indefinitely until they are cancelled again.

Returns the remaining number of cancellation requests.
"""
if self._num_cancels_requested > 0:
self._num_cancels_requested -= 1
return self._num_cancels_requested

def __step(self, exc=None):
if self.done():
Expand Down
45 changes: 18 additions & 27 deletions Modules/_asynciomodule.c
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Original file line number Diff line number Diff line change
Expand Up @@ -91,7 +91,7 @@ typedef struct {
PyObject *task_context;
int task_must_cancel;
int task_log_destroy_pending;
int task_cancel_requested;
int task_num_cancels_requested;
} TaskObj;

typedef struct {
Expand Down Expand Up @@ -2036,7 +2036,7 @@ _asyncio_Task___init___impl(TaskObj *self, PyObject *coro, PyObject *loop,
Py_CLEAR(self->task_fut_waiter);
self->task_must_cancel = 0;
self->task_log_destroy_pending = 1;
self->task_cancel_requested = 0;
self->task_num_cancels_requested = 0;
Py_INCREF(coro);
Py_XSETREF(self->task_coro, coro);

Expand Down Expand Up @@ -2191,22 +2191,24 @@ 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).

This also increases the task's count of cancellation requests.
[clinic start generated code]*/

static PyObject *
_asyncio_Task_cancel_impl(TaskObj *self, PyObject *msg)
/*[clinic end generated code: output=c66b60d41c74f9f1 input=f4ff8e8ffc5f1c00]*/
/*[clinic end generated code: output=c66b60d41c74f9f1 input=7bb51bf25974c783]*/
{
self->task_log_tb = 0;

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

if (self->task_cancel_requested) {
self->task_num_cancels_requested += 1;
if (self->task_num_cancels_requested > 1) {
Py_RETURN_FALSE;
}
self->task_cancel_requested = 1;

if (self->task_fut_waiter) {
PyObject *res;
Expand Down Expand Up @@ -2238,51 +2240,40 @@ _asyncio_Task_cancel_impl(TaskObj *self, PyObject *msg)
/*[clinic input]
_asyncio.Task.cancelling

Return True if the task is in the process of being cancelled.

This is set once .cancel() is called
and remains set until .uncancel() is called.
Return the count of the task's cancellation requests.

As long as this flag is set, further .cancel() calls will be ignored,
until .uncancel() is called to reset it.
This count is incremented when .cancel() is called
and may be decremented using .uncancel().
[clinic start generated code]*/

static PyObject *
_asyncio_Task_cancelling_impl(TaskObj *self)
/*[clinic end generated code: output=803b3af96f917d7e input=c50e50f9c3ca4676]*/
/*[clinic end generated code: output=803b3af96f917d7e input=b625224d310cbb17]*/
/*[clinic end generated code]*/
{
if (self->task_cancel_requested) {
Py_RETURN_TRUE;
}
else {
Py_RETURN_FALSE;
}
return PyLong_FromLong(self->task_num_cancels_requested);
}

/*[clinic input]
_asyncio.Task.uncancel

Reset the flag returned by cancelling().
Decrement the task's count of cancellation requests.

This should be used by tasks that catch CancelledError
and wish to continue indefinitely until they are cancelled again.

Returns the previous value of the flag.
Returns the remaining number of cancellation requests.
[clinic start generated code]*/

static PyObject *
_asyncio_Task_uncancel_impl(TaskObj *self)
/*[clinic end generated code: output=58184d236a817d3c input=5db95e28fcb6f7cd]*/
/*[clinic end generated code: output=58184d236a817d3c input=68f81a4b90b46be2]*/
/*[clinic end generated code]*/
{
if (self->task_cancel_requested) {
self->task_cancel_requested = 0;
Py_RETURN_TRUE;
}
else {
Py_RETURN_FALSE;
if (self->task_num_cancels_requested > 0) {
self->task_num_cancels_requested -= 1;
}
return PyLong_FromLong(self->task_num_cancels_requested);
}

/*[clinic input]
Expand Down
19 changes: 9 additions & 10 deletions Modules/clinic/_asynciomodule.c.h
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