New submission from Sam Martin :
Whilst working with concurrent.futures and ThreadPoolExecutors, my colleague
and I have noted some undocumented behaviour.
When adding a done_callback to a future that has already completed, we note
that that callback is executed directly, outside of any try...except statement.
However, both the docs and the _invoke_callbacks methods will wrap any
done_callbacks in a try...except statement which logs the result and returns.
Could we either update the documentation to mention that callback behaviour may
raise if added to an already completed future, or simply add the same
try...except wrapping that the _invoke_callback method already uses please?
(Preferably the latter)
The two pieces of the futures library I am referring to can be viewed here:
_invoke_callbacks:
https://github.com/python/cpython/blob/master/Lib/concurrent/futures/_base.py#L323
add_done_callback:
https://github.com/python/cpython/blob/master/Lib/concurrent/futures/_base.py#L403
I would note, that the test code which covers this area of the code, doesn't
currently exercise this particular condition. The closest test I could find is
test_done_callback_already_failed, which checks that a callback can retrieve an
exception from a future, but it does not validate what happens when a callback
raises when the future it is attached to is already complete.
Source:
https://github.com/python/cpython/blob/c3d9508ff22ece9a96892b628dd5813e2fb0cd80/Lib/test/test_concurrent_futures.py#L1012
The other test closely related is test_done_callback_raises, however this
doesn't check the behaviour of a callback when added to an already completed
future. We should be able to simulate this by moving the f.set_result line to
above the f.add_done_callback lines?
Source:
https://github.com/python/cpython/blob/c3d9508ff22ece9a96892b628dd5813e2fb0cd80/Lib/test/test_concurrent_futures.py#L990
--
components: Library (Lib)
messages: 314151
nosy: samm
priority: normal
severity: normal
status: open
title: Adding a done callback to a concurrent.futures Future once it has
already completed, may raise an exception, contrary to docs
type: behavior
versions: Python 3.6
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<https://bugs.python.org/issue33110>
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