date: Implement setting the date on Unix & Windows (#1798)

* date: implement set date for unix and windows

Parsing the date string is not fully implemented yet, as in it relies
on the internals of chrono - things like "Mon, 14 Aug 2006 02:34:56 -0600"
do not work, nor does "2006-08-14 02:34:56" (no TZ / local time). This
is no different to using the "--date" option however, and will get fixed
when `parse_date` is a bit smarter.

Only supports unix and Windows platforms for now.

tests/by-util/test_date.rs

@@ -2,6 +2,8 @@ extern crate regex;
 
 use self::regex::Regex;
 use crate::common::util::*;
+#[cfg(all(unix, not(target_os = "macos")))]
+use rust_users::*;
 
 #[test]
 fn test_date_email() {
@@ -131,3 +133,42 @@ fn test_date_format_full_day() {
     let re = Regex::new(r"\S+ \d{4}-\d{2}-\d{2}").unwrap();
     assert!(re.is_match(&result.stdout.trim()));
 }
+
+#[test]
+#[cfg(all(unix, not(target_os = "macos")))]
+fn test_date_set_valid() {
+    if get_effective_uid() == 0 {
+        let (_, mut ucmd) = at_and_ucmd!();
+        let result = ucmd.arg("--set").arg("2020-03-12 13:30:00+08:00").succeeds();
+        result.no_stdout().no_stderr();
+    }
+}
+
+#[test]
+#[cfg(any(windows, all(unix, not(target_os = "macos"))))]
+fn test_date_set_invalid() {
+    let (_, mut ucmd) = at_and_ucmd!();
+    let result = ucmd.arg("--set").arg("123abcd").fails();
+    let result = result.no_stdout();
+    assert!(result.stderr.starts_with("date: invalid date "));
+}
+
+#[test]
+#[cfg(all(unix, not(target_os = "macos")))]
+fn test_date_set_permissions_error() {
+    if !(get_effective_uid() == 0 || is_wsl()) {
+        let (_, mut ucmd) = at_and_ucmd!();
+        let result = ucmd.arg("--set").arg("2020-03-11 21:45:00+08:00").fails();
+        let result = result.no_stdout();
+        assert!(result.stderr.starts_with("date: cannot set date: "));
+    }
+}
+
+#[test]
+#[cfg(target_os = "macos")]
+fn test_date_set_mac_unavailable() {
+    let (_, mut ucmd) = at_and_ucmd!();
+    let result = ucmd.arg("--set").arg("2020-03-11 21:45:00+08:00").fails();
+    let result = result.no_stdout();
+    assert!(result.stderr.starts_with("date: setting the date is not supported by macOS"));
+}