AssertionError on await of Future returned by asyncio.wrap_future

[jhaydaman]

BPO	33238
Nosy	@asvetlov, @1st1, @jhaydaman
PRs	bpo-33238: Add InvalidStateError to concurrent.futures. #7056

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2018-05-30.17:53:49.619>
created_at = <Date 2018-04-06.18:02:45.599>
labels = ['3.8', 'library', 'expert-asyncio']
title = 'AssertionError on await of Future returned by asyncio.wrap_future'
updated_at = <Date 2018-05-30.17:53:49.619>
user = 'https://github.com/jhaydaman'

bugs.python.org fields:

activity = <Date 2018-05-30.17:53:49.619>
actor = 'asvetlov'
assignee = 'none'
closed = True
closed_date = <Date 2018-05-30.17:53:49.619>
closer = 'asvetlov'
components = ['Library (Lib)', 'asyncio']
creation = <Date 2018-04-06.18:02:45.599>
creator = 'jhaydaman'
dependencies = []
files = []
hgrepos = []
issue_num = 33238
keywords = ['patch']
message_count = 4.0
messages = ['315030', '315031', '317224', '318157']
nosy_count = 3.0
nosy_names = ['asvetlov', 'yselivanov', 'jhaydaman']
pr_nums = ['7056']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue33238'
versions = ['Python 3.8']

[jhaydaman]

When the concurrent Future wrapped by asyncio.wrap_future has set_result or set_exception called multiple times, I get the following traceback:

Traceback (most recent call last):
  File "/usr/local/lib/python3.6/asyncio/events.py", line 145, in _run
    self._callback(*self._args)
  File "/usr/local/lib/python3.6/asyncio/futures.py", line 399, in _set_state
    _copy_future_state(other, future)
  File "/usr/local/lib/python3.6/asyncio/futures.py", line 369, in _copy_future_state
    assert not dest.done()
AssertionError

Minimal reproducible example:

import asyncio
import concurrent.futures

f = concurrent.futures.Future()
async_f = asyncio.wrap_future(f)
f.set_result(1)
f.set_result(1)

loop = asyncio.get_event_loop()
loop.run_until_complete(async_f)

The documentation says that set_result and set_exception are only meant for Executors, so, arguably using them outside of that context would fall into undocumented behavior rather than a bug. But it still seems like it should be the second set_result that raises something like InvalidStateError. Alternatively, this can be avoided if _copy_future_state checks for done in addition to cancellation. What should happen here?

[jhaydaman]

May also be worth pointing out that even in the case of only calling set_result once, _done_callbacks still has _chain_future in it:

import asyncio
import concurrent.futures


f = concurrent.futures.Future()
async_f = asyncio.wrap_future(f)

f.set_result(1)
loop = asyncio.get_event_loop()
print(loop.run_until_complete(async_f))
print(f._done_callbacks)

>> 1
>> [<function _chain_future.<locals>._call_set_state at 0x7f1687f3c620>]

Should that be cleared by that point?

[asvetlov]

Raising InvalidStateError sounds perfect.

Would you make a pull request?

[asvetlov]

New changeset 0a28c0d by Andrew Svetlov (jhaydaman) in branch 'master':
bpo-33238: Add InvalidStateError to concurrent.futures. (GH-7056)
0a28c0d