GH-95704: Don't suppress errors from tasks when TG is cancelled (#95761)

When a task catches CancelledError and raises some other error,
the other error should not silently be suppressed.

Any scenario where a task crashes in cleanup upon cancellation
will now result in an ExceptionGroup wrapping the crash(es)
instead of propagating CancelledError and ignoring the side errors.

NOTE: This represents a change in behavior (hence the need to
change several tests).  But it is only an edge case.

Co-authored-by: Thomas Grainger <tagrain@gmail.com>

1 files changed, 3 insertions, 4 deletions

diff --git a/Lib/asyncio/taskgroups.py b/Lib/asyncio/taskgroups.py
index 9be4838e3c..5d5e2a8a85 100644
--- a/Lib/asyncio/taskgroups.py
+++ b/Lib/asyncio/taskgroups.py
@@ -116,10 +116,9 @@ class TaskGroup:
         if self._base_error is not None:
             raise self._base_error
 
-        if propagate_cancellation_error is not None:
-            # The wrapping task was cancelled; since we're done with
-            # closing all child tasks, just propagate the cancellation
-            # request now.
+        # Propagate CancelledError if there is one, except if there
+        # are other errors -- those have priority.
+        if propagate_cancellation_error and not self._errors:
             raise propagate_cancellation_error
 
         if et is not None and et is not exceptions.CancelledError: