bug#69532: mv's new -x option should be made orthogonal to -t/-T/default

[Paul Eggert]

On 2024-03-05 06:16, Pádraig Brady wrote:
> I think I'll remove the as yet unreleased mv --swap from coreutils, 
> given that
> util-linux is as widely available as coreutils on GNU/Linux platforms.

Although removing that "mv --swap" implementation was a win, I don't 
think we can simply delegate this to util-linux's exch command. 
Exchanging files via a renameat-like call is not limited to the Linux 
kernel; it's also possible on macOS via renameatx_np with RENAME_SWAP, 
and there have been noises about adding similar things to other 
operating systems.

I just now added support for macOS renameatx_np to Gnulib, so coreutils 
does not need to worry about the macOS details; it can simply use 
renameatu with the Linux flags. See:

https://git.savannah.gnu.org/cgit/gnulib.git/commit/?id=af32ee824ee18255839f9812b8ed61aa5257a82b

Even with Linux it's dicey. People may have older util-linux installed 
and so lack the 'exch' utility; this is true for both Fedora 39 and 
Ubuntu 23.10, the current releases. Ubuntu is also odd in that it 
doesn't install all the util-linux utilities as part of the util-linux 
package, so it's not clear what they will do with 'exch'.

So I propose that we implement the idea in coreutils in a better way, 
that interacts more nicely with -t, -T, etc. Also, I suggest using the 
Linuxish name "--exchange" instead of the macOSish name "--swap", and 
(for now at least) not giving the option a single-letter equivalent as I 
expect it to be useful from scripts, not interactively.

After looking at various ways to do it I came up with the attached 
proposed patch. This should work on both GNU/Linux and macOS, if your OS 
is recent enough and the file system supports atomic exchange.

0001-mv-new-option-exchange.patch
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