bpo-32528: Make asyncio.CancelledError a BaseException. (GH-13528)

This will address the common mistake many asyncio users make:
an "except Exception" clause breaking Tasks cancellation.

In addition to this change, we stop inheriting asyncio.TimeoutError
and asyncio.InvalidStateError from their concurrent.futures.*
counterparts.  There's no point for these exceptions to share the
inheritance chain.

In 3.9 we'll focus on implementing supervisors and cancel scopes,
which should allow better handling of all exceptions, including
SystemExit and KeyboardInterrupt

1 files changed, 6 insertions, 4 deletions

diff --git a/Lib/asyncio/tasks.py b/Lib/asyncio/tasks.py
index 1dc5952..78e7600 100644
--- a/Lib/asyncio/tasks.py
+++ b/Lib/asyncio/tasks.py
@@ -260,11 +260,11 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
                 super().set_result(exc.value)
         except exceptions.CancelledError:
             super().cancel()  # I.e., Future.cancel(self).
-        except Exception as exc:
+        except (KeyboardInterrupt, SystemExit) as exc:
             super().set_exception(exc)
+            raise
         except BaseException as exc:
             super().set_exception(exc)
-            raise
         else:
             blocking = getattr(result, '_asyncio_future_blocking', None)
             if blocking is not None:
@@ -318,7 +318,7 @@ class Task(futures._PyFuture):  # Inherit Python Task implementation
     def __wakeup(self, future):
         try:
             future.result()
-        except Exception as exc:
+        except BaseException as exc:
             # This may also be a cancellation.
             self.__step(exc)
         else:
@@ -858,7 +858,9 @@ def run_coroutine_threadsafe(coro, loop):
     def callback():
         try:
             futures._chain_future(ensure_future(coro, loop=loop), future)
-        except Exception as exc:
+        except (SystemExit, KeyboardInterrupt):
+            raise
+        except BaseException as exc:
             if future.set_running_or_notify_cancel():
                 future.set_exception(exc)
             raise