bug#55029: Simple backup swaps source and destination files

[Pádraig Brady]

On 21/04/2022 03:53, Paul Eggert wrote:
> On 4/19/22 16:05, Steve Ward wrote:
> > When doing mv or cp with --backup=simple, if an existing file in
> > DIRECTORY has the same name as SOURCE, the files appear to be swapped
> > instead of an in-place backup of the original file in DIRECTORY being
> > made.
>
> Thanks for the bug report. That's new to coreutils 9.1, and is a big
> enough fail that it suggests we'll need a 9.2 sooner rather than later.
> I introduced the bug when fixing an earlier bug (sorry).
>
> I installed the attached Gnulib patch, which should fix the bug in
> Coreutils, with the attached two Coreutils patches to update to the
> latest Gnulib, and to add a test case for the bug.

Thanks for the fix.

For the record, another failure mode is with different file systems:
  $ echo 1 > /tmp/a
  $ echo 2 > a
  $ echo 3 > a~
  $ src/mv -v --backup=simple a /tmp
  src/mv: cannot backup '/tmp/a': Invalid cross-device link
In this case the failure is not silent, and no changes are made.
This is a bit better, in that there is no possibility of silent data loss.

For the same file system case you might lose data in a setup like:
  $ cd workdir
  $ gen_data1 > data
  $ mv --backup=simple data1/
  $ gen_data2 > data
  $ mv --backup=simple data2/
In which case you'd lose the data~ backups in each of the data?/ dirs.

Another case which might be more problematic is with install(1).
  $ install --backup=simple blah /usr/local/dir
In which case we'd lose the blah~ backup upon removal of the build dir.

Note --backup without a specific type will default to simple
or numbered as appropriate, but this does _not_ have the
issue when defaulting to simple.

I've applied the fix to Fedora rawhide which had already moved to 9.1.
I've reported git issue at https://bugs.archlinux.org/task/74544

This should induce a coreutils 9.2 sooner,
but I don't think we need an immediate release just yet.

thanks,
Pádraig