concurrent.futures.wait() blocks forever when given duplicate Futures

[glangford]

BPO	20369
Nosy	@tim-one, @vstinner, @pablogsal, @miss-islington, @iritkatriel, @kumaraditya303
PRs	bpo-20369: concurrent.futures.wait() now deduplicates futures given a… #30168 [3.10] bpo-20369: concurrent.futures.wait() now deduplicates futures given a… (GH-30168) #30400 [3.9] bpo-20369: concurrent.futures.wait() now deduplicates futures given a… (GH-30168) #30401
Files	test_dupfuture_wait.py issue20369.patch issue20369.patch issue20369.patch

^{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 2022-01-04.14:35:22.675>
created_at = <Date 2014-01-23.15:33:51.643>
labels = ['easy', 'type-bug', '3.9', '3.10', '3.11', 'library']
title = 'concurrent.futures.wait() blocks forever when given duplicate Futures'
updated_at = <Date 2022-01-04.14:35:22.675>
user = 'https://bugs.python.org/glangford'

bugs.python.org fields:

activity = <Date 2022-01-04.14:35:22.675>
actor = 'AlexWaygood'
assignee = 'none'
closed = True
closed_date = <Date 2022-01-04.14:35:22.675>
closer = 'AlexWaygood'
components = ['Library (Lib)']
creation = <Date 2014-01-23.15:33:51.643>
creator = 'glangford'
dependencies = []
files = ['33660', '33665', '33708', '33751']
hgrepos = []
issue_num = 20369
keywords = ['patch', 'easy']
message_count = 7.0
messages = ['208960', '208979', '209241', '209454', '408390', '409676', '409681', '409682']
nosy_count = 6.0
nosy_names = ['tim.peters', 'vstinner', 'pablogsal', 'miss-islington', 'iritkatriel', 'kumaraditya']
pr_nums = ['30168', '30400', '30401']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue20369'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[glangford]

For a Future f which has already completed,
wait( [f,f], return_when=ALL_COMPLETED )
blocks forever.

This is because the test in wait():

if len(done) == len(fs)

is comparing the length of a set to the length of a list.

If f has not completed, wait( [f,f] ) will yield f once. The behaviour should be consistent with as_completed() - see issue bpo-20367.

[glangford]

Proposed patch...please treat with an appropriate level of suspicion since this is my first patch submission. :-)

A corresponding change will be made to as_completed() for bpo-20367. Suggestions welcome.

[glangford]

Updated patch with a test case, and added a minor note to the docstring to clarify behaviour.

The use of sleep() in the test is not great, but it is the most obvious way to test and it is consistent with the approach used in other concurrent test cases.

[glangford]

Updated patch with change to Doc/library/concurrent.futures.rst.

[iritkatriel]

Reproduced on 3.11.

[pablogsal]

New changeset 7d7817c by Kumar Aditya in branch 'main':
bpo-20369: concurrent.futures.wait() now deduplicates futures given a… (GH-30168)
7d7817c

[miss-islington]

New changeset 9a9061d by Miss Islington (bot) in branch '3.9':
bpo-20369: concurrent.futures.wait() now deduplicates futures given a… (GH-30168)
9a9061d

[miss-islington]

New changeset ba12467 by Miss Islington (bot) in branch '3.10':
bpo-20369: concurrent.futures.wait() now deduplicates futures given a… (GH-30168)
ba12467