gh-97696: asyncio eager tasks factory (#102853)

Co-authored-by: Jacob Bower <jbower@meta.com>
Co-authored-by: Carol Willing <carolcode@willingconsulting.com>

3 files changed, 450 insertions, 26 deletions

diff --git a/Lib/asyncio/base_tasks.py b/Lib/asyncio/base_tasks.py
index 26298e638c..c907b68341 100644
--- a/Lib/asyncio/base_tasks.py
+++ b/Lib/asyncio/base_tasks.py
@@ -15,11 +15,13 @@ def _task_repr_info(task):
 
     info.insert(1, 'name=%r' % task.get_name())
 
-    coro = coroutines._format_coroutine(task._coro)
-    info.insert(2, f'coro=<{coro}>')
-
     if task._fut_waiter is not None:
-        info.insert(3, f'wait_for={task._fut_waiter!r}')
+        info.insert(2, f'wait_for={task._fut_waiter!r}')
+
+    if task._coro:
+        coro = coroutines._format_coroutine(task._coro)
+        info.insert(2, f'coro=<{coro}>')
+
     return info
 
 
diff --git a/Lib/asyncio/tasks.py b/Lib/asyncio/tasks.py
index c90d32c97a..aa5269ade1 100644
--- a/Lib/asyncio/tasks.py
+++ b/Lib/asyncio/tasks.py
@@ -6,6 +6,7 @@ __all__ = (
     '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',
 )
 
@@ -43,22 +44,26 @@ def all_tasks(loop=None):
     """Return a set of all tasks for the loop."""
     if loop is None:
         loop = events.get_running_loop()
-    # Looping over a WeakSet (_all_tasks) isn't safe as it can be updated from another
-    # thread while we do so. 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.
+    # 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:
-            tasks = list(_all_tasks)
+            scheduled_tasks = list(_scheduled_tasks)
         except RuntimeError:
             i += 1
             if i >= 1000:
                 raise
         else:
             break
-    return {t for t in tasks
+    return {t for t in itertools.chain(scheduled_tasks, eager_tasks)
             if futures._get_loop(t) is loop and not t.done()}
 
 
@@ -93,7 +98,8 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
     # status is still pending
     _log_destroy_pending = True
 
-    def __init__(self, coro, *, loop=None, name=None, context=None):
+    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]
@@ -117,8 +123,11 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
         else:
             self._context = context
 
-        self._loop.call_soon(self.__step, context=self._context)
-        _register_task(self)
+        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:
@@ -250,6 +259,25 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
             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(
@@ -258,11 +286,17 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
             if not isinstance(exc, exceptions.CancelledError):
                 exc = self._make_cancelled_error()
             self._must_cancel = False
-        coro = self._coro
         self._fut_waiter = None
 
         _enter_task(self._loop, self)
-        # Call either coro.throw(exc) or coro.send(None).
+        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
@@ -334,7 +368,6 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
                 self._loop.call_soon(
                     self.__step, new_exc, context=self._context)
         finally:
-            _leave_task(self._loop, self)
             self = None  # Needed to break cycles when an exception occurs.
 
     def __wakeup(self, future):
@@ -897,8 +930,27 @@ def run_coroutine_threadsafe(coro, loop):
     return future
 
 
-# WeakSet containing all alive tasks.
-_all_tasks = weakref.WeakSet()
+def create_eager_task_factory(custom_task_constructor):
+
+    if "eager_start" not in inspect.signature(custom_task_constructor).parameters:
+        raise TypeError(
+            "Provided constructor does not support eager task execution")
+
+    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}
@@ -906,8 +958,13 @@ _current_tasks = {}
 
 
 def _register_task(task):
-    """Register a new task in asyncio as executed by loop."""
-    _all_tasks.add(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):
@@ -926,28 +983,49 @@ def _leave_task(loop, task):
     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 task."""
-    _all_tasks.discard(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, _unregister_task,
-                          _enter_task, _leave_task,
-                          _all_tasks, _current_tasks,
+    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
diff --git a/Lib/test/test_asyncio/test_eager_task_factory.py b/Lib/test/test_asyncio/test_eager_task_factory.py
new file mode 100644
index 0000000000..fe69093429
--- /dev/null
+++ b/Lib/test/test_asyncio/test_eager_task_factory.py
@@ -0,0 +1,344 @@
+"""Tests for base_events.py"""
+
+import asyncio
+import contextvars
+import gc
+import time
+import unittest
+
+from types import GenericAlias
+from unittest import mock
+from asyncio import base_events
+from asyncio import tasks
+from test.test_asyncio import utils as test_utils
+from test.test_asyncio.test_tasks import get_innermost_context
+from test import support
+
+MOCK_ANY = mock.ANY
+
+
+def tearDownModule():
+    asyncio.set_event_loop_policy(None)
+
+
+class EagerTaskFactoryLoopTests:
+
+    Task = None
+
+    def run_coro(self, coro):
+        """
+        Helper method to run the `coro` coroutine in the test event loop.
+        It helps with making sure the event loop is running before starting
+        to execute `coro`. This is important for testing the eager step
+        functionality, since an eager step is taken only if the event loop
+        is already running.
+        """
+
+        async def coro_runner():
+            self.assertTrue(asyncio.get_event_loop().is_running())
+            return await coro
+
+        return self.loop.run_until_complete(coro)
+
+    def setUp(self):
+        super().setUp()
+        self.loop = asyncio.new_event_loop()
+        self.eager_task_factory = asyncio.create_eager_task_factory(self.Task)
+        self.loop.set_task_factory(self.eager_task_factory)
+        self.set_event_loop(self.loop)
+
+    def test_eager_task_factory_set(self):
+        self.assertIsNotNone(self.eager_task_factory)
+        self.assertIs(self.loop.get_task_factory(), self.eager_task_factory)
+
+        async def noop(): pass
+
+        async def run():
+            t = self.loop.create_task(noop())
+            self.assertIsInstance(t, self.Task)
+            await t
+
+        self.run_coro(run())
+
+    def test_await_future_during_eager_step(self):
+
+        async def set_result(fut, val):
+            fut.set_result(val)
+
+        async def run():
+            fut = self.loop.create_future()
+            t = self.loop.create_task(set_result(fut, 'my message'))
+            # assert the eager step completed the task
+            self.assertTrue(t.done())
+            return await fut
+
+        self.assertEqual(self.run_coro(run()), 'my message')
+
+    def test_eager_completion(self):
+
+        async def coro():
+            return 'hello'
+
+        async def run():
+            t = self.loop.create_task(coro())
+            # assert the eager step completed the task
+            self.assertTrue(t.done())
+            return await t
+
+        self.assertEqual(self.run_coro(run()), 'hello')
+
+    def test_block_after_eager_step(self):
+
+        async def coro():
+            await asyncio.sleep(0.1)
+            return 'finished after blocking'
+
+        async def run():
+            t = self.loop.create_task(coro())
+            self.assertFalse(t.done())
+            result = await t
+            self.assertTrue(t.done())
+            return result
+
+        self.assertEqual(self.run_coro(run()), 'finished after blocking')
+
+    def test_cancellation_after_eager_completion(self):
+
+        async def coro():
+            return 'finished without blocking'
+
+        async def run():
+            t = self.loop.create_task(coro())
+            t.cancel()
+            result = await t
+            # finished task can't be cancelled
+            self.assertFalse(t.cancelled())
+            return result
+
+        self.assertEqual(self.run_coro(run()), 'finished without blocking')
+
+    def test_cancellation_after_eager_step_blocks(self):
+
+        async def coro():
+            await asyncio.sleep(0.1)
+            return 'finished after blocking'
+
+        async def run():
+            t = self.loop.create_task(coro())
+            t.cancel('cancellation message')
+            self.assertGreater(t.cancelling(), 0)
+            result = await t
+
+        with self.assertRaises(asyncio.CancelledError) as cm:
+            self.run_coro(run())
+
+        self.assertEqual('cancellation message', cm.exception.args[0])
+
+    def test_current_task(self):
+        captured_current_task = None
+
+        async def coro():
+            nonlocal captured_current_task
+            captured_current_task = asyncio.current_task()
+            # verify the task before and after blocking is identical
+            await asyncio.sleep(0.1)
+            self.assertIs(asyncio.current_task(), captured_current_task)
+
+        async def run():
+            t = self.loop.create_task(coro())
+            self.assertIs(captured_current_task, t)
+            await t
+
+        self.run_coro(run())
+        captured_current_task = None
+
+    def test_all_tasks_with_eager_completion(self):
+        captured_all_tasks = None
+
+        async def coro():
+            nonlocal captured_all_tasks
+            captured_all_tasks = asyncio.all_tasks()
+
+        async def run():
+            t = self.loop.create_task(coro())
+            self.assertIn(t, captured_all_tasks)
+            self.assertNotIn(t, asyncio.all_tasks())
+
+        self.run_coro(run())
+
+    def test_all_tasks_with_blocking(self):
+        captured_eager_all_tasks = None
+
+        async def coro(fut1, fut2):
+            nonlocal captured_eager_all_tasks
+            captured_eager_all_tasks = asyncio.all_tasks()
+            await fut1
+            fut2.set_result(None)
+
+        async def run():
+            fut1 = self.loop.create_future()
+            fut2 = self.loop.create_future()
+            t = self.loop.create_task(coro(fut1, fut2))
+            self.assertIn(t, captured_eager_all_tasks)
+            self.assertIn(t, asyncio.all_tasks())
+            fut1.set_result(None)
+            await fut2
+            self.assertNotIn(t, asyncio.all_tasks())
+
+        self.run_coro(run())
+
+    def test_context_vars(self):
+        cv = contextvars.ContextVar('cv', default=0)
+
+        coro_first_step_ran = False
+        coro_second_step_ran = False
+
+        async def coro():
+            nonlocal coro_first_step_ran
+            nonlocal coro_second_step_ran
+            self.assertEqual(cv.get(), 1)
+            cv.set(2)
+            self.assertEqual(cv.get(), 2)
+            coro_first_step_ran = True
+            await asyncio.sleep(0.1)
+            self.assertEqual(cv.get(), 2)
+            cv.set(3)
+            self.assertEqual(cv.get(), 3)
+            coro_second_step_ran = True
+
+        async def run():
+            cv.set(1)
+            t = self.loop.create_task(coro())
+            self.assertTrue(coro_first_step_ran)
+            self.assertFalse(coro_second_step_ran)
+            self.assertEqual(cv.get(), 1)
+            await t
+            self.assertTrue(coro_second_step_ran)
+            self.assertEqual(cv.get(), 1)
+
+        self.run_coro(run())
+
+
+class PyEagerTaskFactoryLoopTests(EagerTaskFactoryLoopTests, test_utils.TestCase):
+    Task = tasks._PyTask
+
+
+@unittest.skipUnless(hasattr(tasks, '_CTask'),
+                     'requires the C _asyncio module')
+class CEagerTaskFactoryLoopTests(EagerTaskFactoryLoopTests, test_utils.TestCase):
+    Task = getattr(tasks, '_CTask', None)
+
+
+class AsyncTaskCounter:
+    def __init__(self, loop, *, task_class, eager):
+        self.suspense_count = 0
+        self.task_count = 0
+
+        def CountingTask(*args, eager_start=False, **kwargs):
+            if not eager_start:
+                self.task_count += 1
+            kwargs["eager_start"] = eager_start
+            return task_class(*args, **kwargs)
+
+        if eager:
+            factory = asyncio.create_eager_task_factory(CountingTask)
+        else:
+            def factory(loop, coro, **kwargs):
+                return CountingTask(coro, loop=loop, **kwargs)
+        loop.set_task_factory(factory)
+
+    def get(self):
+        return self.task_count
+
+
+async def awaitable_chain(depth):
+    if depth == 0:
+        return 0
+    return 1 + await awaitable_chain(depth - 1)
+
+
+async def recursive_taskgroups(width, depth):
+    if depth == 0:
+        return
+
+    async with asyncio.TaskGroup() as tg:
+        futures = [
+            tg.create_task(recursive_taskgroups(width, depth - 1))
+            for _ in range(width)
+        ]
+
+
+async def recursive_gather(width, depth):
+    if depth == 0:
+        return
+
+    await asyncio.gather(
+        *[recursive_gather(width, depth - 1) for _ in range(width)]
+    )
+
+
+class BaseTaskCountingTests:
+
+    Task = None
+    eager = None
+    expected_task_count = None
+
+    def setUp(self):
+        super().setUp()
+        self.loop = asyncio.new_event_loop()
+        self.counter = AsyncTaskCounter(self.loop, task_class=self.Task, eager=self.eager)
+        self.set_event_loop(self.loop)
+
+    def test_awaitables_chain(self):
+        observed_depth = self.loop.run_until_complete(awaitable_chain(100))
+        self.assertEqual(observed_depth, 100)
+        self.assertEqual(self.counter.get(), 0 if self.eager else 1)
+
+    def test_recursive_taskgroups(self):
+        num_tasks = self.loop.run_until_complete(recursive_taskgroups(5, 4))
+        self.assertEqual(self.counter.get(), self.expected_task_count)
+
+    def test_recursive_gather(self):
+        self.loop.run_until_complete(recursive_gather(5, 4))
+        self.assertEqual(self.counter.get(), self.expected_task_count)
+
+
+class BaseNonEagerTaskFactoryTests(BaseTaskCountingTests):
+    eager = False
+    expected_task_count = 781  # 1 + 5 + 5^2 + 5^3 + 5^4
+
+
+class BaseEagerTaskFactoryTests(BaseTaskCountingTests):
+    eager = True
+    expected_task_count = 0
+
+
+class NonEagerTests(BaseNonEagerTaskFactoryTests, test_utils.TestCase):
+    Task = asyncio.Task
+
+
+class EagerTests(BaseEagerTaskFactoryTests, test_utils.TestCase):
+    Task = asyncio.Task
+
+
+class NonEagerPyTaskTests(BaseNonEagerTaskFactoryTests, test_utils.TestCase):
+    Task = tasks._PyTask
+
+
+class EagerPyTaskTests(BaseEagerTaskFactoryTests, test_utils.TestCase):
+    Task = tasks._PyTask
+
+
+@unittest.skipUnless(hasattr(tasks, '_CTask'),
+                     'requires the C _asyncio module')
+class NonEagerCTaskTests(BaseNonEagerTaskFactoryTests, test_utils.TestCase):
+    Task = getattr(tasks, '_CTask', None)
+
+
+@unittest.skipUnless(hasattr(tasks, '_CTask'),
+                     'requires the C _asyncio module')
+class EagerCTaskTests(BaseEagerTaskFactoryTests, test_utils.TestCase):
+    Task = getattr(tasks, '_CTask', None)
+
+if __name__ == '__main__':
+    unittest.main()