Have os.unlink remove junction points

[KimGrsman]

BPO	18314
Nosy	@tim-one, @terryjreedy, @tjguk, @briancurtin, @zware, @eryksun
Files	unlink_junction.patch unlink-junctions.patch unlink_junctions_r2.patch

^{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 = 'https://github.com/tjguk'
closed_at = <Date 2014-05-05.20:01:05.083>
created_at = <Date 2013-06-27.12:15:21.053>
labels = ['type-feature', 'OS-windows']
title = 'Have os.unlink remove junction points'
updated_at = <Date 2014-05-07.00:33:11.076>
user = 'https://bugs.python.org/KimGrsman'

bugs.python.org fields:

activity = <Date 2014-05-07.00:33:11.076>
actor = 'eryksun'
assignee = 'tim.golden'
closed = True
closed_date = <Date 2014-05-05.20:01:05.083>
closer = 'tim.golden'
components = ['Windows']
creation = <Date 2013-06-27.12:15:21.053>
creator = 'Kim.Gr\xc3\xa4sman'
dependencies = []
files = ['31915', '33288', '35095']
hgrepos = []
issue_num = 18314
keywords = ['patch']
message_count = 45.0
messages = ['191945', '191946', '192177', '195114', '195121', '195124', '198416', '198500', '198647', '198649', '200058', '201018', '201054', '201055', '207097', '207157', '208500', '211008', '213278', '216579', '217287', '217288', '217293', '217311', '217312', '217318', '217404', '217463', '217464', '217471', '217479', '217488', '217538', '217940', '217941', '217943', '217948', '217949', '217953', '217955', '217966', '217969', '217975', '217976', '218030']
nosy_count = 8.0
nosy_names = ['tim.peters', 'terry.reedy', 'tim.golden', 'brian.curtin', 'python-dev', 'zach.ware', 'eryksun', 'Kim.Gr\xc3\xa4sman']
pr_nums = []
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue18314'
versions = ['Python 3.5']

[KimGrsman]

os.unlink currently raises a WindowsError (Access Denied) if I attempt to unlink an NTFS junction point.

It looks trivial to allow Py_DeleteFileW [1] to remove junction points as well as proper symbolic links, as far as I can tell.

For example, the ntfslink-python library [2] only checks if both FILE_ATTRIBUTE_DIRECTORY and FILE_ATTRIBUTE_REPARSE_POINT are set.

RemoveDirectoryW is documented to handle junction points transparently, so it should just be a matter of passing the path on if it's a junction point or a symbolic link.

My motivation for this is that I have used external tools to create junction points, and am now switching to symbolic links. When deleting a directory, I need to do:

try:
    os.unlink(link_path)
except WindowsError as detail:
    # BACKWARD COMPATIBILITY HACK
    if detail.winerror == 5:
        _delete_junction_point(link_path)
    else:
        raise

which is a little funky. It seems like os.unlink semantics work just as well for junction points, even if they can't be created with os.symlink.

Love it/hate it?

[1] http://hg.python.org/cpython/file/44f455e6163d/Modules/posixmodule.c#l4105
[2] https://github.com/Juntalis/ntfslink-python/blob/2f6ff903f9b22942de8aa93a32a3d817124f359e/ntfslink/internals/__init__.py#L32

[KimGrsman]

Also, I believe the reason os.unlink raises "access denied" is because a junction point does not currently qualify as a directory && link, so its path is passed directly to DeleteFileW, which in turn refuses to delete a directory.

[KimGrsman]

This comment outlines how to tell junction points from other mount points:
http://www.codeproject.com/Articles/21202/Reparse-Points-in-Vista?msg=3651130#xx3651130xx

This should port straight into Py_DeleteFileW.

Would anyone be interested in a patch?

[vstinner]

What is "an NTFS junction point"? Is it possible to delete it in cmd.exe with the del command?

[tim-one]

Victor, Wikipedia has a readable explanation:

http://en.wikipedia.org/wiki/NTFS_junction_point

I haven't used them much. From cmd.exe, I've been able to delete them, not with "del" but with "rmdir". You can create one from cmd.exe with the "mklink" command.

[KimGrsman]

Victor,

Junction points are like links between directories only. They've been around since the NTFS that came with Windows 2000, but integration with OS tools has been generally poor (e.g. Explorer wouldn't see the difference between a junction point and a regular folder.) As of Windows 7, this works better.

[terryjreedy]

I am not sure if this is a bug or enhancement. It is a moot point until there is a patch to apply.

A patch would need a test that fails now and passes with the patch. From the Wikipedia article, it appears that a test using mklink /J would not run on XP and would have to be skipped. I would not expect an XP buildbot to necessarily have the Server 2003 Resource Kit needed for an XP test.

Does _delete_junction_point(link_path) work on XP or should the feature be restricted to Vista+?

[KimGrsman]

_delete_junction_point currently shells out to a command-line tool, junction.exe, from SysInternals. That ran fine on XP.

As I understand it, RemoveDirectoryW on XP also takes care of junction points, but I'll find a machine to verify.

[KimGrsman]

Attached is a patch that considers directory symlinks and junction points equivalent.

I'm struggling with the test -- would it be acceptable to only run this test on platforms that have mklink /j (Windows Vista and higher)?

I've looked at programmatic means of creating junction points, but it involves enough Win32 interop to make it a candidate for a module all by itself (it's REALLY messy.)

Any ideas?

Thanks!

[tjguk]

I'll try to pick this one up over the next few days. Feel free to ping me if it drops into silence!