shutil.move fails to move symlink (Invalid cross-device link)

[Unode]

BPO	26791
Nosy	@tarekziade, @websurfer5, @iritkatriel
PRs	bpo-26791: update shutil.move() to provide the same symlink move behavior as the mv shell when moving a symlink into a directory that is the target of the symlink #21759

^{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 2016-04-17.16:58:17.148>
labels = ['type-bug', 'library', '3.9', '3.10', '3.11']
title = 'shutil.move fails to move symlink (Invalid cross-device link)'
updated_at = <Date 2021-11-30.12:28:21.452>
user = 'https://bugs.python.org/Unode'

bugs.python.org fields:

activity = <Date 2021-11-30.12:28:21.452>
actor = 'iritkatriel'
assignee = 'none'
closed = False
closed_date = None
closer = None
components = ['Library (Lib)']
creation = <Date 2016-04-17.16:58:17.148>
creator = 'Unode'
dependencies = []
files = []
hgrepos = []
issue_num = 26791
keywords = ['patch']
message_count = 6.0
messages = ['263616', '263617', '263654', '374648', '374916', '407371']
nosy_count = 5.0
nosy_names = ['tarek', 'Unode', 'Jeffrey.Kintscher', 'iritkatriel', 'Murray Wilson']
pr_nums = ['21759']
priority = 'normal'
resolution = None
stage = 'patch review'
status = 'open'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue26791'
versions = ['Python 3.9', 'Python 3.10', 'Python 3.11']

[Unode]

Hi everyone,

I'm not really sure if this is a new issue but digging through the bug reports from the past I couldn't find an answer.
There's http://bugs.python.org/issue1438480 but this seems to be a different issue.
I also found http://bugs.python.org/issue9993 that addressed problems with symlinks but didn't correct the behavior reported here.

The problem can be visualized with the following code.
Code fails on python 2.7 as well as python 3.4+. Not tested in python <2.7 and <3.4.

    import shutil
    import os
    
    TMPDIR = "/tmp/tmpdir"
    TESTLINK = "test_dir"
    
    if not os.path.isdir(TMPDIR):
        os.mkdir(TMPDIR)
    
    if not os.path.islink(TESTLINK):
        os.symlink(TMPDIR, TESTLINK)
    
    shutil.move(TESTLINK, TMPDIR)

When executed it gives me:

    % python3 test.py
    Traceback (most recent call last):
      File "test.py", line 14, in <module>
        shutil.move(TESTLINK, TMPDIR)
      File "/usr/lib64/python3.4/shutil.py", line 516, in move
        os.rename(src, dst)
    OSError: [Errno 18] Invalid cross-device link: 'test_dir' -> '/tmp/tmpdir'

This happens because /tmp is:

tmpfs on /tmp type tmpfs (rw,nosuid,nodev,noatime,nodiratime)

In the past the recommendation to handle this problem was to stop using os.rename and use shutil.move instead.
This was even discussed in a bug report - http://bugs.python.org/issue14848

If one searches for this exception there's plenty of advice [1][2][3][4] in the same direction.
However, given that shutil.move uses os.rename internally, the problem returns.

On the other end doing the equivalent action in the shell with 'mv' works fine.

[1] - http://stackoverflow.com/a/15300474
[2] - https://mail.python.org/pipermail/python-list/2005-February/342892.html
[3] - http://www.thecodingforums.com/threads/errno-18-invalid-cross-device-link-using-os-rename.341597/
[4] - pypa/pip#103

[Unode]

Also related to http://bugs.python.org/issue212317

[SilentGhost]

This seems to be only triggered when moving a symlink into its destination. Assumption in the code is that when _samefile returns True, it must be due to case-insensitive filesystem, rather than this edge case.

[MurrayWilson]

It also happens when moving a file in linux from an xfs filesystem to a NFS mounted filesystem.

[websurfer5]

SilentGhost's analysis is correct. The provided example code causes the error because it is trying to move the symlink into its target when the target is a directory. Any cross-device moving issues are unrelated to this example code. Here is the relevant code in the master branch:

    if os.path.isdir(dst):
        if _samefile(src, dst):
            # We might be on a case insensitive filesystem,
            # perform the rename anyway.
            os.rename(src, dst)
            return

shutil._samefile() considers the example link and its target to be the same. When _samefile() returns False, this code gets executed:

        real_dst = os.path.join(dst, _basename(src))

        if os.path.exists(real_dst):
            raise Error("Destination path '%s' already exists" % real_dst)
    try:
        os.rename(src, real_dst)
    except OSError:
        if os.path.islink(src):
            linkto = os.readlink(src)
            os.symlink(linkto, real_dst)
            os.unlink(src)

A simple fix is to check whether src is a symlink when _samefile() returns True. The "Destination path...already exists" error isn't a problem for our symlink case because the shell mv command also returns an error.

$ ls -l /tmp/tmpdir/
total 0
lrwxr-xr-x  1 jeff  staff  11 Aug  5 23:36 test_dir -> /tmp/tmpdir
$ mv test_dir /tmp/tmpdir
mv: test_dir and /tmp/tmpdir/test_dir are identical

I will generate a pull request.

[iritkatriel]

Removing old versions.