gh-124309: Revert eager task factory fix to prevent breaking downstream (#124810)

* Revert "GH-124639: add back loop param to staggered_race (#124700)"

This reverts commit e0a41a5dd1.

* Revert "gh-124309: Modernize the `staggered_race` implementation to support eager task factories (#124390)"

This reverts commit de929f353c.

Lib/asyncio/base_events.py

@@ -1144,7 +1144,7 @@ class BaseEventLoop(events.AbstractEventLoop):
                     (functools.partial(self._connect_sock,
                                        exceptions, addrinfo, laddr_infos)
                      for addrinfo in infos),
-                    happy_eyeballs_delay)
+                    happy_eyeballs_delay, loop=self)
 
             if sock is None:
                 exceptions = [exc for sub in exceptions for exc in sub]

Lib/asyncio/staggered.py

@@ -4,12 +4,11 @@ __all__ = 'staggered_race',
 
 import contextlib
 
+from . import events
+from . import exceptions as exceptions_mod
 from . import locks
 from . import tasks
-from . import taskgroups
 
-class _Done(Exception):
-    pass
 
 async def staggered_race(coro_fns, delay, *, loop=None):
     """Run coroutines with staggered start times and take the first to finish.
@@ -43,6 +42,8 @@ async def staggered_race(coro_fns, delay, *, loop=None):
         delay: amount of time, in seconds, between starting coroutines. If
             ``None``, the coroutines will run sequentially.
 
+        loop: the event loop to use.
+
     Returns:
         tuple *(winner_result, winner_index, exceptions)* where
 
@@ -61,11 +62,36 @@ async def staggered_race(coro_fns, delay, *, loop=None):
 
     """
     # TODO: when we have aiter() and anext(), allow async iterables in coro_fns.
+    loop = loop or events.get_running_loop()
+    enum_coro_fns = enumerate(coro_fns)
     winner_result = None
     winner_index = None
     exceptions = []
+    running_tasks = []
+
+    async def run_one_coro(previous_failed) -> None:
+        # Wait for the previous task to finish, or for delay seconds
+        if previous_failed is not None:
+            with contextlib.suppress(exceptions_mod.TimeoutError):
+                # Use asyncio.wait_for() instead of asyncio.wait() here, so
+                # that if we get cancelled at this point, Event.wait() is also
+                # cancelled, otherwise there will be a "Task destroyed but it is
+                # pending" later.
+                await tasks.wait_for(previous_failed.wait(), delay)
+        # Get the next coroutine to run
+        try:
+            this_index, coro_fn = next(enum_coro_fns)
+        except StopIteration:
+            return
+        # Start task that will run the next coroutine
+        this_failed = locks.Event()
+        next_task = loop.create_task(run_one_coro(this_failed))
+        running_tasks.append(next_task)
+        assert len(running_tasks) == this_index + 2
+        # Prepare place to put this coroutine's exceptions if not won
+        exceptions.append(None)
+        assert len(exceptions) == this_index + 1
 
-    async def run_one_coro(this_index, coro_fn, this_failed):
         try:
             result = await coro_fn()
         except (SystemExit, KeyboardInterrupt):
@@ -79,23 +105,34 @@ async def staggered_race(coro_fns, delay, *, loop=None):
             assert winner_index is None
             winner_index = this_index
             winner_result = result
-            raise _Done
+            # Cancel all other tasks. We take care to not cancel the current
+            # task as well. If we do so, then since there is no `await` after
+            # here and CancelledError are usually thrown at one, we will
+            # encounter a curious corner case where the current task will end
+            # up as done() == True, cancelled() == False, exception() ==
+            # asyncio.CancelledError. This behavior is specified in
+            # https://bugs.python.org/issue30048
+            for i, t in enumerate(running_tasks):
+                if i != this_index:
+                    t.cancel()
 
+    first_task = loop.create_task(run_one_coro(None))
+    running_tasks.append(first_task)
     try:
-        tg = taskgroups.TaskGroup()
-        # Intentionally override the loop in the TaskGroup to avoid
-        # using the running loop, preserving backwards compatibility
-        # TaskGroup only starts using `_loop` after `__aenter__`
-        # so overriding it here is safe.
-        tg._loop = loop
-        async with tg:
-            for this_index, coro_fn in enumerate(coro_fns):
-                this_failed = locks.Event()
-                exceptions.append(None)
-                tg.create_task(run_one_coro(this_index, coro_fn, this_failed))
-                with contextlib.suppress(TimeoutError):
-                    await tasks.wait_for(this_failed.wait(), delay)
-    except* _Done:
-        pass
-
-    return winner_result, winner_index, exceptions
+        # Wait for a growing list of tasks to all finish: poor man's version of
+        # curio's TaskGroup or trio's nursery
+        done_count = 0
+        while done_count != len(running_tasks):
+            done, _ = await tasks.wait(running_tasks)
+            done_count = len(done)
+            # If run_one_coro raises an unhandled exception, it's probably a
+            # programming error, and I want to see it.
+            if __debug__:
+                for d in done:
+                    if d.done() and not d.cancelled() and d.exception():
+                        raise d.exception()
+        return winner_result, winner_index, exceptions
+    finally:
+        # Make sure no tasks are left running if we leave this function
+        for t in running_tasks:
+            t.cancel()