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.