chown: allow setting arbitrary numeric user ID

Update `chown` to allow setting the owner of a file to a numeric user
ID regardless of whether a corresponding username exists on the
system.

For example,

    $ touch f && sudo chown 12345 f

succeeds even though there is no named user with ID 12345.

Fixes #3380.

src/uu/chown/src/chown.rs

@@ -197,10 +197,17 @@ fn parse_spec(spec: &str, sep: char) -> UResult<(Option<u32>, Option<u32>)> {
                     // So, try to parse it this way
                     return parse_spec(spec, '.');
                 } else {
+                    // It's possible that the `user` string contains a
+                    // numeric user ID, in which case, we respect that.
+                    match user.parse() {
+                        Ok(uid) => uid,
+                        Err(_) => {
                             return Err(USimpleError::new(
                                 1,
                                 format!("invalid user: {}", spec.quote()),
-                    ));
+                            ))
+                        }
+                    }
                 }
             }
         })

tests/by-util/test_chown.rs

@@ -418,6 +418,29 @@ fn test_chown_only_user_id() {
         .stderr_contains(&"failed to change");
 }
 
+/// Test for setting the owner to a user ID for a user that does not exist.
+///
+/// For example:
+///
+///     $ touch f && chown 12345 f
+///
+/// succeeds with exit status 0 and outputs nothing. The owner of the
+/// file is set to 12345, even though no user with that ID exists.
+///
+/// This test must be run as root, because only the root user can
+/// transfer ownership of a file.
+#[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");
+    }
+}
+
 #[test]
 // FixME: stderr = chown: ownership of 'test_chown_file1' retained as cuuser:wheel
 #[cfg(not(target_os = "freebsd"))]