move_file() fails if source and destination are not on the same file system

[frankenjoe]

I just encountered the following error when trying to use audeer.move_file() to move a file from one file system to another:

OSError: [Errno 18] Invalid cross-device link: '/tmp/tmpvgh8upvf/audio/002.wav' -> '/media/jwagner/Data/Git/pyaudb/my/build/audio/002.wav'

We should either fix it or mention in the docstring that it's not possible.

[hagenw]

That's unfortunate. It seems there exists no secure and fast methods to move files around under Python.
We could have another look, otherwise I would recommend to add a note to the docstring explaining what to use if you want to move files across file systems.

[frankenjoe]

I think it is expected that moving a file from one system to another will be slower as it basically requires to copy the file and then delete in the old destination. So maybe we should simply catch the error and fallback to another move function.

[hagenw]

In principle, that's a good idea. But we have to make sure, that OSError is not thrown for other problems as well. Or can we maybe check the returned error message if it contains "Invalid cross-device link"? The only problem is that the error message might vary between operating systems and it might also be not possible to write tests for it.

[frankenjoe]

If there is a cheap way to find out if the two files are on different disks we could also check that first.

On the other hand - if it raises a OSError due to some other reason, it will probably raise it again when we call the other move function.

[frankenjoe]

Here's my current workaround (on Linux):

def safe_move(src: str, dst: str):
    try:
        audeer.move(src, dst)
    except OSError as ex:
        if 'Invalid cross-device link' in str(ex):
            shutil.move(src, dst)
        else:
            raise ex

[hagenw]

One problem might be that the error messages are usually different for file system operations on different operating systems.
So it might be that this will not work under Windows and maybe under MacOS.

The ideal solution would be to be able to create a test on the runners for having different file systems, but I'm not sure if this is possible (BTW, does this mean simply different partitions, or does this mean having EXT4 on one partition and NTFS on another one?).

[frankenjoe]

In my case I have two disks, both EXT4 formatted.