pathlib.resolve() causes infinite loop on Windows

[GeorgMischler]

BPO	29079
Nosy	@pfmoore, @pitrou, @tjguk, @zware, @serhiy-storchaka, @zooba
PRs	[Do Not Merge] Convert Misc/NEWS so that it is managed by towncrier #552

^{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/zooba'
closed_at = <Date 2016-12-29.00:04:11.270>
created_at = <Date 2016-12-26.20:46:14.262>
labels = ['3.7', 'type-bug', 'library', 'OS-windows']
title = 'pathlib.resolve() causes infinite loop on Windows'
updated_at = <Date 2017-03-31.16:36:34.212>
user = 'https://bugs.python.org/GeorgMischler'

bugs.python.org fields:

activity = <Date 2017-03-31.16:36:34.212>
actor = 'dstufft'
assignee = 'steve.dower'
closed = True
closed_date = <Date 2016-12-29.00:04:11.270>
closer = 'steve.dower'
components = ['Library (Lib)', 'Windows']
creation = <Date 2016-12-26.20:46:14.262>
creator = 'Georg Mischler'
dependencies = []
files = []
hgrepos = []
issue_num = 29079
keywords = []
message_count = 5.0
messages = ['284045', '284048', '284217', '286760', '286806']
nosy_count = 8.0
nosy_names = ['paul.moore', 'pitrou', 'tim.golden', 'python-dev', 'zach.ware', 'serhiy.storchaka', 'steve.dower', 'Georg Mischler']
pr_nums = ['552']
priority = 'normal'
resolution = 'fixed'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue29079'
versions = ['Python 3.6', 'Python 3.7']

[GeorgMischler]

When pathlib.resolve() is invoked on Windows(10) with an absolute path including a non-existing drive, it gets caught in an infinite loop.

To reproduce:
Select a drive letter that doesn't exist on the system (in my case H:).
Run the following line of code:
pathlib.Path('h:\\').resolve()

Expected result:
returns the input string unchanged.

Actual result:
pathlib.resolve() ends up in an infinite loop, repeatedly calling _getfinalpathname() on the same string.

[zooba]

We should break out of the "while True" loop in _WindowsFlavour.resolve when joining with ".." doesn't result in a different path.

[python-dev]

New changeset af8c8551ea45 by Steve Dower in branch '3.6':
Issue bpo-29079: Prevent infinite loop in pathlib.resolve() on Windows
https://hg.python.org/cpython/rev/af8c8551ea45

New changeset 9de7bf6c60d2 by Steve Dower in branch 'default':
Issue bpo-29079: Prevent infinite loop in pathlib.resolve() on Windows
https://hg.python.org/cpython/rev/9de7bf6c60d2

[serhiy-storchaka]

I'm not sure the fix is correct. os.path.dirname(s) can point to different place than os.path.abspath(os.path.join(s, os.pardir)) if the last component of s is "..", "." or a symbolic link.

Would be nice to add tests.

[zooba]

At the point this code is running, it doesn't matter. The path doesn't exist, so trimming irrelevant segments from it will just cause a few extra iterations through resolve until we clear out enough of the absent segments to find something that does exist.

abspath just prepends the current working directory unless the path is rooted, so we essentially have unbounded concatenation of "\.." in that case.