gh-111085: Fix invalid state handling in TaskGroup and Timeout (#111111)

asyncio.TaskGroup and asyncio.Timeout classes now raise proper RuntimeError
if they are improperly used.

* When they are used without entering the context manager.
* When they are used after finishing.
* When the context manager is entered more than once (simultaneously or
  sequentially).
* If there is no current task when entering the context manager.

They now remain in a consistent state after an exception is thrown,
so subsequent operations can be performed correctly (if they are allowed).

Co-authored-by: James Hilton-Balfe <gobot1234yt@gmail.com>

Lib/test/test_asyncio/test_timeouts.py

@@ -5,11 +5,12 @@ import time
 
 import asyncio
 
+from test.test_asyncio.utils import await_without_task
+
 
 def tearDownModule():
     asyncio.set_event_loop_policy(None)
 
-
 class TimeoutTests(unittest.IsolatedAsyncioTestCase):
 
     async def test_timeout_basic(self):
@@ -257,6 +258,51 @@ class TimeoutTests(unittest.IsolatedAsyncioTestCase):
         cause = exc.exception.__cause__
         assert isinstance(cause, asyncio.CancelledError)
 
+    async def test_timeout_already_entered(self):
+        async with asyncio.timeout(0.01) as cm:
+            with self.assertRaisesRegex(RuntimeError, "has already been entered"):
+                async with cm:
+                    pass
+
+    async def test_timeout_double_enter(self):
+        async with asyncio.timeout(0.01) as cm:
+            pass
+        with self.assertRaisesRegex(RuntimeError, "has already been entered"):
+            async with cm:
+                pass
+
+    async def test_timeout_finished(self):
+        async with asyncio.timeout(0.01) as cm:
+            pass
+        with self.assertRaisesRegex(RuntimeError, "finished"):
+            cm.reschedule(0.02)
+
+    async def test_timeout_expired(self):
+        with self.assertRaises(TimeoutError):
+            async with asyncio.timeout(0.01) as cm:
+                await asyncio.sleep(1)
+        with self.assertRaisesRegex(RuntimeError, "expired"):
+            cm.reschedule(0.02)
+
+    async def test_timeout_expiring(self):
+        async with asyncio.timeout(0.01) as cm:
+            with self.assertRaises(asyncio.CancelledError):
+                await asyncio.sleep(1)
+            with self.assertRaisesRegex(RuntimeError, "expiring"):
+                cm.reschedule(0.02)
+
+    async def test_timeout_not_entered(self):
+        cm = asyncio.timeout(0.01)
+        with self.assertRaisesRegex(RuntimeError, "has not been entered"):
+            cm.reschedule(0.02)
+
+    async def test_timeout_without_task(self):
+        cm = asyncio.timeout(0.01)
+        with self.assertRaisesRegex(RuntimeError, "task"):
+            await await_without_task(cm.__aenter__())
+        with self.assertRaisesRegex(RuntimeError, "has not been entered"):
+            cm.reschedule(0.02)
+
 
 if __name__ == '__main__':
     unittest.main()