cp: preserve hard links when target already exists

Prevent a panic in `cp -a` when the target of a hard link already
exists in the target directory structure.

For example,

    $ mkdir -p src dest/src
    $ touch src/f dest/src/f
    $ ln src/f src/link
    $ cp -a src dest

The `cp` command now succeeds without error.

tests/by-util/test_cp.rs

@@ -2363,3 +2363,32 @@ fn test_reflink_never_sparse_always() {
     assert_eq!(src_metadata.blocks(), dest_metadata.blocks());
     assert_eq!(dest_metadata.len(), 1024 * 1024);
 }
+
+/// Test for preserving attributes of a hard link in a directory.
+#[test]
+fn test_preserve_hardlink_attributes_in_directory() {
+    let (at, mut ucmd) = at_and_ucmd!();
+
+    // The source directory tree.
+    at.mkdir("src");
+    at.touch("src/f");
+    at.hard_link("src/f", "src/link");
+
+    // The destination directory tree.
+    //
+    // The file `f` already exists, but the `link` does not.
+    at.mkdir_all("dest/src");
+    at.touch("dest/src/f");
+
+    ucmd.args(&["-a", "src", "dest"]).succeeds().no_output();
+
+    // The hard link should now appear in the destination directory tree.
+    //
+    // A hard link should have the same inode as the target file.
+    at.file_exists("dest/src/link");
+    #[cfg(unix)]
+    assert_eq!(
+        at.metadata("dest/src/f").ino(),
+        at.metadata("dest/src/link").ino()
+    );
+}

tests/common/util.rs

@@ -228,6 +228,11 @@ impl CmdResult {
         self
     }
 
+    /// Assert that there is output to neither stderr nor stdout.
+    pub fn no_output(&self) -> &Self {
+        self.no_stdout().no_stderr()
+    }
+
     /// asserts that the command resulted in stdout stream output that equals the
     /// passed in value, trailing whitespace are kept to force strict comparison (#1235)
     /// stdout_only is a better choice unless stderr may or will be non-empty