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os.readlink produces wrong result on windows #87123
Description
| BPO | 42957 |
|---|---|
| Nosy | @pfmoore, @tjguk, @ethanfurman, @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 2021-01-22.19:44:43.542>
created_at = <Date 2021-01-18.19:13:09.085>
labels = ['type-bug', '3.8', '3.9', '3.10', 'invalid', 'library', 'OS-windows']
title = 'os.readlink produces wrong result on windows'
updated_at = <Date 2021-04-27.20:06:34.457>
user = 'https://bugs.python.org/simonmackenzie'bugs.python.org fields:
activity = <Date 2021-04-27.20:06:34.457>
actor = 'steve.dower'
assignee = 'none'
closed = True
closed_date = <Date 2021-01-22.19:44:43.542>
closer = 'steve.dower'
components = ['Library (Lib)', 'Windows']
creation = <Date 2021-01-18.19:13:09.085>
creator = 'simon mackenzie'
dependencies = []
files = []
hgrepos = []
issue_num = 42957
keywords = []
message_count = 6.0
messages = ['385218', '385232', '385285', '385287', '385308', '385510']
nosy_count = 7.0
nosy_names = ['paul.moore', 'tim.golden', 'ethan.furman', 'zach.ware', 'eryksun', 'steve.dower', 'simon mackenzie']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue42957'
versions = ['Python 3.8', 'Python 3.9', 'Python 3.10']
Activity
simonmackenzie commentedon Jan 18, 2021
os.readlink gives wrong result on python 3.8 onwards for windows
Should read d:\\v1
eryksun commentedon Jan 18, 2021
Symlinks and mountpoints (aka junctions) contain two forms of the target path. There's a path that's intended for display in the shell, and there's the actual substitute path to which the link resolves. os.readlink() was changed to return the substitute path because the display form is not mandated by filesystem protocols (it's sometimes missing, especially for junctions) and not reliable (e.g. the display path may be a long path or contain reserved names such that it's not valid without the \\?\ prefix). It was decided to keep the C implementation of os.readlink() simple. Whether to retain the \\?\ prefix was shifted to high-level functions that consume the result of os.readlink(), such as os.path.realpath().
There was a previous issue related to this, in that the shutil module copies symlinks via os.readlink() and os.symlink(), which thus copies only the substitute path now. The issue was closed as not a bug, but had it been resolved with new functionality, I would have preferred to do so with a low-level function to copy a reparse point, not by reverting the behavior of os.readlink(). I also see no reason against adding an option to readlink() to return the display path instead of the substitute path, or to just remove the prefix. But I'd vote against making it the default behavior.
simonmackenzie commentedon Jan 19, 2021
For most people the expectation would be that it returns a path in the same
format as any other path. Furthermore it seems odd to change the default
behaviour after years when it worked as expected. I never heard of this
substitute path before and it does not work in some circumstances e.g.
docker does not recognise it.
Note also that os.path.realpath(os.path.readlink("v1")) still returns
\\\\?\\d:\\v1. There needs to be some way of getting to the path that
everyone actually uses.
On Mon, 18 Jan 2021 at 21:25, Eryk Sun <report@bugs.python.org> wrote:
simonmackenzie commentedon Jan 19, 2021
I note os.path.realpath("v1") does produce the right path in windows. Maybe
that is what you meant. Will that work cross-platform?
On Tue, 19 Jan 2021 at 18:48, simon mackenzie <simonm3@gmail.com> wrote:
eryksun commentedon Jan 20, 2021
os.readlink() was generalized to support mountpoints (junctions) as well as symlinks, and it's more common for mountpoints to lack the print name field in the reparse data buffer [1]. For example, PowerShell's new-item creates junctions that only have a substitute path. This is allowed since the filesystem protocols only require that the substitute path is valid in the reparse data buffer, since that's all that the system actually uses when resolving the reparse point.
The substitute path in the reparse point is a \\?\ prefixed path (actually a \??\ NT path, but they're effectively the same for our purposes). This type of path is usually called an extended path -- or an extended device path, or verbatim path. It's a device path, like the \\.\ prefix, except that (1) it's verbatim (i.e. not normalized), (2) its length is never limited to MAX_PATH (260) characters, and (3) the Windows file API supports the \\?\ prefix more broadly than the \\.\ prefix.
You're right that some programs can't grok an extended path. Some can't even handle any type of UNC path. I agree that we need a simple way to remove the prefix. I just don't agree that removing it should be the default behavior in nt.readlink(), which I prefer to keep efficient and free of race conditions.
os.path.realpath() isn't necessarily what you want since it resolves the final path. The link may target a path that traverses any number of reparse points and mapped drives, so the final path may be completely different from the os.readlink() result. We simply need an option to remove the \\?\ or \\?\UNC prefix, either always or only when the path doesn't require it. It could be implemented by a high-level wrapper function in os.py.
---
Reasons the prefix may be required
If the length of the target path exceeds MAX_PATH, then removing the prefix may render the path inaccessible if the current process doesn't support long paths without it (e.g. Windows 10 without long paths enabled at the system level, or any version prior to Windows 10).
Also, reserved DOS device names are only accessible using an extended path. Say I have the following "spam" junction:
>>> print(os.readlink('spam')) \\?\C:\Temp\conThe junction allows accessing the target directory normally:
>>> stat.S_ISDIR(os.stat('spam').st_mode) TrueBut look what happens when I try to access the target path without the prefix:
>>> stat.S_ISDIR(os.stat(r'C:\Temp\con').st_mode) False >>> stat.S_ISCHR(os.stat(r'C:\Temp\con').st_mode) TrueInstead of the directory that one might expect, it's actually a character device!? Let's see what Windows opens:
>>> print(os.path.abspath(r'C:\Temp\con')) \\.\conIt opens the "CON" device, for console I/O. It turns out that a bunch of names are reserved, including NUL, CON, CONIN$, CONOUT$, AUX, PRN, COM<1-9>, and LPT<1-9>. They're reserved even with an extension introduced by a colon or dot, preceded by zero or more spaces. For example:
>>> print(os.path.abspath(r'C:\Temp\con :whatever')) \\.\conDirectly accessing such a name in the filesystem requires a verbatim path. For example:
>>> stat.S_ISDIR(os.stat(r'\\?\C:\Temp\con').st_mode) TrueUsing reserved names is cautioned against, but in the real world we have to be defensive. We can't simply remove the prefix and hope for the best.
---
[1] https://docs.microsoft.com/en-us/windows-hardware/drivers/ddi/ntifs/ns-ntifs-_reparse_data_buffer
zooba commentedon Jan 22, 2021
I agree with Eryk (unsurprisingly, we discussed this change *a lot* back when it was made ~3 years ago).
os.readlink is the lowest-level API that gives a reliable result.
os.path.realpath is the high-level API that probably does what most users want most of the time (and relies on os.readlink being reliable).
We can't change either of these to more of a middle-ground without taking away the ability for users to implement their own behaviour. So there's no behavioural issue here. (The behavioural issue we fixed in 3.8 was that realpath didn't resolve anything at all, so we're not reverting back to that.)
If you'd like a new API added to either retrieve the display path or to read a link and try to partially normalise the result without resolving any more links, please open an issue specifically for that (targeting Python 3.10). You probably also want to prototype a wrapper around readlink() to do it, and be prepared to have many potential edge cases thrown against it.