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

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

Lib/test/test_asyncio/test_waitfor.py

@@ -237,33 +237,6 @@ class AsyncioWaitForTest(unittest.IsolatedAsyncioTestCase):
         with self.assertRaises(FooException):
             await foo()
 
-    async def test_wait_for_self_cancellation(self):
-        async def inner():
-            try:
-                await asyncio.sleep(0.3)
-            except asyncio.CancelledError:
-                try:
-                    await asyncio.sleep(0.3)
-                except asyncio.CancelledError:
-                    await asyncio.sleep(0.3)
-
-            return 42
-
-        inner_task = asyncio.create_task(inner())
-
-        wait = asyncio.wait_for(inner_task, timeout=0.1)
-
-        # Test that wait_for itself is properly cancellable
-        # even when the initial task holds up the initial cancellation.
-        task = asyncio.create_task(wait)
-        await asyncio.sleep(0.2)
-        task.cancel()
-
-        with self.assertRaises(asyncio.CancelledError):
-            await task
-
-        self.assertEqual(await inner_task, 42)
-
     async def _test_cancel_wait_for(self, timeout):
         loop = asyncio.get_running_loop()
 
@@ -289,6 +262,106 @@ class AsyncioWaitForTest(unittest.IsolatedAsyncioTestCase):
     async def test_cancel_wait_for(self):
         await self._test_cancel_wait_for(60.0)
 
+    async def test_wait_for_cancel_suppressed(self):
+        # GH-86296: Supressing CancelledError is discouraged
+        # but if a task subpresses CancelledError and returns a value,
+        # `wait_for` should return the value instead of raising CancelledError.
+        # This is the same behavior as `asyncio.timeout`.
+
+        async def return_42():
+            try:
+                await asyncio.sleep(10)
+            except asyncio.CancelledError:
+                return 42
+
+        res = await asyncio.wait_for(return_42(), timeout=0.1)
+        self.assertEqual(res, 42)
+
+
+    async def test_wait_for_issue86296(self):
+        # GH-86296: The task should get cancelled and not run to completion.
+        # inner completes in one cycle of the event loop so it
+        # completes before the task is cancelled.
+
+        async def inner():
+            return 'done'
+
+        inner_task = asyncio.create_task(inner())
+        reached_end = False
+
+        async def wait_for_coro():
+            await asyncio.wait_for(inner_task, timeout=100)
+            await asyncio.sleep(1)
+            nonlocal reached_end
+            reached_end = True
+
+        task = asyncio.create_task(wait_for_coro())
+        self.assertFalse(task.done())
+        # Run the task
+        await asyncio.sleep(0)
+        task.cancel()
+        with self.assertRaises(asyncio.CancelledError):
+            await task
+        self.assertTrue(inner_task.done())
+        self.assertEqual(await inner_task, 'done')
+        self.assertFalse(reached_end)
+
+
+class WaitForShieldTests(unittest.IsolatedAsyncioTestCase):
+
+    async def test_zero_timeout(self):
+        # `asyncio.shield` creates a new task which wraps the passed in
+        # awaitable and shields it from cancellation so with timeout=0
+        # the task returned by `asyncio.shield` aka shielded_task gets
+        # cancelled immediately and the task wrapped by it is scheduled
+        # to run.
+
+        async def coro():
+            await asyncio.sleep(0.01)
+            return 'done'
+
+        task = asyncio.create_task(coro())
+        with self.assertRaises(asyncio.TimeoutError):
+            shielded_task = asyncio.shield(task)
+            await asyncio.wait_for(shielded_task, timeout=0)
+
+        # Task is running in background
+        self.assertFalse(task.done())
+        self.assertFalse(task.cancelled())
+        self.assertTrue(shielded_task.cancelled())
+
+        # Wait for the task to complete
+        await asyncio.sleep(0.1)
+        self.assertTrue(task.done())
+
+
+    async def test_none_timeout(self):
+        # With timeout=None the timeout is disabled so it
+        # runs till completion.
+        async def coro():
+            await asyncio.sleep(0.1)
+            return 'done'
+
+        task = asyncio.create_task(coro())
+        await asyncio.wait_for(asyncio.shield(task), timeout=None)
+
+        self.assertTrue(task.done())
+        self.assertEqual(await task, "done")
+
+    async def test_shielded_timeout(self):
+        # shield prevents the task from being cancelled.
+        async def coro():
+            await asyncio.sleep(0.1)
+            return 'done'
+
+        task = asyncio.create_task(coro())
+        with self.assertRaises(asyncio.TimeoutError):
+            await asyncio.wait_for(asyncio.shield(task), timeout=0.01)
+
+        self.assertFalse(task.done())
+        self.assertFalse(task.cancelled())
+        self.assertEqual(await task, "done")
+
 
 if __name__ == '__main__':
     unittest.main()