bpo-46752: Add TaskGroup; add Task..cancelled(),.uncancel() (GH-31270)

asyncio/taskgroups.py is an adaptation of taskgroup.py from EdgeDb, with the following key changes:

- Allow creating new tasks as long as the last task hasn't finished
- Raise [Base]ExceptionGroup (directly) rather than TaskGroupError deriving from MultiError
- Instead of monkey-patching the parent task's cancel() method,
  add a new public API to Task

The Task class has a new internal flag, `_cancel_requested`, which is set when `.cancel()` is called successfully. The `.cancelling()` method returns the value of this flag. Further `.cancel()` calls while this flag is set return False. To reset this flag, call `.uncancel()`.

Thus, a Task that catches and ignores `CancelledError` should call `.uncancel()` if it wants to be cancellable again; until it does so, it is deemed to be busy with uninterruptible cleanup.

This new Task API helps solve the problem where TaskGroup needs to distinguish between whether the parent task being cancelled "from the outside" vs. "from inside".

Co-authored-by: Yury Selivanov <yury@edgedb.com>
Co-authored-by: Andrew Svetlov <andrew.svetlov@gmail.com>

1 files changed, 15 insertions, 1 deletions

diff --git a/Lib/asyncio/tasks.py b/Lib/asyncio/tasks.py
index 2bee5c050de..c11d0daaefe 100644
--- a/Lib/asyncio/tasks.py
+++ b/Lib/asyncio/tasks.py
@@ -105,6 +105,7 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
         else:
             self._name = str(name)
 
+        self._cancel_requested = False
         self._must_cancel = False
         self._fut_waiter = None
         self._coro = coro
@@ -201,6 +202,9 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
         self._log_traceback = False
         if self.done():
             return False
+        if self._cancel_requested:
+            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
@@ -212,6 +216,16 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
         self._cancel_message = msg
         return True
 
+    def cancelling(self):
+        return self._cancel_requested
+
+    def uncancel(self):
+        if self._cancel_requested:
+            self._cancel_requested = False
+            return True
+        else:
+            return False
+
     def __step(self, exc=None):
         if self.done():
             raise exceptions.InvalidStateError(
@@ -634,7 +648,7 @@ def _ensure_future(coro_or_future, *, loop=None):
         loop = events._get_event_loop(stacklevel=4)
     try:
         return loop.create_task(coro_or_future)
-    except RuntimeError: 
+    except RuntimeError:
         if not called_wrap_awaitable:
             coro_or_future.close()
         raise