Implement os.unsetenv() on Windows

[vstinner]

BPO	39413
Nosy	@pfmoore, @vstinner, @tjguk, @zware, @eryksun, @zooba, @miss-islington
PRs	bpo-39413: Implement os.unsetenv() on Windows #18104 bpo-39413: Use _wputenv(), not SetEnvironmentVariableW() #18107 [3.8] bpo-39413: os.unsetenv() is not available on Windows #18108 [3.7] bpo-39413: os.unsetenv() is not available on Windows (GH-18108) #18114 [WIP] bpo-39413: os.unsetenv() uses _wputenv() on Windows #18115 Revert "bpo-39413: Implement os.unsetenv() on Windows (GH-18104)" #18124 bpo-39413: Implement os.unsetenv() on Windows #18163

^{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 2020-01-28.02:08:26.530>
created_at = <Date 2020-01-21.14:39:20.002>
labels = ['library', '3.9', 'OS-windows']
title = 'Implement os.unsetenv() on Windows'
updated_at = <Date 2020-01-28.02:08:26.529>
user = 'https://github.com/vstinner'

bugs.python.org fields:

activity = <Date 2020-01-28.02:08:26.529>
actor = 'vstinner'
assignee = 'none'
closed = True
closed_date = <Date 2020-01-28.02:08:26.530>
closer = 'vstinner'
components = ['Library (Lib)', 'Windows']
creation = <Date 2020-01-21.14:39:20.002>
creator = 'vstinner'
dependencies = []
files = []
hgrepos = []
issue_num = 39413
keywords = ['patch']
message_count = 10.0
messages = ['360402', '360403', '360404', '360418', '360420', '360423', '360462', '360463', '360503', '360609']
nosy_count = 7.0
nosy_names = ['paul.moore', 'vstinner', 'tim.golden', 'zach.ware', 'eryksun', 'steve.dower', 'miss-islington']
pr_nums = ['18104', '18107', '18108', '18114', '18115', '18124', '18163']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39413'
versions = ['Python 3.9']

[vstinner]

os.unsetenv() is documented to be available on Windows, but it's not.

In Python 3.8, "del os.environ[key]" is implemented as:

os.putenv(key.upper(), "")

Attached PR implements it using SetEnvironmentVariableW(name, NULL).

[vstinner]

New changeset 56cd371 by Victor Stinner in branch 'master':
bpo-39413: Implement os.unsetenv() on Windows (GH-18104)
56cd371

[eryksun]

This bypasses the CRT's copy of the environment. If any C code calls [_w]getenv, it will still see the variable defined. The only way it won't is if setting the value also bypassed the CRT environment by directly calling SetEnvironmentVariableW.

[vstinner]

This bypasses the CRT's copy of the environment. If any C code calls [_w]getenv, it will still see the variable defined. The only way it won't is if setting the value also bypassed the CRT environment by directly calling SetEnvironmentVariableW.

Oh. That's a valid concern. I reopen the issue.

[vstinner]

I checked the doc: _putenv("VAR=") removes the environment variable "VAR". It's well documented:

https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/putenv-wputenv?view=vs-2019

"You can remove a variable from the environment by specifying an empty value_string, or in other words, by specifying only varname=."

So Python 3.8 os._unsetenv() is correct, but this function was not exposed, and so this issue remains relevant: Python 3.8 documents that os.unsetenv() is available on Windows, but it's not.

[vstinner]

PR 18107 still fix this issue (add os.unsetenv() to Python 3.9 on Windows), but fix the implementation (use _wputenv() rather than SetEnvironmentVariableW(), to update the CRT).

[vstinner]

New changeset f84f65b by Victor Stinner in branch '3.8':
bpo-39413: os.unsetenv() is not available on Windows (GH-18108)
f84f65b

[miss-islington]

New changeset 5707246 by Miss Islington (bot) in branch '3.7':
bpo-39413: os.unsetenv() is not available on Windows (GH-18108)
5707246

[vstinner]

New changeset b73dd02 by Victor Stinner in branch 'master':
Revert "bpo-39413: Implement os.unsetenv() on Windows (GH-18104)" (GH-18124)
b73dd02

[vstinner]

New changeset 161e7b3 by Victor Stinner in branch 'master':
bpo-39413: Implement os.unsetenv() on Windows (GH-18163)
161e7b3