os.dup2() tests dup3() availability on each call

[izbyshev]

BPO	32859
Nosy	@vstinner, @benjaminp, @izbyshev
PRs	bpo-32859: Don't retry dup3() if it is not available at runtime #5708 [3.7] closes bpo-32859: Don't retry dup3() if it is not available at runtime (GH-5708) #5768 [3.6] closes bpo-32859: Don't retry dup3() if it is not available at runtime (GH-5708) #5769

^{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 2018-02-20.07:25:54.407>
created_at = <Date 2018-02-16.23:10:17.305>
labels = ['extension-modules', '3.7', 'library', 'performance']
title = 'os.dup2() tests dup3() availability on each call'
updated_at = <Date 2018-02-21.05:21:14.552>
user = 'https://github.com/izbyshev'

bugs.python.org fields:

activity = <Date 2018-02-21.05:21:14.552>
actor = 'benjamin.peterson'
assignee = 'none'
closed = True
closed_date = <Date 2018-02-20.07:25:54.407>
closer = 'benjamin.peterson'
components = ['Extension Modules', 'Library (Lib)']
creation = <Date 2018-02-16.23:10:17.305>
creator = 'izbyshev'
dependencies = []
files = []
hgrepos = []
issue_num = 32859
keywords = ['patch']
message_count = 4.0
messages = ['312256', '312400', '312401', '312460']
nosy_count = 3.0
nosy_names = ['vstinner', 'benjamin.peterson', 'izbyshev']
pr_nums = ['5708', '5768', '5769']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'performance'
url = 'https://bugs.python.org/issue32859'
versions = ['Python 3.6', 'Python 3.7']

[izbyshev]

os.dup2() tests for dup3() system call availability at runtime, but doesn't remember the result across calls, repeating the test on each call with inheritable=False even if the test fails.

Judging by the code, 'dup3_works' was intended to be static (the first time its value is checked it's always an initial one).

[benjaminp]

New changeset b3caf38 by Benjamin Peterson (Alexey Izbyshev) in branch 'master':
closes bpo-32859: Don't retry dup3() if it is not available at runtime (GH-5708)
b3caf38

[benjaminp]

New changeset bab4fe3 by Benjamin Peterson (Miss Islington (bot)) in branch '3.7':
closes bpo-32859: Don't retry dup3() if it is not available at runtime (GH-5708)
bab4fe3

[benjaminp]

New changeset 16de2a9 by Benjamin Peterson (Miss Islington (bot)) in branch '3.6':
closes bpo-32859: Don't retry dup3() if it is not available at runtime (GH-5708)
16de2a9