cp: truncate destination when --reflink is set

[pimzero]

This is needed in order to allow overriding an existing file. closes #3758

$ truncate -s 512M /tmp/disk.img
$ mkfs.btrfs /tmp/disk.img
[...]
$ mkdir /tmp/disk
$ sudo mount /tmp/disk.img /tmp/disk
$ sudo chown $(id -u):$(id -g) -R /tmp/disk
$ for i in $(seq 0 8192); do echo -ne 'a' >>/tmp/disk/src1; done
$ echo "success" >/tmp/disk/src2
$
$ # GNU ls supports overriding files created with `--reflink`
$ cp --reflink=always /tmp/disk/src1 /tmp/disk/dst1
$ cp --reflink=always /tmp/disk/src2 /tmp/disk/dst1
$ cat /tmp/disk/dst1
success
$
$ # Now testing with uutils
$ cargo run cp --reflink=always /tmp/disk/src1 /tmp/disk/dst2
    Finished dev [unoptimized + debuginfo] target(s) in 0.25s
     Running `target/debug/coreutils cp --reflink=always /tmp/disk/src1 /tmp/disk/dst2`
$ cargo run cp --reflink=always /tmp/disk/src2 /tmp/disk/dst2
    Finished dev [unoptimized + debuginfo] target(s) in 0.26s
     Running `target/debug/coreutils cp --reflink=always /tmp/disk/src2 /tmp/disk/dst2`
cp: failed to clone "/tmp/disk/src2" from "/tmp/disk/dst2": Invalid argument (os error 22)
$ cat /tmp/disk/dst2
[lots of 'a']
$
$ # With truncate(true)
$ cargo run cp --reflink=always /tmp/disk/src1 /tmp/disk/dst3
    Finished dev [unoptimized + debuginfo] target(s) in 7.98s
     Running `target/debug/coreutils cp --reflink=always /tmp/disk/src1 /tmp/disk/dst3`
$ cargo run cp --reflink=always /tmp/disk/src2 /tmp/disk/dst3
    Finished dev [unoptimized + debuginfo] target(s) in 0.27s
     Running `target/debug/coreutils cp --reflink=always /tmp/disk/src2 /tmp/disk/dst3`
$ cat /tmp/disk/dst3
success

[sylvestre]

cool, could you please add a test for this ? thanks

[sylvestre]

need test :)

[pimzero]

I am not completely happy with the testing (it looks clunky, needs root, and may leak mount points in case of test failures) but I don't see a better way to check this (I believe tmpfs don't handle reflink).

[sylvestre]

Can you do like in

coreutils/tests/by-util/test_chown.rs

Lines 432 to 442 in 40095e1

	#[test]
	fn test_chown_only_user_id_nonexistent_user() {
	let ts = TestScenario::new(util_name!());
	let at = ts.fixtures.clone();
	at.touch("f");
	if let Ok(result) = run_ucmd_as_root(&ts, &["12345", "f"]) {
	result.success().no_stdout().no_stderr();
	} else {
	print!("Test skipped; requires root user");
	}
	}

instead?

thanks

[pimzero]

I am not sure this allows me to do what I want. My understanding is that only the tested command (cp) can be run as root with run_ucmd_as_root, while we need only need root for mount/umount in this test. Nevertheless, I tried to take into account this idea to allow running the test as non root (running sudo only for mount and umount)