islink and stat follow_symlinks are inconsistent on Windows

[eryksun]

BPO	29250
Nosy	@pfmoore, @tjguk, @zware, @eryksun, @zooba

^{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 2019-08-22.04:55:31.440>
created_at = <Date 2017-01-12.10:42:45.874>
labels = ['3.7', 'type-feature', 'library', 'OS-windows']
title = 'islink and stat follow_symlinks are inconsistent on Windows'
updated_at = <Date 2019-08-22.04:55:31.439>
user = 'https://github.com/eryksun'

bugs.python.org fields:

activity = <Date 2019-08-22.04:55:31.439>
actor = 'eryksun'
assignee = 'none'
closed = True
closed_date = <Date 2019-08-22.04:55:31.440>
closer = 'eryksun'
components = ['Library (Lib)', 'Windows']
creation = <Date 2017-01-12.10:42:45.874>
creator = 'eryksun'
dependencies = []
files = []
hgrepos = []
issue_num = 29250
keywords = []
message_count = 1.0
messages = ['285299']
nosy_count = 5.0
nosy_names = ['paul.moore', 'tim.golden', 'zach.ware', 'eryksun', 'steve.dower']
pr_nums = []
priority = 'normal'
resolution = 'out of date'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'enhancement'
url = 'https://bugs.python.org/issue29250'
versions = ['Python 3.7']

[eryksun]

win32_xstat_impl in Modules/posixmodule.c doesn't check the value of reparse_tag to limit traversal to only tags that are considered links. So we have cases in which os.path.islink returns False (e.g. junctions, pending the resolution of bpo-23407), but os.stat and os.lstat disagree. For example:

    >>> os.path.islink(r'C:\Documents and Settings')
    False
    >>> s = os.stat(r'C:\Documents and Settings')
    >>> ls = os.lstat(r'C:\Documents and Settings')
    >>> s.st_ino == ls.st_ino
    False

At its strictest, traversal would be limited to just symbolic links. It could be relaxed to also include junctions, and even further to include all tags with the name surrogate bit set 1. This includes the following Microsoft tags

IO_REPARSE_TAG_MOUNT_POINT (junctions)
IO_REPARSE_TAG_SYMLINK
IO_REPARSE_TAG_IIS_CACHE

and non-Microsoft tags:

IO_REPARSE_TAG_SOLUTIONSOFT
IO_REPARSE_TAG_OSR_SAMPLE
IO_REPARSE_TAG_QI_TECH_HSM
IO_REPARSE_TAG_MAXISCALE_HSM

I'm creating this issue as an enhancement for 3.7. The problem has existed for a long time, but it hasn't been a thorn in anyone's side as far as I know.