bpo-46752: Introduce task groups in asyncio

Lib/asyncio/__init__.py

@@ -17,6 +17,7 @@
 from .streams import *
 from .subprocess import *
 from .tasks import *
+from .taskgroups import *
 from .threads import *
 from .transports import *
 

Lib/asyncio/base_tasks.py

@@ -8,7 +8,7 @@
 def _task_repr_info(task):
     info = base_futures._future_repr_info(task)
 
-    if task._must_cancel:
+    if task.cancelling() and not task.done():
         # replace status
         info[0] = 'cancelling'
 

Lib/asyncio/taskgroups.py

@@ -0,0 +1,235 @@
+# Adapted with permission from the EdgeDB project.
+
+
+__all__ = ["TaskGroup"]
+
+import itertools
+import textwrap
+import traceback
+import types
+import weakref
+
+from . import events
+from . import exceptions
+from . import tasks
+
+class TaskGroup:
+
+    def __init__(self, *, name=None):
+        if name is None:
+            self._name = f'tg-{_name_counter()}'
+        else:
+            self._name = str(name)
+
+        self._entered = False
+        self._exiting = False
+        self._aborting = False
+        self._loop = None
+        self._parent_task = None
+        self._parent_cancel_requested = False
+        self._tasks = weakref.WeakSet()
+        self._unfinished_tasks = 0
+        self._errors = []
+        self._base_error = None
+        self._on_completed_fut = None
+
+    def get_name(self):
+        return self._name
+
+    def __repr__(self):
+        msg = f'<TaskGroup {self._name!r}'
+        if self._tasks:
+            msg += f' tasks:{len(self._tasks)}'
+        if self._unfinished_tasks:
+            msg += f' unfinished:{self._unfinished_tasks}'
+        if self._errors:
+            msg += f' errors:{len(self._errors)}'
+        if self._aborting:
+            msg += ' cancelling'
+        elif self._entered:
+            msg += ' entered'
+        msg += '>'
+        return msg
+
+    async def __aenter__(self):
+        if self._entered:
+            raise RuntimeError(
+                f"TaskGroup {self!r} has been already entered")
+        self._entered = True
+
+        if self._loop is None:
+            self._loop = events.get_running_loop()
+
+        self._parent_task = tasks.current_task(self._loop)
+        if self._parent_task is None:
+            raise RuntimeError(
+                f'TaskGroup {self!r} cannot determine the parent task')
+
+        return self
+
+    async def __aexit__(self, et, exc, tb):
+        self._exiting = True
+        propagate_cancellation_error = None
+
+        if (exc is not None and
+                self._is_base_error(exc) and
+                self._base_error is None):
+            self._base_error = exc
+
+        if et is exceptions.CancelledError:
+            if self._parent_cancel_requested:
+                # Only if we did request task to cancel ourselves
+                # we mark it as no longer cancelled.
+                self._parent_task.uncancel()
+            else:
+                propagate_cancellation_error = et
+
+        if et is not None and not self._aborting:
+            # Our parent task is being cancelled:
+            #
+            #    async with TaskGroup() as g:
+            #        g.create_task(...)
+            #        await ...  # <- CancelledError
+            #
+            if et is exceptions.CancelledError:
+                propagate_cancellation_error = et
+
+            # or there's an exception in "async with":
+            #
+            #    async with TaskGroup() as g:
+            #        g.create_task(...)
+            #        1 / 0
+            #
+            self._abort()
+
+        # We use while-loop here because "self._on_completed_fut"
+        # can be cancelled multiple times if our parent task
+        # is being cancelled repeatedly (or even once, when
+        # our own cancellation is already in progress)
+        while self._unfinished_tasks:
+            if self._on_completed_fut is None:
+                self._on_completed_fut = self._loop.create_future()
+
+            try:
+                await self._on_completed_fut
+            except exceptions.CancelledError as ex:
+                if not self._aborting:
+                    # Our parent task is being cancelled:
+                    #
+                    #    async def wrapper():
+                    #        async with TaskGroup() as g:
+                    #            g.create_task(foo)
+                    #
+                    # "wrapper" is being cancelled while "foo" is
+                    # still running.
+                    propagate_cancellation_error = ex
+                    self._abort()
+
+            self._on_completed_fut = None
+
+        assert self._unfinished_tasks == 0
+        self._on_completed_fut = None  # no longer needed
+
+        if self._base_error is not None:
+            raise self._base_error
+
+        if propagate_cancellation_error is not None:
+            # The wrapping task was cancelled; since we're done with
+            # closing all child tasks, just propagate the cancellation
+            # request now.
+            raise propagate_cancellation_error
+
+        if et is not None and et is not exceptions.CancelledError:
+            self._errors.append(exc)
+
+        if self._errors:
+            # Exceptions are heavy objects that can have object
+            # cycles (bad for GC); let's not keep a reference to
+            # a bunch of them.
+            errors = self._errors
+            self._errors = None
+
+            me = BaseExceptionGroup('unhandled errors in a TaskGroup', errors)
+            raise me from None
+
+    def create_task(self, coro):
+        if not self._entered:
+            raise RuntimeError(f"TaskGroup {self!r} has not been entered")
+        if self._exiting and self._unfinished_tasks == 0:
+            raise RuntimeError(f"TaskGroup {self!r} is finished")
+        task = self._loop.create_task(coro)
+        task.add_done_callback(self._on_task_done)
+        self._unfinished_tasks += 1
+        self._tasks.add(task)
+        return task
+
+    # Since Python 3.8 Tasks propagate all exceptions correctly,
+    # except for KeyboardInterrupt and SystemExit which are
+    # still considered special.
+
+    def _is_base_error(self, exc: BaseException) -> bool:
+        assert isinstance(exc, BaseException)
+        return isinstance(exc, (SystemExit, KeyboardInterrupt))
+
+    def _abort(self):
+        self._aborting = True
+
+        for t in self._tasks:
+            if not t.done():
+                t.cancel()
+
+    def _on_task_done(self, task):
+        self._unfinished_tasks -= 1
+        assert self._unfinished_tasks >= 0
+
+        if self._on_completed_fut is not None and not self._unfinished_tasks:
+            if not self._on_completed_fut.done():
+                self._on_completed_fut.set_result(True)
+
+        if task.cancelled():
+            return
+
+        exc = task.exception()
+        if exc is None:
+            return
+
+        self._errors.append(exc)
+        if self._is_base_error(exc) and self._base_error is None:
+            self._base_error = exc
+
+        if self._parent_task.done():
+            # Not sure if this case is possible, but we want to handle
+            # it anyways.
+            self._loop.call_exception_handler({
+                'message': f'Task {task!r} has errored out but its parent '
+                           f'task {self._parent_task} is already completed',
+                'exception': exc,
+                'task': task,
+            })
+            return
+
+        self._abort()
+        if not self._parent_task.cancelling():
+            # If parent task *is not* being cancelled, it means that we want
+            # to manually cancel it to abort whatever is being run right now
+            # in the TaskGroup.  But we want to mark parent task as
+            # "not cancelled" later in __aexit__.  Example situation that
+            # we need to handle:
+            #
+            #    async def foo():
+            #        try:
+            #            async with TaskGroup() as g:
+            #                g.create_task(crash_soon())
+            #                await something  # <- this needs to be canceled
+            #                                 #    by the TaskGroup, e.g.
+            #                                 #    foo() needs to be cancelled
+            #        except Exception:
+            #            # Ignore any exceptions raised in the TaskGroup
+            #            pass
+            #        await something_else     # this line has to be called
+            #                                 # after TaskGroup is finished.
+            self._parent_cancel_requested = True
+            self._parent_task.cancel()
+
+
+_name_counter = itertools.count(1).__next__

Lib/asyncio/tasks.py

@@ -105,6 +105,7 @@ def __init__(self, coro, *, loop=None, name=None):
         else:
             self._name = str(name)
 
+        self._cancel_requested = False
         self._must_cancel = False
         self._fut_waiter = None
         self._coro = coro
@@ -201,6 +202,9 @@ def cancel(self, msg=None):
         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 @@ def cancel(self, msg=None):
         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