gh-155233: Fix asyncio.Barrier reusing a cancelled task's index (alternative to GH-155235)

Lib/asyncio/locks.py

@@ -489,6 +489,7 @@ def __init__(self, parties):
         self._parties = parties
         self._state = _BarrierState.FILLING
         self._count = 0       # count tasks in Barrier
+        self._index = self._parties - 1 # index number returned when barrier drains
 
     def __repr__(self):
         res = super().__repr__()
@@ -515,13 +516,14 @@ async def wait(self):
         async with self._cond:
             await self._block() # Block while the barrier drains or resets.
             try:
-                index = self._count
                 self._count += 1
-                if index + 1 == self._parties:
+                if self._count == self._parties:
                     # We release the barrier
                     await self._release()
                 else:
                     await self._wait()
+                index = self._index
+                self._index = (self._index + 1) % self.parties
                 return index
             finally:
                 self._count -= 1
@@ -569,6 +571,7 @@ def _exit(self):
         if self._count == 0:
             if self._state in (_BarrierState.RESETTING, _BarrierState.DRAINING):
                 self._state = _BarrierState.FILLING
+                self._index = self.parties - 1
             self._cond.notify_all()
 
     async def reset(self):
@@ -584,6 +587,7 @@ async def reset(self):
                     self._state = _BarrierState.RESETTING
             else:
                 self._state = _BarrierState.FILLING
+                self._index = self.parties - 1
             self._cond.notify_all()
 
     async def abort(self):

Lib/test/test_asyncio/test_locks.py

@@ -1820,6 +1820,35 @@ async def coro():
 
         self.assertEqual(barrier1.n_waiting, 0)
 
+    async def test_filling_tasks_cancel_one_index_still_unique(self):
+        # See gh-155233: a task cancelled while the barrier is still
+        # filling used to leave its index available for reuse.
+        # Index counter is now calculated on demand during the draining phasis.
+        self.N = 3
+        barrier = asyncio.Barrier(self.N)
+        results = []
+
+        async def coro():
+            i = await barrier.wait()
+            results.append(i)
+
+        t1 = asyncio.create_task(coro())
+        t2 = asyncio.create_task(coro())
+        await asyncio.sleep(0)
+        self.assertEqual(barrier.n_waiting, 2)
+
+        t1.cancel()
+        with self.assertRaises(asyncio.CancelledError):
+            await t1
+        await asyncio.sleep(0)
+        self.assertEqual(barrier.n_waiting, 1)
+
+        t3 = asyncio.create_task(coro())
+        t4 = asyncio.create_task(coro())
+        await asyncio.gather(t2, t3, t4)
+
+        self.assertEqual(sorted(results), list(range(self.N)))
+        self.assertEqual(barrier.n_waiting, 0)
 
 if __name__ == '__main__':
     unittest.main()

Misc/NEWS.d/next/Library/2026-10-07-10-59-27.gh-issue-155233.YGuQGK.rst

@@ -0,0 +1 @@
+Fix the returned value of the :meth:`asyncio.Barrier.wait`. This value is now calculated on demand during the *draining* phasis.