@@ -587,6 +587,17 @@ def with_timeout(
     def error_callback(future: Future) -> None:
         try:
             future.result()
+        except BaseException as e:
+            # TODO(bdarnell): How should we handle BaseExceptions here?
+            # asyncio.CancelledError changed from an Exception to a BaseException
+            # in Python 3.8, but we (sometimes) included it as a quiet_exception.
+            # We preserve the pre-3.8 behavior by raising all BaseExceptions
+            # unless explicitly quieted, but I'm not sure this is the right behavior.
+            # BaseExceptions should not usually be caught, but i'm not sure there is a
+            # better way to prevent log spam in this case. Should asyncio.CancelledError
+            # always be "quiet"?
+            if not isinstance(e, quiet_exceptions):
+                raise
         except Exception as e:
             if not isinstance(e, quiet_exceptions):
                 app_log.error(
 
@@ -13,7 +13,7 @@
 # under the License.
 
 import collections
-from concurrent.futures import CancelledError
+from asyncio import CancelledError
 import datetime
 import types