subprocess: reusing STARTUPINFO breaks under 3.7 (Windows)

[xflr6]

BPO	34044
Nosy	@vstinner, @eryksun, @xflr6
PRs	bpo-34044: subprocess.Popen copies startupinfo #8090 [3.7] bpo-34044: subprocess.Popen copies startupinfo (GH-8090) #8120 [3.7] bpo-34044: subprocess.Popen copies startupinfo (GH-8090) #8121

^{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 2018-07-05.21:20:00.049>
created_at = <Date 2018-07-04.14:55:46.660>
labels = ['3.7', '3.8', 'type-bug', 'library']
title = 'subprocess: reusing STARTUPINFO breaks under 3.7 (Windows)'
updated_at = <Date 2018-07-05.22:27:55.180>
user = 'https://github.com/xflr6'

bugs.python.org fields:

activity = <Date 2018-07-05.22:27:55.180>
actor = 'xflr6'
assignee = 'none'
closed = True
closed_date = <Date 2018-07-05.21:20:00.049>
closer = 'vstinner'
components = ['Library (Lib)']
creation = <Date 2018-07-04.14:55:46.660>
creator = 'xflr6'
dependencies = []
files = []
hgrepos = []
issue_num = 34044
keywords = ['patch']
message_count = 5.0
messages = ['321047', '321131', '321132', '321133', '321138']
nosy_count = 3.0
nosy_names = ['vstinner', 'eryksun', 'xflr6']
pr_nums = ['8090', '8120', '8121']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue34044'
versions = ['Python 3.7', 'Python 3.8']

[xflr6]

AFAIU, the change for https://bugs.python.org/issue19764 broke the following usage of subprocess on Windows (re-using a subprocess.STARTUPINFO instance to hide the command window):

    import os, subprocess

    STARTUPINFO = subprocess.STARTUPINFO()
    STARTUPINFO.dwFlags |= subprocess.STARTF_USESHOWWINDOW
    STARTUPINFO.wShowWindow = subprocess.SW_HIDE

    # raises OSError: [WinError 87]
    # in the second loop iteration starting with Python 3.7
    for i in range(2):
        print(i)
        with open(os.devnull, 'w') as stderr:
            subprocess.check_call(['attrib'], stderr=stderr,
                                  startupinfo=STARTUPINFO)

AFAICT, this works on Python 2.7, 3.4, 3.5, and 3.6

I think the documentation in

https://docs.python.org/3/library/subprocess.html#windows-popen-helpers

does not mention that every Popen call should be done with a fresh instance, so either the documentation needs to be changed, or the implementation (e.g. by deep-copying the instance).

See also https://bugs.python.org/issue19764#msg320784

[vstinner]

New changeset 483422f by Victor Stinner in branch 'master':
bpo-34044: subprocess.Popen copies startupinfo (GH-8090)
483422f

[vstinner]

New changeset 29be3bd by Victor Stinner in branch '3.7':
bpo-34044: subprocess.Popen copies startupinfo (GH-8090) (GH-8121)
29be3bd

[vstinner]

Sebastian Bank: Thank you for your bug report and your example! I used your example to write an unit test. I fixed the bug in 3.7 and master branches. The fix will be part of the future 3.7.1 release.

[xflr6]

Perfect, thanks for the quick fix.