concurrent.futures.ThreadPoolExecutor: tasks in queue not marked as done

[pyfm]

BPO	28650
Files	thread.py.patch: Proposal of a patch

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assignee = None
closed_at = None
created_at = <Date 2016-11-09.16:05:49.685>
labels = ['type-bug']
title = 'concurrent.futures.ThreadPoolExecutor: tasks in queue not marked as done'
updated_at = <Date 2016-11-09.16:05:49.685>
user = 'https://bugs.python.org/pyfm'

bugs.python.org fields:

activity = <Date 2016-11-09.16:05:49.685>
actor = 'pyfm'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = []
creation = <Date 2016-11-09.16:05:49.685>
creator = 'pyfm'
dependencies = []
files = ['45409']
hgrepos = []
issue_num = 28650
keywords = ['patch']
message_count = 1.0
messages = ['280410']
nosy_count = 1.0
nosy_names = ['pyfm']
pr_nums = []
priority = 'normal'
resolution = None
stage = None
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue28650'
versions = ['Python 3.5', 'Python 3.6']

[pyfm]

Hi,

I just realized that the ThreadPoolExecutor's workers do not call task_done() on the work_queue from which they took their task.
Or is there a reason for not calling task_done that I am missing?

Calling task_done decrements the queue's unfinished_tasks counter which could be used to improve the reuse of idle threads (see bquinlan's comment, Lib/concurrent/futures/thread.py:l124).

I am thinking of something like
if self._work_queue.unfinished_tasks > len(self._threads) and len(self._threads) < self._max_workers:

One could still construct cases in which threads are created unnecessarily but it should improve the situation in most cases.

(proposed patch is based on Python-3.6.0b3)