GH-96764: rewrite asyncio.wait_for to use asyncio.timeout (#98518)

Changes `asyncio.wait_for` to use `asyncio.timeout` as its underlying implementation.

Lib/asyncio/tasks.py

@@ -24,6 +24,7 @@ from . import coroutines
 from . import events
 from . import exceptions
 from . import futures
+from . import timeouts
 from .coroutines import _is_coroutine
 
 # Helper to generate new task names
@@ -437,65 +438,44 @@ async def wait_for(fut, timeout):
 
     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.
     """
-    loop = events.get_running_loop()
+    # 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 None:
-        return await fut
 
-    if timeout <= 0:
-        fut = ensure_future(fut, loop=loop)
+    if timeout is not None and timeout <= 0:
+        fut = ensure_future(fut)
 
         if fut.done():
             return fut.result()
 
-        await _cancel_and_wait(fut, loop=loop)
+        await _cancel_and_wait(fut)
         try:
             return fut.result()
         except exceptions.CancelledError as exc:
-            raise exceptions.TimeoutError() from exc
-
-    waiter = loop.create_future()
-    timeout_handle = loop.call_later(timeout, _release_waiter, waiter)
-    cb = functools.partial(_release_waiter, waiter)
-
-    fut = ensure_future(fut, loop=loop)
-    fut.add_done_callback(cb)
-
-    try:
-        # wait until the future completes or the timeout
-        try:
-            await waiter
-        except exceptions.CancelledError:
-            if fut.done():
-                return fut.result()
-            else:
-                fut.remove_done_callback(cb)
-                # We must ensure that the task is not running
-                # after wait_for() returns.
-                # See https://bugs.python.org/issue32751
-                await _cancel_and_wait(fut, loop=loop)
-                raise
-
-        if fut.done():
-            return fut.result()
-        else:
-            fut.remove_done_callback(cb)
-            # We must ensure that the task is not running
-            # after wait_for() returns.
-            # See https://bugs.python.org/issue32751
-            await _cancel_and_wait(fut, loop=loop)
-            # In case task cancellation failed with some
-            # exception, we should re-raise it
-            # See https://bugs.python.org/issue40607
-            try:
-                return fut.result()
-            except exceptions.CancelledError as exc:
-                raise exceptions.TimeoutError() from exc
-    finally:
-        timeout_handle.cancel()
+            raise TimeoutError from exc
 
+    async with timeouts.timeout(timeout):
+        return await fut
 
 async def _wait(fs, timeout, return_when, loop):
     """Internal helper for wait().
@@ -541,9 +521,10 @@ async def _wait(fs, timeout, return_when, loop):
     return done, pending
 
 
-async def _cancel_and_wait(fut, loop):
+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)