Closes #21886, #21447: Fix a race condition in asyncio when setting the result

of a Future with call_soon(). Add an helper, a private method, to set the
result only if the future was not cancelled.

1 files changed, 6 insertions, 0 deletions

diff --git a/Lib/asyncio/futures.py b/Lib/asyncio/futures.py
index fcc90d1..022fef7 100644
--- a/Lib/asyncio/futures.py
+++ b/Lib/asyncio/futures.py
@@ -316,6 +316,12 @@ class Future:
 
     # So-called internal methods (note: no set_running_or_notify_cancel()).
 
+    def _set_result_unless_cancelled(self, result):
+        """Helper setting the result only if the future was not cancelled."""
+        if self.cancelled():
+            return
+        self.set_result(result)
+
     def set_result(self, result):
         """Mark the future done and set its result.