scandir.dirfd() method

[giampaolo]

BPO	39099
Nosy	@giampaolo, @benjaminp, @eryksun
PRs	bpo-39099: add scandir.dirfd() #17664

^{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 2019-12-19.19:08:15.551>
created_at = <Date 2019-12-19.18:29:01.546>
labels = ['library', '3.9']
title = 'scandir.dirfd() method'
updated_at = <Date 2019-12-20.01:11:30.167>
user = 'https://github.com/giampaolo'

bugs.python.org fields:

activity = <Date 2019-12-20.01:11:30.167>
actor = 'eryksun'
assignee = 'none'
closed = True
closed_date = <Date 2019-12-19.19:08:15.551>
closer = 'giampaolo.rodola'
components = ['Library (Lib)']
creation = <Date 2019-12-19.18:29:01.546>
creator = 'giampaolo.rodola'
dependencies = []
files = []
hgrepos = []
issue_num = 39099
keywords = ['patch']
message_count = 4.0
messages = ['358686', '358687', '358688', '358692']
nosy_count = 3.0
nosy_names = ['giampaolo.rodola', 'benjamin.peterson', 'eryksun']
pr_nums = ['17664']
priority = 'normal'
resolution = None
stage = 'resolved'
status = 'closed'
superseder = None
type = None
url = 'https://bugs.python.org/issue39099'
versions = ['Python 3.9']

[giampaolo]

PR in attachment adds a new dirfd() method to the scandir() object (POSIX only). This can be be passed to os.* functions supporting the "dir_fd" parameter, and avoid opening a new fd as in:

>>> dirfd = os.open("basename", os.O_RDONLY, dir_fd=topfd)

At the moment I am not sure if it's possible to also support Windows.

[benjaminp]

Why not just os.open the directory yourself and pass it to os.scandir?

[giampaolo]

Good point, I didn't consider that. I suppose you're right. =)
Closing.

[eryksun]

I am not sure if it's possible to also support Windows.

For reference, FindFirstFileW doesn't support handle-relative names, and neither does CreateFileW. At a lower level in Windows NT, NtCreateFile has always supported handle-relative names, but CPython doesn't use the NT API.

fd support (not dirfd) could be added to listdir and scandir in Windows. A directory can be listed via GetFileInformationByHandleEx: FileFullDirectoryInfo. This doesn't query the short name, so it may perform better than FindFirstFileW. However, opening a directory with os.open can't be supported. VC++ still hasn't documented the _O_OBTAIN_DIR (0x2000) flag.