mv: fails to copy special files across filesystems

[jfinkels]

Environment: Ubuntu 20.04, uutils main branch (git commit 1e8e16b), GNU coreutils v8.30

Steps to reproduce:

# create a directory on a different filesystem
mkdir /dev/shm/tmp
# create a special file, in this case a FIFO
mknod pipe p
mv pipe /dev/shm/tmp

What happens now: uutils mv hangs indefinitely and never terminates.

What I expected to happen: GNU mv copies the special file to the target directory

Notes: this is causing a failure in the GNU test file tests/mv/mv-special-1.sh.

Because the source file and the destination are on different filesystems, the file is copied to the destination and then the source file is removed; this can be seen by passing the --verbose option:

$ mv --verbose pipe /dev/shm/tmp
copied 'pipe' -> '/dev/shm/tmp/pipe'
removed 'pipe'

[DaringCuteSeal]

I think we need to create a new file and copy its attributes instead of solely relying on fs::copy, kinda like how cp does it when the --no-dereference option is passed. if we do that now we'd get like three identical functions both in cp and mv relating to the attributes copying operation.

this makes me want to put the entire cp into uucore.. think the whole app. but minus the progressbar, though i will have no idea of how to expose the internal progress of the copy call to the caller. a closure should be fine? but seriously this is some refactor worthy of consideration imho