Sync contrib/tools/python3 layout with upstream

* Move src/ subdir contents to the top of the layout
* Rename self-written lib -> lib2 to avoid CaseFolding warning from the VCS
* Regenerate contrib/libs/python proxy-headers accordingly
4ccc62ac1511abcf0fed14ccade38e984e088f1e

1 files changed, 1065 insertions, 0 deletions

diff --git a/contrib/tools/python3/Lib/asyncio/tasks.py b/contrib/tools/python3/Lib/asyncio/tasks.py
new file mode 100644
index 0000000000..65f2a6ef80
--- /dev/null
+++ b/contrib/tools/python3/Lib/asyncio/tasks.py
@@ -0,0 +1,1065 @@
+"""Support for tasks, coroutines and the scheduler."""
+
+__all__ = (
+    'Task', 'create_task',
+    'FIRST_COMPLETED', 'FIRST_EXCEPTION', 'ALL_COMPLETED',
+    'wait', 'wait_for', 'as_completed', 'sleep',
+    'gather', 'shield', 'ensure_future', 'run_coroutine_threadsafe',
+    'current_task', 'all_tasks',
+    'create_eager_task_factory', 'eager_task_factory',
+    '_register_task', '_unregister_task', '_enter_task', '_leave_task',
+)
+
+import concurrent.futures
+import contextvars
+import functools
+import inspect
+import itertools
+import types
+import warnings
+import weakref
+from types import GenericAlias
+
+from . import base_tasks
+from . import coroutines
+from . import events
+from . import exceptions
+from . import futures
+from . import timeouts
+
+# Helper to generate new task names
+# This uses itertools.count() instead of a "+= 1" operation because the latter
+# is not thread safe. See bpo-11866 for a longer explanation.
+_task_name_counter = itertools.count(1).__next__
+
+
+def current_task(loop=None):
+    """Return a currently executed task."""
+    if loop is None:
+        loop = events.get_running_loop()
+    return _current_tasks.get(loop)
+
+
+def all_tasks(loop=None):
+    """Return a set of all tasks for the loop."""
+    if loop is None:
+        loop = events.get_running_loop()
+    # capturing the set of eager tasks first, so if an eager task "graduates"
+    # to a regular task in another thread, we don't risk missing it.
+    eager_tasks = list(_eager_tasks)
+    # Looping over the WeakSet isn't safe as it can be updated from another
+    # thread, therefore we cast it to list prior to filtering. The list cast
+    # itself requires iteration, so we repeat it several times ignoring
+    # RuntimeErrors (which are not very likely to occur).
+    # See issues 34970 and 36607 for details.
+    scheduled_tasks = None
+    i = 0
+    while True:
+        try:
+            scheduled_tasks = list(_scheduled_tasks)
+        except RuntimeError:
+            i += 1
+            if i >= 1000:
+                raise
+        else:
+            break
+    return {t for t in itertools.chain(scheduled_tasks, eager_tasks)
+            if futures._get_loop(t) is loop and not t.done()}
+
+
+def _set_task_name(task, name):
+    if name is not None:
+        try:
+            set_name = task.set_name
+        except AttributeError:
+            warnings.warn("Task.set_name() was added in Python 3.8, "
+                      "the method support will be mandatory for third-party "
+                      "task implementations since 3.13.",
+                      DeprecationWarning, stacklevel=3)
+        else:
+            set_name(name)
+
+
+class Task(futures._PyFuture):  # Inherit Python Task implementation
+                                # from a Python Future implementation.
+
+    """A coroutine wrapped in a Future."""
+
+    # An important invariant maintained while a Task not done:
+    # _fut_waiter is either None or a Future.  The Future
+    # can be either done() or not done().
+    # The task can be in any of 3 states:
+    #
+    # - 1: _fut_waiter is not None and not _fut_waiter.done():
+    #      __step() is *not* scheduled and the Task is waiting for _fut_waiter.
+    # - 2: (_fut_waiter is None or _fut_waiter.done()) and __step() is scheduled:
+    #       the Task is waiting for __step() to be executed.
+    # - 3:  _fut_waiter is None and __step() is *not* scheduled:
+    #       the Task is currently executing (in __step()).
+    #
+    # * In state 1, one of the callbacks of __fut_waiter must be __wakeup().
+    # * The transition from 1 to 2 happens when _fut_waiter becomes done(),
+    #   as it schedules __wakeup() to be called (which calls __step() so
+    #   we way that __step() is scheduled).
+    # * It transitions from 2 to 3 when __step() is executed, and it clears
+    #   _fut_waiter to None.
+
+    # If False, don't log a message if the task is destroyed while its
+    # status is still pending
+    _log_destroy_pending = True
+
+    def __init__(self, coro, *, loop=None, name=None, context=None,
+                 eager_start=False):
+        super().__init__(loop=loop)
+        if self._source_traceback:
+            del self._source_traceback[-1]
+        if not coroutines.iscoroutine(coro):
+            # raise after Future.__init__(), attrs are required for __del__
+            # prevent logging for pending task in __del__
+            self._log_destroy_pending = False
+            raise TypeError(f"a coroutine was expected, got {coro!r}")
+
+        if name is None:
+            self._name = f'Task-{_task_name_counter()}'
+        else:
+            self._name = str(name)
+
+        self._num_cancels_requested = 0
+        self._must_cancel = False
+        self._fut_waiter = None
+        self._coro = coro
+        if context is None:
+            self._context = contextvars.copy_context()
+        else:
+            self._context = context
+
+        if eager_start and self._loop.is_running():
+            self.__eager_start()
+        else:
+            self._loop.call_soon(self.__step, context=self._context)
+            _register_task(self)
+
+    def __del__(self):
+        if self._state == futures._PENDING and self._log_destroy_pending:
+            context = {
+                'task': self,
+                'message': 'Task was destroyed but it is pending!',
+            }
+            if self._source_traceback:
+                context['source_traceback'] = self._source_traceback
+            self._loop.call_exception_handler(context)
+        super().__del__()
+
+    __class_getitem__ = classmethod(GenericAlias)
+
+    def __repr__(self):
+        return base_tasks._task_repr(self)
+
+    def get_coro(self):
+        return self._coro
+
+    def get_context(self):
+        return self._context
+
+    def get_name(self):
+        return self._name
+
+    def set_name(self, value):
+        self._name = str(value)
+
+    def set_result(self, result):
+        raise RuntimeError('Task does not support set_result operation')
+
+    def set_exception(self, exception):
+        raise RuntimeError('Task does not support set_exception operation')
+
+    def get_stack(self, *, limit=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.
+        """
+        return base_tasks._task_get_stack(self, limit)
+
+    def print_stack(self, *, limit=None, file=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.
+        """
+        return base_tasks._task_print_stack(self, limit, file)
+
+    def cancel(self, msg=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).
+
+        This also increases the task's count of cancellation requests.
+        """
+        self._log_traceback = False
+        if self.done():
+            return False
+        self._num_cancels_requested += 1
+        # These two lines are controversial.  See discussion starting at
+        # https://github.com/python/cpython/pull/31394#issuecomment-1053545331
+        # Also remember that this is duplicated in _asynciomodule.c.
+        # if self._num_cancels_requested > 1:
+        #     return False
+        if self._fut_waiter is not None:
+            if self._fut_waiter.cancel(msg=msg):
+                # Leave self._fut_waiter; it may be a Task that
+                # catches and ignores the cancellation so we may have
+                # to cancel it again later.
+                return True
+        # It must be the case that self.__step is already scheduled.
+        self._must_cancel = True
+        self._cancel_message = msg
+        return True
+
+    def cancelling(self):
+        """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):
+        """Decrement the task's count of cancellation requests.
+
+        This should be called by the party that called `cancel()` on the task
+        beforehand.
+
+        Returns the remaining number of cancellation requests.
+        """
+        if self._num_cancels_requested > 0:
+            self._num_cancels_requested -= 1
+        return self._num_cancels_requested
+
+    def __eager_start(self):
+        prev_task = _swap_current_task(self._loop, self)
+        try:
+            _register_eager_task(self)
+            try:
+                self._context.run(self.__step_run_and_handle_result, None)
+            finally:
+                _unregister_eager_task(self)
+        finally:
+            try:
+                curtask = _swap_current_task(self._loop, prev_task)
+                assert curtask is self
+            finally:
+                if self.done():
+                    self._coro = None
+                    self = None  # Needed to break cycles when an exception occurs.
+                else:
+                    _register_task(self)
+
+    def __step(self, exc=None):
+        if self.done():
+            raise exceptions.InvalidStateError(
+                f'_step(): already done: {self!r}, {exc!r}')
+        if self._must_cancel:
+            if not isinstance(exc, exceptions.CancelledError):
+                exc = self._make_cancelled_error()
+            self._must_cancel = False
+        self._fut_waiter = None
+
+        _enter_task(self._loop, self)
+        try:
+            self.__step_run_and_handle_result(exc)
+        finally:
+            _leave_task(self._loop, self)
+            self = None  # Needed to break cycles when an exception occurs.
+
+    def __step_run_and_handle_result(self, exc):
+        coro = self._coro
+        try:
+            if exc is None:
+                # We use the `send` method directly, because coroutines
+                # don't have `__iter__` and `__next__` methods.
+                result = coro.send(None)
+            else:
+                result = coro.throw(exc)
+        except StopIteration as exc:
+            if self._must_cancel:
+                # Task is cancelled right before coro stops.
+                self._must_cancel = False
+                super().cancel(msg=self._cancel_message)
+            else:
+                super().set_result(exc.value)
+        except exceptions.CancelledError as exc:
+            # Save the original exception so we can chain it later.
+            self._cancelled_exc = exc
+            super().cancel()  # I.e., Future.cancel(self).
+        except (KeyboardInterrupt, SystemExit) as exc:
+            super().set_exception(exc)
+            raise
+        except BaseException as exc:
+            super().set_exception(exc)
+        else:
+            blocking = getattr(result, '_asyncio_future_blocking', None)
+            if blocking is not None:
+                # Yielded Future must come from Future.__iter__().
+                if futures._get_loop(result) is not self._loop:
+                    new_exc = RuntimeError(
+                        f'Task {self!r} got Future '
+                        f'{result!r} attached to a different loop')
+                    self._loop.call_soon(
+                        self.__step, new_exc, context=self._context)
+                elif blocking:
+                    if result is self:
+                        new_exc = RuntimeError(
+                            f'Task cannot await on itself: {self!r}')
+                        self._loop.call_soon(
+                            self.__step, new_exc, context=self._context)
+                    else:
+                        result._asyncio_future_blocking = False
+                        result.add_done_callback(
+                            self.__wakeup, context=self._context)
+                        self._fut_waiter = result
+                        if self._must_cancel:
+                            if self._fut_waiter.cancel(
+                                    msg=self._cancel_message):
+                                self._must_cancel = False
+                else:
+                    new_exc = RuntimeError(
+                        f'yield was used instead of yield from '
+                        f'in task {self!r} with {result!r}')
+                    self._loop.call_soon(
+                        self.__step, new_exc, context=self._context)
+
+            elif result is None:
+                # Bare yield relinquishes control for one event loop iteration.
+                self._loop.call_soon(self.__step, context=self._context)
+            elif inspect.isgenerator(result):
+                # Yielding a generator is just wrong.
+                new_exc = RuntimeError(
+                    f'yield was used instead of yield from for '
+                    f'generator in task {self!r} with {result!r}')
+                self._loop.call_soon(
+                    self.__step, new_exc, context=self._context)
+            else:
+                # Yielding something else is an error.
+                new_exc = RuntimeError(f'Task got bad yield: {result!r}')
+                self._loop.call_soon(
+                    self.__step, new_exc, context=self._context)
+        finally:
+            self = None  # Needed to break cycles when an exception occurs.
+
+    def __wakeup(self, future):
+        try:
+            future.result()
+        except BaseException as exc:
+            # This may also be a cancellation.
+            self.__step(exc)
+        else:
+            # Don't pass the value of `future.result()` explicitly,
+            # as `Future.__iter__` and `Future.__await__` don't need it.
+            # If we call `_step(value, None)` instead of `_step()`,
+            # Python eval loop would use `.send(value)` method call,
+            # instead of `__next__()`, which is slower for futures
+            # that return non-generator iterators from their `__iter__`.
+            self.__step()
+        self = None  # Needed to break cycles when an exception occurs.
+
+
+_PyTask = Task
+
+
+try:
+    import _asyncio
+except ImportError:
+    pass
+else:
+    # _CTask is needed for tests.
+    Task = _CTask = _asyncio.Task
+
+
+def create_task(coro, *, name=None, context=None):
+    """Schedule the execution of a coroutine object in a spawn task.
+
+    Return a Task object.
+    """
+    loop = events.get_running_loop()
+    if context is None:
+        # Use legacy API if context is not needed
+        task = loop.create_task(coro)
+    else:
+        task = loop.create_task(coro, context=context)
+
+    _set_task_name(task, name)
+    return task
+
+
+# wait() and as_completed() similar to those in PEP 3148.
+
+FIRST_COMPLETED = concurrent.futures.FIRST_COMPLETED
+FIRST_EXCEPTION = concurrent.futures.FIRST_EXCEPTION
+ALL_COMPLETED = concurrent.futures.ALL_COMPLETED
+
+
+async def wait(fs, *, timeout=None, return_when=ALL_COMPLETED):
+    """Wait for the Futures or Tasks given by fs to complete.
+
+    The fs iterable must not be empty.
+
+    Coroutines will be wrapped in Tasks.
+
+    Returns two sets of Future: (done, pending).
+
+    Usage:
+
+        done, pending = await asyncio.wait(fs)
+
+    Note: This does not raise TimeoutError! Futures that aren't done
+    when the timeout occurs are returned in the second set.
+    """
+    if futures.isfuture(fs) or coroutines.iscoroutine(fs):
+        raise TypeError(f"expect a list of futures, not {type(fs).__name__}")
+    if not fs:
+        raise ValueError('Set of Tasks/Futures is empty.')
+    if return_when not in (FIRST_COMPLETED, FIRST_EXCEPTION, ALL_COMPLETED):
+        raise ValueError(f'Invalid return_when value: {return_when}')
+
+    fs = set(fs)
+
+    if any(coroutines.iscoroutine(f) for f in fs):
+        raise TypeError("Passing coroutines is forbidden, use tasks explicitly.")
+
+    loop = events.get_running_loop()
+    return await _wait(fs, timeout, return_when, loop)
+
+
+def _release_waiter(waiter, *args):
+    if not waiter.done():
+        waiter.set_result(None)
+
+
+async def wait_for(fut, timeout):
+    """Wait for the single Future or coroutine to complete, with timeout.
+
+    Coroutine will be wrapped in Task.
+
+    Returns result of the Future or coroutine.  When a timeout occurs,
+    it cancels the task and raises TimeoutError.  To avoid the task
+    cancellation, wrap it in shield().
+
+    If the wait is cancelled, the task is also cancelled.
+
+    If the task supresses the cancellation and returns a value instead,
+    that value is returned.
+
+    This function is a coroutine.
+    """
+    # The special case for timeout <= 0 is for the following case:
+    #
+    # async def test_waitfor():
+    #     func_started = False
+    #
+    #     async def func():
+    #         nonlocal func_started
+    #         func_started = True
+    #
+    #     try:
+    #         await asyncio.wait_for(func(), 0)
+    #     except asyncio.TimeoutError:
+    #         assert not func_started
+    #     else:
+    #         assert False
+    #
+    # asyncio.run(test_waitfor())
+
+
+    if timeout is not None and timeout <= 0:
+        fut = ensure_future(fut)
+
+        if fut.done():
+            return fut.result()
+
+        await _cancel_and_wait(fut)
+        try:
+            return fut.result()
+        except exceptions.CancelledError as exc:
+            raise TimeoutError from exc
+
+    async with timeouts.timeout(timeout):
+        return await fut
+
+async def _wait(fs, timeout, return_when, loop):
+    """Internal helper for wait().
+
+    The fs argument must be a collection of Futures.
+    """
+    assert fs, 'Set of Futures is empty.'
+    waiter = loop.create_future()
+    timeout_handle = None
+    if timeout is not None:
+        timeout_handle = loop.call_later(timeout, _release_waiter, waiter)
+    counter = len(fs)
+
+    def _on_completion(f):
+        nonlocal counter
+        counter -= 1
+        if (counter <= 0 or
+            return_when == FIRST_COMPLETED or
+            return_when == FIRST_EXCEPTION and (not f.cancelled() and
+                                                f.exception() is not None)):
+            if timeout_handle is not None:
+                timeout_handle.cancel()
+            if not waiter.done():
+                waiter.set_result(None)
+
+    for f in fs:
+        f.add_done_callback(_on_completion)
+
+    try:
+        await waiter
+    finally:
+        if timeout_handle is not None:
+            timeout_handle.cancel()
+        for f in fs:
+            f.remove_done_callback(_on_completion)
+
+    done, pending = set(), set()
+    for f in fs:
+        if f.done():
+            done.add(f)
+        else:
+            pending.add(f)
+    return done, pending
+
+
+async def _cancel_and_wait(fut):
+    """Cancel the *fut* future or task and wait until it completes."""
+
+    loop = events.get_running_loop()
+    waiter = loop.create_future()
+    cb = functools.partial(_release_waiter, waiter)
+    fut.add_done_callback(cb)
+
+    try:
+        fut.cancel()
+        # We cannot wait on *fut* directly to make
+        # sure _cancel_and_wait itself is reliably cancellable.
+        await waiter
+    finally:
+        fut.remove_done_callback(cb)
+
+
+# This is *not* a @coroutine!  It is just an iterator (yielding Futures).
+def as_completed(fs, *, timeout=None):
+    """Return an iterator whose values are coroutines.
+
+    When waiting for the yielded coroutines you'll get the results (or
+    exceptions!) of the original Futures (or coroutines), in the order
+    in which and as soon as they complete.
+
+    This differs from PEP 3148; the proper way to use this is:
+
+        for f in as_completed(fs):
+            result = await f  # The 'await' may raise.
+            # Use result.
+
+    If a timeout is specified, the 'await' will raise
+    TimeoutError when the timeout occurs before all Futures are done.
+
+    Note: The futures 'f' are not necessarily members of fs.
+    """
+    if futures.isfuture(fs) or coroutines.iscoroutine(fs):
+        raise TypeError(f"expect an iterable of futures, not {type(fs).__name__}")
+
+    from .queues import Queue  # Import here to avoid circular import problem.
+    done = Queue()
+
+    loop = events.get_event_loop()
+    todo = {ensure_future(f, loop=loop) for f in set(fs)}
+    timeout_handle = None
+
+    def _on_timeout():
+        for f in todo:
+            f.remove_done_callback(_on_completion)
+            done.put_nowait(None)  # Queue a dummy value for _wait_for_one().
+        todo.clear()  # Can't do todo.remove(f) in the loop.
+
+    def _on_completion(f):
+        if not todo:
+            return  # _on_timeout() was here first.
+        todo.remove(f)
+        done.put_nowait(f)
+        if not todo and timeout_handle is not None:
+            timeout_handle.cancel()
+
+    async def _wait_for_one():
+        f = await done.get()
+        if f is None:
+            # Dummy value from _on_timeout().
+            raise exceptions.TimeoutError
+        return f.result()  # May raise f.exception().
+
+    for f in todo:
+        f.add_done_callback(_on_completion)
+    if todo and timeout is not None:
+        timeout_handle = loop.call_later(timeout, _on_timeout)
+    for _ in range(len(todo)):
+        yield _wait_for_one()
+
+
+@types.coroutine
+def __sleep0():
+    """Skip one event loop run cycle.
+
+    This is a private helper for 'asyncio.sleep()', used
+    when the 'delay' is set to 0.  It uses a bare 'yield'
+    expression (which Task.__step knows how to handle)
+    instead of creating a Future object.
+    """
+    yield
+
+
+async def sleep(delay, result=None):
+    """Coroutine that completes after a given time (in seconds)."""
+    if delay <= 0:
+        await __sleep0()
+        return result
+
+    loop = events.get_running_loop()
+    future = loop.create_future()
+    h = loop.call_later(delay,
+                        futures._set_result_unless_cancelled,
+                        future, result)
+    try:
+        return await future
+    finally:
+        h.cancel()
+
+
+def ensure_future(coro_or_future, *, loop=None):
+    """Wrap a coroutine or an awaitable in a future.
+
+    If the argument is a Future, it is returned directly.
+    """
+    if futures.isfuture(coro_or_future):
+        if loop is not None and loop is not futures._get_loop(coro_or_future):
+            raise ValueError('The future belongs to a different loop than '
+                            'the one specified as the loop argument')
+        return coro_or_future
+    should_close = True
+    if not coroutines.iscoroutine(coro_or_future):
+        if inspect.isawaitable(coro_or_future):
+            async def _wrap_awaitable(awaitable):
+                return await awaitable
+
+            coro_or_future = _wrap_awaitable(coro_or_future)
+            should_close = False
+        else:
+            raise TypeError('An asyncio.Future, a coroutine or an awaitable '
+                            'is required')
+
+    if loop is None:
+        loop = events.get_event_loop()
+    try:
+        return loop.create_task(coro_or_future)
+    except RuntimeError:
+        if should_close:
+            coro_or_future.close()
+        raise
+
+
+class _GatheringFuture(futures.Future):
+    """Helper for gather().
+
+    This overrides cancel() to cancel all the children and act more
+    like Task.cancel(), which doesn't immediately mark itself as
+    cancelled.
+    """
+
+    def __init__(self, children, *, loop):
+        assert loop is not None
+        super().__init__(loop=loop)
+        self._children = children
+        self._cancel_requested = False
+
+    def cancel(self, msg=None):
+        if self.done():
+            return False
+        ret = False
+        for child in self._children:
+            if child.cancel(msg=msg):
+                ret = True
+        if ret:
+            # If any child tasks were actually cancelled, we should
+            # propagate the cancellation request regardless of
+            # *return_exceptions* argument.  See issue 32684.
+            self._cancel_requested = True
+        return ret
+
+
+def gather(*coros_or_futures, return_exceptions=False):
+    """Return a future aggregating results from the given coroutines/futures.
+
+    Coroutines will be wrapped in a future and scheduled in the event
+    loop. They will not necessarily be scheduled in the same order as
+    passed in.
+
+    All futures must share the same event loop.  If all the tasks are
+    done successfully, the returned future's result is the list of
+    results (in the order of the original sequence, not necessarily
+    the order of results arrival).  If *return_exceptions* is True,
+    exceptions in the tasks are treated the same as successful
+    results, and gathered in the result list; otherwise, the first
+    raised exception will be immediately propagated to the returned
+    future.
+
+    Cancellation: if the outer Future is cancelled, all children (that
+    have not completed yet) are also cancelled.  If any child is
+    cancelled, this is treated as if it raised CancelledError --
+    the outer Future is *not* cancelled in this case.  (This is to
+    prevent the cancellation of one child to cause other children to
+    be cancelled.)
+
+    If *return_exceptions* is False, cancelling gather() after it
+    has been marked done won't cancel any submitted awaitables.
+    For instance, gather can be marked done after propagating an
+    exception to the caller, therefore, calling ``gather.cancel()``
+    after catching an exception (raised by one of the awaitables) from
+    gather won't cancel any other awaitables.
+    """
+    if not coros_or_futures:
+        loop = events.get_event_loop()
+        outer = loop.create_future()
+        outer.set_result([])
+        return outer
+
+    def _done_callback(fut):
+        nonlocal nfinished
+        nfinished += 1
+
+        if outer is None or outer.done():
+            if not fut.cancelled():
+                # Mark exception retrieved.
+                fut.exception()
+            return
+
+        if not return_exceptions:
+            if fut.cancelled():
+                # Check if 'fut' is cancelled first, as
+                # 'fut.exception()' will *raise* a CancelledError
+                # instead of returning it.
+                exc = fut._make_cancelled_error()
+                outer.set_exception(exc)
+                return
+            else:
+                exc = fut.exception()
+                if exc is not None:
+                    outer.set_exception(exc)
+                    return
+
+        if nfinished == nfuts:
+            # All futures are done; create a list of results
+            # and set it to the 'outer' future.
+            results = []
+
+            for fut in children:
+                if fut.cancelled():
+                    # Check if 'fut' is cancelled first, as 'fut.exception()'
+                    # will *raise* a CancelledError instead of returning it.
+                    # Also, since we're adding the exception return value
+                    # to 'results' instead of raising it, don't bother
+                    # setting __context__.  This also lets us preserve
+                    # calling '_make_cancelled_error()' at most once.
+                    res = exceptions.CancelledError(
+                        '' if fut._cancel_message is None else
+                        fut._cancel_message)
+                else:
+                    res = fut.exception()
+                    if res is None:
+                        res = fut.result()
+                results.append(res)
+
+            if outer._cancel_requested:
+                # If gather is being cancelled we must propagate the
+                # cancellation regardless of *return_exceptions* argument.
+                # See issue 32684.
+                exc = fut._make_cancelled_error()
+                outer.set_exception(exc)
+            else:
+                outer.set_result(results)
+
+    arg_to_fut = {}
+    children = []
+    nfuts = 0
+    nfinished = 0
+    done_futs = []
+    loop = None
+    outer = None  # bpo-46672
+    for arg in coros_or_futures:
+        if arg not in arg_to_fut:
+            fut = ensure_future(arg, loop=loop)
+            if loop is None:
+                loop = futures._get_loop(fut)
+            if fut is not arg:
+                # 'arg' was not a Future, therefore, 'fut' is a new
+                # Future created specifically for 'arg'.  Since the caller
+                # can't control it, disable the "destroy pending task"
+                # warning.
+                fut._log_destroy_pending = False
+
+            nfuts += 1
+            arg_to_fut[arg] = fut
+            if fut.done():
+                done_futs.append(fut)
+            else:
+                fut.add_done_callback(_done_callback)
+
+        else:
+            # There's a duplicate Future object in coros_or_futures.
+            fut = arg_to_fut[arg]
+
+        children.append(fut)
+
+    outer = _GatheringFuture(children, loop=loop)
+    # Run done callbacks after GatheringFuture created so any post-processing
+    # can be performed at this point
+    # optimization: in the special case that *all* futures finished eagerly,
+    # this will effectively complete the gather eagerly, with the last
+    # callback setting the result (or exception) on outer before returning it
+    for fut in done_futs:
+        _done_callback(fut)
+    return outer
+
+
+def shield(arg):
+    """Wait for a future, shielding it from cancellation.
+
+    The statement
+
+        task = asyncio.create_task(something())
+        res = await shield(task)
+
+    is exactly equivalent to the statement
+
+        res = await something()
+
+    *except* that if the coroutine containing it is cancelled, the
+    task running in something() is not cancelled.  From the POV of
+    something(), the cancellation did not happen.  But its caller is
+    still cancelled, so the yield-from expression still raises
+    CancelledError.  Note: If something() is cancelled by other means
+    this will still cancel shield().
+
+    If you want to completely ignore cancellation (not recommended)
+    you can combine shield() with a try/except clause, as follows:
+
+        task = asyncio.create_task(something())
+        try:
+            res = await shield(task)
+        except CancelledError:
+            res = None
+
+    Save a reference to tasks passed to this function, to avoid
+    a task disappearing mid-execution. The event loop only keeps
+    weak references to tasks. A task that isn't referenced elsewhere
+    may get garbage collected at any time, even before it's done.
+    """
+    inner = ensure_future(arg)
+    if inner.done():
+        # Shortcut.
+        return inner
+    loop = futures._get_loop(inner)
+    outer = loop.create_future()
+
+    def _inner_done_callback(inner):
+        if outer.cancelled():
+            if not inner.cancelled():
+                # Mark inner's result as retrieved.
+                inner.exception()
+            return
+
+        if inner.cancelled():
+            outer.cancel()
+        else:
+            exc = inner.exception()
+            if exc is not None:
+                outer.set_exception(exc)
+            else:
+                outer.set_result(inner.result())
+
+
+    def _outer_done_callback(outer):
+        if not inner.done():
+            inner.remove_done_callback(_inner_done_callback)
+
+    inner.add_done_callback(_inner_done_callback)
+    outer.add_done_callback(_outer_done_callback)
+    return outer
+
+
+def run_coroutine_threadsafe(coro, loop):
+    """Submit a coroutine object to a given event loop.
+
+    Return a concurrent.futures.Future to access the result.
+    """
+    if not coroutines.iscoroutine(coro):
+        raise TypeError('A coroutine object is required')
+    future = concurrent.futures.Future()
+
+    def callback():
+        try:
+            futures._chain_future(ensure_future(coro, loop=loop), future)
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
+            if future.set_running_or_notify_cancel():
+                future.set_exception(exc)
+            raise
+
+    loop.call_soon_threadsafe(callback)
+    return future
+
+
+def create_eager_task_factory(custom_task_constructor):
+    """Create a function suitable for use as a task factory on an event-loop.
+
+        Example usage:
+
+            loop.set_task_factory(
+                asyncio.create_eager_task_factory(my_task_constructor))
+
+        Now, tasks created will be started immediately (rather than being first
+        scheduled to an event loop). The constructor argument can be any callable
+        that returns a Task-compatible object and has a signature compatible
+        with `Task.__init__`; it must have the `eager_start` keyword argument.
+
+        Most applications will use `Task` for `custom_task_constructor` and in
+        this case there's no need to call `create_eager_task_factory()`
+        directly. Instead the  global `eager_task_factory` instance can be
+        used. E.g. `loop.set_task_factory(asyncio.eager_task_factory)`.
+        """
+
+    def factory(loop, coro, *, name=None, context=None):
+        return custom_task_constructor(
+            coro, loop=loop, name=name, context=context, eager_start=True)
+
+    return factory
+
+
+eager_task_factory = create_eager_task_factory(Task)
+
+
+# Collectively these two sets hold references to the complete set of active
+# tasks. Eagerly executed tasks use a faster regular set as an optimization
+# but may graduate to a WeakSet if the task blocks on IO.
+_scheduled_tasks = weakref.WeakSet()
+_eager_tasks = set()
+
+# Dictionary containing tasks that are currently active in
+# all running event loops.  {EventLoop: Task}
+_current_tasks = {}
+
+
+def _register_task(task):
+    """Register an asyncio Task scheduled to run on an event loop."""
+    _scheduled_tasks.add(task)
+
+
+def _register_eager_task(task):
+    """Register an asyncio Task about to be eagerly executed."""
+    _eager_tasks.add(task)
+
+
+def _enter_task(loop, task):
+    current_task = _current_tasks.get(loop)
+    if current_task is not None:
+        raise RuntimeError(f"Cannot enter into task {task!r} while another "
+                           f"task {current_task!r} is being executed.")
+    _current_tasks[loop] = task
+
+
+def _leave_task(loop, task):
+    current_task = _current_tasks.get(loop)
+    if current_task is not task:
+        raise RuntimeError(f"Leaving task {task!r} does not match "
+                           f"the current task {current_task!r}.")
+    del _current_tasks[loop]
+
+
+def _swap_current_task(loop, task):
+    prev_task = _current_tasks.get(loop)
+    if task is None:
+        del _current_tasks[loop]
+    else:
+        _current_tasks[loop] = task
+    return prev_task
+
+
+def _unregister_task(task):
+    """Unregister a completed, scheduled Task."""
+    _scheduled_tasks.discard(task)
+
+
+def _unregister_eager_task(task):
+    """Unregister a task which finished its first eager step."""
+    _eager_tasks.discard(task)
+
+
+_py_current_task = current_task
+_py_register_task = _register_task
+_py_register_eager_task = _register_eager_task
+_py_unregister_task = _unregister_task
+_py_unregister_eager_task = _unregister_eager_task
+_py_enter_task = _enter_task
+_py_leave_task = _leave_task
+_py_swap_current_task = _swap_current_task
+
+
+try:
+    from _asyncio import (_register_task, _register_eager_task,
+                          _unregister_task, _unregister_eager_task,
+                          _enter_task, _leave_task, _swap_current_task,
+                          _scheduled_tasks, _eager_tasks, _current_tasks,
+                          current_task)
+except ImportError:
+    pass
+else:
+    _c_current_task = current_task
+    _c_register_task = _register_task
+    _c_register_eager_task = _register_eager_task
+    _c_unregister_task = _unregister_task
+    _c_unregister_eager_task = _unregister_eager_task
+    _c_enter_task = _enter_task
+    _c_leave_task = _leave_task
+    _c_swap_current_task = _swap_current_task