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>

2 files changed, 10 insertions, 8 deletions

diff --git a/Lib/asyncio/taskgroups.py b/Lib/asyncio/taskgroups.py
index 24238c4..91be0de 100644
--- a/Lib/asyncio/taskgroups.py
+++ b/Lib/asyncio/taskgroups.py
@@ -54,16 +54,14 @@ class TaskGroup:
     async def __aenter__(self):
         if self._entered:
             raise RuntimeError(
-                f"TaskGroup {self!r} has been already entered")
-        self._entered = True
-
+                f"TaskGroup {self!r} has already been entered")
         if self._loop is None:
             self._loop = events.get_running_loop()
-
         self._parent_task = tasks.current_task(self._loop)
         if self._parent_task is None:
             raise RuntimeError(
                 f'TaskGroup {self!r} cannot determine the parent task')
+        self._entered = True
 
         return self
 
diff --git a/Lib/asyncio/timeouts.py b/Lib/asyncio/timeouts.py
index 029c468..30042ab 100644
--- a/Lib/asyncio/timeouts.py
+++ b/Lib/asyncio/timeouts.py
@@ -49,8 +49,9 @@ class Timeout:
 
     def reschedule(self, when: Optional[float]) -> None:
         """Reschedule the timeout."""
-        assert self._state is not _State.CREATED
         if self._state is not _State.ENTERED:
+            if self._state is _State.CREATED:
+                raise RuntimeError("Timeout has not been entered")
             raise RuntimeError(
                 f"Cannot change state of {self._state.value} Timeout",
             )
@@ -82,11 +83,14 @@ class Timeout:
         return f"<Timeout [{self._state.value}]{info_str}>"
 
     async def __aenter__(self) -> "Timeout":
+        if self._state is not _State.CREATED:
+            raise RuntimeError("Timeout has already been entered")
+        task = tasks.current_task()
+        if task is None:
+            raise RuntimeError("Timeout should be used inside a task")
         self._state = _State.ENTERED
-        self._task = tasks.current_task()
+        self._task = task
         self._cancelling = self._task.cancelling()
-        if self._task is None:
-            raise RuntimeError("Timeout should be used inside a task")
         self.reschedule(self._when)
         return self