Add optional argument to Semaphore.release for releasing multiple threads

[rhettinger]

BPO	10978
Nosy	@tim-one, @rhettinger, @pitrou, @MojoVampire
PRs	bpo-10978: Semaphores can release multiple threads at a time #15588
Files	sem.diff: Patch to threading.py multirelease.diff: Update patch to Py3.5. Add test and docs

^{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 = 'https://github.com/rhettinger'
closed_at = <Date 2019-08-29.08:45:42.904>
created_at = <Date 2011-01-22.00:15:34.025>
labels = ['type-feature', 'library', '3.9']
title = 'Add optional argument to Semaphore.release for releasing multiple threads'
updated_at = <Date 2019-08-29.08:45:42.904>
user = 'https://github.com/rhettinger'

bugs.python.org fields:

activity = <Date 2019-08-29.08:45:42.904>
actor = 'rhettinger'
assignee = 'rhettinger'
closed = True
closed_date = <Date 2019-08-29.08:45:42.904>
closer = 'rhettinger'
components = ['Library (Lib)']
creation = <Date 2011-01-22.00:15:34.025>
creator = 'rhettinger'
dependencies = []
files = ['20483', '35718']
hgrepos = []
issue_num = 10978
keywords = ['patch']
message_count = 6.0
messages = ['126804', '135005', '221028', '221213', '221585', '350757']
nosy_count = 5.0
nosy_names = ['tim.peters', 'rhettinger', 'pitrou', 'neologix', 'josh.r']
pr_nums = ['15588']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue10978'
versions = ['Python 3.9']

[rhettinger]

Call sem.release(5) would have the same effect as:

with lock:
    for i in range(5):
         sem.release()

The use case is when a single semaphore is holding up multiple threads and needs to release them all. According to "The Little Book of Semaphores <http://greenteapress.com/semaphores/\>", this is a common design pattern.

Basic patch attached. If the proposal meets with acceptance, will add tests and a doc update.

[pitrou]

Your patch uses tabs for indentation. Otherwise, looks good on the principle.

[BreamoreBoy]

Seems good to proceed as there are no dissenters.

[rhettinger]

Updated patch (with tests and docs).

[MojoVampire]

Never know whether to comment on issue itself, but just in case:

There are issues with the patch when n < 0 is passed, as n is not sanity checked, which would break the Semaphore invariant (value must be >= 0). n == 0 is also a weird value, but harmless if passed; release(0) would acquire and release the lock but otherwise act as a noop.

[rhettinger]

New changeset 35f6301 by Raymond Hettinger in branch 'master':
bpo-10978: Semaphores can release multiple threads at a time (GH-15588)
35f6301