[3.6] bpo-33110: Catch errors raised when running add_done_callback on already completed futures (GH-13141) by miss-islington · Pull Request #13509

[3.6] bpo-33110: Catch errors raised when running add_done_callback on already completed futures (GH-13141) by miss-islington · Pull Request #13509 · python/cpython

…ady completed futures (pythonGH-13141)

Wrap the callback call within the `add_done_callback` function within concurrent.futures, in order to behave in an identical manner to callbacks added to a running future are triggered once it has completed.
(cherry picked from commit 2a3a2ec)

Co-authored-by: Sam Martin <ABitMoreDepth@users.noreply.github.com>