ensure greenlet_spawn propagates BaseException

Fixed bug in asyncio implementation where the greenlet adaptation system
failed to propagate ``BaseException`` subclasses, most notably including
``asyncio.CancelledError``, to the exception handling logic used by the
engine to invalidate and clean up the connection, thus preventing
connections from being correctly disposed when a task was cancelled.

Fixes: #6652
Change-Id: Id3809e6c9e7bced46a7a3b5a0d1906c4168dc4fc

1 files changed, 3 insertions, 1 deletions

diff --git a/lib/sqlalchemy/util/_concurrency_py3k.py b/lib/sqlalchemy/util/_concurrency_py3k.py
index 88294557d..c6d1fa5d3 100644
--- a/lib/sqlalchemy/util/_concurrency_py3k.py
+++ b/lib/sqlalchemy/util/_concurrency_py3k.py
@@ -25,6 +25,8 @@ else:
 
 
 def is_exit_exception(e):
+    # note asyncio.CancelledError is already BaseException
+    # so was an exit exception in any case
     return not isinstance(e, Exception) or isinstance(
         e, (asyncio.TimeoutError, asyncio.CancelledError)
     )
@@ -118,7 +120,7 @@ async def greenlet_spawn(
                 # wait for a coroutine from await_ and then return its
                 # result back to it.
                 value = await result
-            except Exception:
+            except BaseException:
                 # this allows an exception to be raised within
                 # the moderated greenlet so that it can continue
                 # its expected flow.