cp: allow removing symbolic link loop destination

Allow `cp --remove-destination` to remove a symbolic link loop (or a
symbolic link that initiates a chain of too many symbolic
links). Before this commit, if `loop` were a symbolic link to itself,
then

    cp --remove-destination file loop

would fail with an error message. After this commit, it succeeds. This
matches the behaviotr of GNU cp.

src/uu/cp/src/cp.rs

@@ -1383,6 +1383,15 @@ fn handle_existing_dest(
     Ok(())
 }
 
+/// Decide whether the given path exists.
+fn file_or_link_exists(path: &Path) -> bool {
+    // Using `Path.exists()` or `Path.try_exists()` is not sufficient,
+    // because if `path` is a symbolic link and there are too many
+    // levels of symbolic link, then those methods will return false
+    // or an OS error.
+    path.symlink_metadata().is_ok()
+}
+
 /// Copy the a file from `source` to `dest`. `source` will be dereferenced if
 /// `options.dereference` is set to true. `dest` will be dereferenced only if
 /// the source was not a symlink.
@@ -1400,7 +1409,7 @@ fn copy_file(
     symlinked_files: &mut HashSet<FileInformation>,
     source_in_command_line: bool,
 ) -> CopyResult<()> {
-    if dest.exists() {
+    if file_or_link_exists(dest) {
         handle_existing_dest(source, dest, options, source_in_command_line)?;
     }
 

tests/by-util/test_cp.rs

@@ -2095,3 +2095,16 @@ fn test_cp_mode_hardlink_no_dereference() {
     assert!(at.symlink_exists("z"));
     assert_eq!(at.read_symlink("z"), "slink");
 }
+
+#[test]
+fn test_remove_destination_symbolic_link_loop() {
+    let (at, mut ucmd) = at_and_ucmd!();
+    at.symlink_file("loop", "loop");
+    at.plus("loop");
+    at.touch("f");
+    ucmd.args(&["--remove-destination", "f", "loop"])
+        .succeeds()
+        .no_stdout()
+        .no_stderr();
+    assert!(at.file_exists("loop"));
+}