support "UNC" device paths in ntpath.splitdrive

[eryksun]

BPO	37609
Nosy	@pfmoore, @tjguk, @zware, @serhiy-storchaka, @eryksun, @zooba, @nsiregar, @barneygale
PRs	bpo-37609: Add device path support in ntpath splitdrive #14841 bpo-37609: Update ntpath.splitdrive to use native functionality where possible #25261 bpo-37609 - Support "UNC" and "GLOBAL" junctions in ntpath.splitdrive(). #31702
Files	splitdrive.py

^{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 = None
created_at = <Date 2019-07-17.11:26:34.050>
labels = ['type-bug', 'library', '3.10', 'OS-windows']
title = 'support "UNC" device paths in ntpath.splitdrive'
updated_at = <Date 2022-03-06.18:47:25.283>
user = 'https://github.com/eryksun'

bugs.python.org fields:

activity = <Date 2022-03-06.18:47:25.283>
actor = 'steve.dower'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)', 'Windows']
creation = <Date 2019-07-17.11:26:34.050>
creator = 'eryksun'
dependencies = []
files = ['49541']
hgrepos = []
issue_num = 37609
keywords = ['patch']
message_count = 9.0
messages = ['348055', '348099', '348206', '352110', '352355', '379780', '390375', '414604', '414621']
nosy_count = 8.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'serhiy.storchaka', 'eryksun', 'steve.dower', 'nsiregar', 'barneygale']
pr_nums = ['14841', '25261', '31702']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue37609'
versions = ['Python 3.10']

[eryksun]

Windows Python includes UNC shares such as "//server/spam" in its definition of a drive. This is natural because Windows supports setting a UNC path as the working directory and handles the share component as the working drive when resolving rooted paths such as "/eggs". For the sake of generality when working with \\?\ extended paths, Python should expand its definition of a UNC drive to include "UNC" device paths.

A practical example is calling glob.glob with a "//?/UNC" device path.

>>> import os, sys, glob
>>> sys.addaudithook(lambda s,a: print('#', a[0]) if s == 'glob.glob' else None)

regular UNC path:

 >>> glob.glob('//localhost/C$/Sys*')
 # //localhost/C$/Sys*
 ['//localhost/C$/System Volume Information']

"UNC" device path:

 >>> glob.glob('//?/UNC/localhost/C$/Sys*')
 # //?/UNC/localhost/C$/Sys*
 # //?/UNC/localhost/C$
 # //?/UNC/localhost
 # //?/UNC/
 []

Since the magic character "?" is in the path (IMO, the drive should be excluded from this check, but that's a separate issue), the internal function glob._iglob calls itself recursively until it reaches the base case of dirname == pathname, where dirname is from os.path.split(pathname). The problem here is that ntpath.split doesn't stop at the proper base case of "//?/UNC/localhost/C$". This is due to ntpath.splitdrive. For example:

 >>> os.path.splitdrive('//?/UNC/localhost/C$/Sys*')
 ('//?/UNC', '/localhost/C$/Sys*')

 >>> os.path.splitdrive('//./UNC/localhost/C$/Sys*')
 ('//./UNC', '/localhost/C$/Sys*')

The results should be "//?/UNC/localhost/C$" and "//./UNC/localhost/C$".

In other cases, returning a device as the drive is fine, if not exactly meaningful (e.g. "//./NUL"). I don't think this needs to change.

[serhiy-storchaka]

Do you want to create a PR Eryk?

[nsiregar]

I was unsure about implementation in the patch, do you have UNC format specification?

[zooba]

For clarity, given Eryk's examples above, both "\\?\UNC\" and "//?/UNC/" are okay (as are any combination of forward and backslashes in the prefix, as normalization will be applied for any except the "\\?\" version). "UNC" is also case-insensitive.

[eryksun]

Please consult the attached file "splitdrive.py". I redesigned splitdrive() to support "UNC" and "GLOBAL" junctions in device paths. I relaxed the design to allow repeated separators everywhere except for the UNC root. IIRC, Windows has supported this since XP. For example:

 >>> print(nt._getfullpathname('//server///share'))
 \\server\share
 >>> print(nt._getfullpathname(r'\\server\\\share'))
 \\server\share

There are also a couple of minor behavior changes in the new implementation.

The old implementation would split "//server/" as ('//server/', ''). Since there's no share, this should not count as a drive. The new implementation splits it as ('', '//server/'). Similarly it splits '//?/UNC/server/' as ('', '//?/UNC/server/').

The old implementation also allowed any character as a drive 'letter'. For example, it would split '/:/spam' as ('/:', '/spam'). The new implementation ensures that the drive letter in a DOS drive is alphabetic.

I also extended test_splitdrive to use a list of test cases in order to avoid having to define each case twice. It calls tester() a second time for each case, with slash and backslash swapped.

[eryksun]

I'm attaching a rewrite of splitdrive() from msg352355. This version uses an internal _next() function to get the indices of the next path component, ignoring repeated separators. It also flattens the nested structure of the previous implementation by adding multiple return statements.

[zooba]

Once bpo-43105 is merged, I've got a fairly simple implementation for this using the (new) nt._path_splitroot native method, as well as improved tests that cover both the native and emulated calculations.