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

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

Author: 3 people

Doc/library/asyncio-task.rst

@@ -527,6 +527,42 @@ Running Tasks Concurrently
       and there is no running event loop.
 
 
+Eager Task Factory
+==================
+
+.. function:: eager_task_factory(loop, coro, *, name=None, context=None)
+
+    A task factory for eager task execution.
+
+    When using this factory (via :meth:`loop.set_task_factory(asyncio.eager_task_factory) <loop.set_task_factory>`),
+    coroutines begin execution synchronously during :class:`Task` construction.
+    Tasks are only scheduled on the event loop if they block.
+    This can be a performance improvement as the overhead of loop scheduling
+    is avoided for coroutines that complete synchronously.
+
+    A common example where this is beneficial is coroutines which employ
+    caching or memoization to avoid actual I/O when possible.
+
+    .. note::
+
+        Immediate execution of the coroutine is a semantic change.
+        If the coroutine returns or raises, the task is never scheduled
+        to the event loop. If the coroutine execution blocks, the task is
+        scheduled to the event loop. This change may introduce behavior
+        changes to existing applications. For example,
+        the application's task execution order is likely to change.
+
+    .. versionadded:: 3.12
+
+.. function:: create_eager_task_factory(custom_task_constructor)
+
+    Create an eager task factory, similar to :func:`eager_task_factory`,
+    using the provided *custom_task_constructor* when creating a new task instead
+    of the default :class:`Task`.
+
+    .. versionadded:: 3.12
+
+
 Shielding From Cancellation
 ===========================
 

Doc/whatsnew/3.12.rst

@@ -613,6 +613,11 @@ Optimizations
 * Speed up :class:`asyncio.Task` creation by deferring expensive string formatting.
   (Contributed by Itamar O in :gh:`103793`.)
 
+* Added :func:`asyncio.eager_task_factory` and :func:`asyncio.create_eager_task_factory`
+  functions to allow opting an event loop in to eager task execution,
+  speeding up some use-cases by up to 50%.
+  (Contributed by Jacob Bower & Itamar O in :gh:`102853`)
+
 
 CPython bytecode changes
 ========================

Include/internal/pycore_global_objects_fini_generated.h

@@ -370,6 +370,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(dst_dir_fd)
         STRUCT_FOR_ID(duration)
         STRUCT_FOR_ID(e)
+        STRUCT_FOR_ID(eager_start)
         STRUCT_FOR_ID(effective_ids)
         STRUCT_FOR_ID(element_factory)
         STRUCT_FOR_ID(encode)
@@ -460,6 +461,7 @@ struct _Py_global_strings {
         STRUCT_FOR_ID(instructions)
         STRUCT_FOR_ID(intern)
         STRUCT_FOR_ID(intersection)
+        STRUCT_FOR_ID(is_running)
         STRUCT_FOR_ID(isatty)
         STRUCT_FOR_ID(isinstance)
         STRUCT_FOR_ID(isoformat)

Include/internal/pycore_global_strings.h

@@ -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
 
 

Include/internal/pycore_runtime_init_generated.h

@@ -6,6 +6,7 @@
     '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 @@ def __init__(self, coro, *, loop=None, name=None, context=None):
         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 @@ def uncancel(self):
             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 @@ def __step(self, exc=None):
             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 @@ def __step(self, exc=None):
                 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):
@@ -897,17 +930,41 @@ def callback():
     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}
 _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