gh-95051: ensure that timeouts scheduled with asyncio.Timeout that have already expired are deliverered promptly

Doc/library/asyncio-task.rst

@@ -625,6 +625,9 @@ Timeouts
 
             If *when* is a float, it is set as the new deadline.
 
+            if *when* is in the past, the timeout will trigger on the next
+            iteration of the event loop.
+
         .. method:: expired() -> bool
 
            Return whether the context manager has exceeded its deadline

Lib/asyncio/timeouts.py

@@ -52,10 +52,10 @@ def reschedule(self, when: Optional[float]) -> None:
             self._timeout_handler = None
         else:
             loop = events.get_running_loop()
-            self._timeout_handler = loop.call_at(
-                when,
-                self._on_timeout,
-            )
+            if when <= loop.time():
+                self._timeout_handler = loop.call_soon(self._on_timeout)
+            else:
+                self._timeout_handler = loop.call_at(when, self._on_timeout)
 
     def expired(self) -> bool:
         """Is timeout expired during execution?"""

Lib/test/test_asyncio/test_timeouts.py

@@ -105,6 +105,30 @@ async def test_timeout_zero(self):
         self.assertLess(t1-t0, 2)
         self.assertTrue(t0 <= cm.when() <= t1)
 
+    async def test_timeout_zero_sleep_zero(self):
+        loop = asyncio.get_running_loop()
+        t0 = loop.time()
+        with self.assertRaises(TimeoutError):
+            async with asyncio.timeout(0) as cm:
+                await asyncio.sleep(0)
+        t1 = loop.time()
+        self.assertTrue(cm.expired())
+        # 2 sec for slow CI boxes
+        self.assertLess(t1-t0, 2)
+        self.assertTrue(t0 <= cm.when() <= t1)
+
+    async def test_timeout_in_the_past_sleep_zero(self):
+        loop = asyncio.get_running_loop()
+        t0 = loop.time()
+        with self.assertRaises(TimeoutError):
+            async with asyncio.timeout(-11) as cm:
+                await asyncio.sleep(0)
+        t1 = loop.time()
+        self.assertTrue(cm.expired())
+        # 2 sec for slow CI boxes
+        self.assertLess(t1-t0, 2)
+        self.assertTrue(t0 >= cm.when() <= t1)
+
     async def test_foreign_exception_passed(self):
         with self.assertRaises(KeyError):
             async with asyncio.timeout(0.01) as cm:

Misc/NEWS.d/next/Library/2022-07-21-22-59-22.gh-issue-95109.usxA9r.rst

@@ -0,0 +1 @@
+Ensure that timeouts scheduled with :class:`asyncio.Timeout` that have already expired are delivered promptly.