Refactor getenvironment() in _winapi.c

[serhiy-storchaka]

BPO	36387
Nosy	@malemburg, @pfmoore, @tjguk, @zware, @serhiy-storchaka, @zooba
PRs	bpo-36387: Refactor getenvironment() in _winapi.c. #12482

^{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 2019-03-28.14:32:41.753>
created_at = <Date 2019-03-21.06:32:57.872>
labels = ['extension-modules', '3.8', 'OS-windows']
title = 'Refactor getenvironment() in _winapi.c'
updated_at = <Date 2019-03-28.14:32:41.752>
user = 'https://github.com/serhiy-storchaka'

bugs.python.org fields:

activity = <Date 2019-03-28.14:32:41.752>
actor = 'serhiy.storchaka'
assignee = 'none'
closed = True
closed_date = <Date 2019-03-28.14:32:41.753>
closer = 'serhiy.storchaka'
components = ['Extension Modules', 'Windows']
creation = <Date 2019-03-21.06:32:57.872>
creator = 'serhiy.storchaka'
dependencies = []
files = []
hgrepos = []
issue_num = 36387
keywords = ['patch']
message_count = 2.0
messages = ['338527', '339046']
nosy_count = 6.0
nosy_names = ['lemburg', 'paul.moore', 'tim.golden', 'zach.ware', 'serhiy.storchaka', 'steve.dower']
pr_nums = ['12482']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue36387'
versions = ['Python 3.8']

[serhiy-storchaka]

Function getenvironment() in Modules/_winapi.c is used for converting the environment mapping to the wchar_t string for passing to CreateProcessW(). It performs the following steps:

• Allocate a Py_UCS4 buffer and copy all keys and values, converting them to Py_UCS4.
• Create a PyUnicode object from the Py_UCS4 buffer.
• Create a wchar_t buffer from the PyUnicode object (unless it has the UCS2 kind).

The proposed PR makes it performing a single steps:

• Allocate a wchar_t buffer and copy all keys and values, converting them to wchar_t.

This needs less memory allocations and smaller memory consumption. In most cases this needs also less and faster memory scans and copies.

In addition, the PR replaces PySequence_Fast C API with faster PyList C API. In the past PyMapping_Keys() and PyMapping_Values() could return a tuple in unlikely case, but now they always return a list (see bpo-28280).

The current code uses the legacy Unicode C API, while the new code uses the newer (added in 3.3) wchar_t based Unicode C API, so something similar to this change should be made sooner or later.

[serhiy-storchaka]

New changeset 8abd7c7 by Serhiy Storchaka in branch 'master':
bpo-36387: Refactor getenvironment() in _winapi.c. (GH-12482)
8abd7c7