bpo-40607: Reraise exception during task cancelation in asyncio.wait_for() (GH-20054)

Currently, if asyncio.wait_for() timeout expires, it cancels
inner future and then always raises TimeoutError. In case
those future is task, it can handle cancelation mannually,
and those process can lead to some other exception. Current
implementation silently loses thoses exception.

To resolve this, wait_for will check was the cancelation
successfull or not. In case there was exception, wait_for
will reraise it.

Co-authored-by: Roman Skurikhin <roman.skurikhin@cruxlab.com>

1 files changed, 9 insertions, 1 deletions

diff --git a/Lib/asyncio/tasks.py b/Lib/asyncio/tasks.py
index 717837d..f5de1a2 100644
--- a/Lib/asyncio/tasks.py
+++ b/Lib/asyncio/tasks.py
@@ -496,7 +496,15 @@ async def wait_for(fut, timeout, *, loop=None):
             # after wait_for() returns.
             # See https://bugs.python.org/issue32751
             await _cancel_and_wait(fut, loop=loop)
-            raise exceptions.TimeoutError()
+            # In case task cancellation failed with some
+            # exception, we should re-raise it
+            # See https://bugs.python.org/issue40607
+            try:
+                fut.result()
+            except exceptions.CancelledError as exc:
+                raise exceptions.TimeoutError() from exc
+            else:
+                raise exceptions.TimeoutError()
     finally:
         timeout_handle.cancel()