gh-97907: use cancellation error from child if possible

Lib/asyncio/tasks.py

@@ -844,6 +844,7 @@ def _done_callback(fut, cur_task=cur_task):
             # All futures are done; create a list of results
             # and set it to the 'outer' future.
             results = []
+            cancelled_child = None
 
             for fut in children:
                 if fut.cancelled():
@@ -853,6 +854,8 @@ def _done_callback(fut, cur_task=cur_task):
                     # to 'results' instead of raising it, don't bother
                     # setting __context__.  This also lets us preserve
                     # calling '_make_cancelled_error()' at most once.
+                    if cancelled_child is None:
+                        cancelled_child = fut
                     res = exceptions.CancelledError(
                         '' if fut._cancel_message is None else
                         fut._cancel_message)
@@ -863,10 +866,15 @@ def _done_callback(fut, cur_task=cur_task):
                 results.append(res)
 
             if outer._cancel_requested:
+                # If one or more children were cancelled, raise the exception
+                # from the first of those encountered, otherwise use the last
+                # child. See issue gh-97907.
+                if cancelled_child is None:
+                    cancelled_child = fut
                 # If gather is being cancelled we must propagate the
                 # cancellation regardless of *return_exceptions* argument.
                 # See issue 32684.
-                exc = fut._make_cancelled_error()
+                exc = cancelled_child._make_cancelled_error()
                 outer.set_exception(exc)
             else:
                 outer.set_result(results)

Lib/test/test_asyncio/test_tasks.py

@@ -2418,6 +2418,27 @@ async def main():
                         'raised by inner task to the gather() caller.'
                     )
 
+    def test_cancel_gather_3(self):
+        loop = asyncio.new_event_loop()
+        self.addCleanup(loop.close)
+        barrier = asyncio.Barrier(2)
+
+        async def f1():
+            await barrier.wait()
+            await asyncio.sleep(1)
+
+        async def f2():
+            return 42
+
+        async def main():
+            gfut = asyncio.gather(f2(), f1(), f2(), return_exceptions=True)
+            await barrier.wait()
+            gfut.cancel("my message")
+            await gfut
+
+        with self.assertRaisesRegex(asyncio.CancelledError, "my message"):
+            loop.run_until_complete(main())
+
     def test_exception_traceback(self):
         # See http://bugs.python.org/issue28843
 

Misc/NEWS.d/next/Library/2025-05-01-21-13-23.gh-issue-97907.MFyz5K.rst

@@ -0,0 +1,2 @@
+Ensure cancellation error is raised by :func:`asyncio.gather` using the error
+from a cancelled child, if there is one.