test_unistd: Try add a test for initgroups on Apple

test/test_unistd.rs

@@ -171,6 +171,44 @@ fn test_initgroups() {
     setgroups(&old_groups).unwrap();
 }
 
+/// `initgroups()` test specific to Apple platforms.
+///
+/// `getgroups()` and `setgroups()` do not behave as expected on Apple
+/// platforms and the use of `setgroups()` is highly discouraged. The only
+/// thing we have to work on is this note in the `getgroups(2)` manpage:
+/// _"Calling initgroups(3) to opt-in for supplementary groups will cause
+/// getgroups() to return a single entry, the GID that was passed to
+/// initgroups(3)"_
+/// This tests that behaviour.
+#[test]
+#[cfg(any(target_os = "ios", target_os = "macos"))]
+fn test_initgroups() {
+    // Skip this test when not run as root as `initgroups()` and `setgroups()`
+    // require root.
+    if !Uid::current().is_root() {
+        let stderr = std::io::stderr();
+        let mut handle = stderr.lock();
+        writeln!(handle, "test_initgroups requires root privileges. Skipping test.").unwrap();
+        return;
+    }
+
+    #[allow(unused_variables)]
+    let m = ::GROUPS_MTX.lock().expect("Mutex got poisoned by another test");
+
+    // It doesn't matter if the root user is not called "root" or if a user
+    // called "root" doesn't exist. We are just checking that the extra,
+    // made-up group, `123`, is set.
+    // FIXME: Test the other half of initgroups' functionality: whether the
+    // groups that the user belongs to are also set.
+    let user = CString::new("root").unwrap();
+    let group = Gid::from_raw(123);
+
+    initgroups(&user, group).unwrap();
+
+    let new_groups = getgroups().unwrap();
+    assert!(new_groups.contains(&group));
+}
+
 macro_rules! execve_test_factory(
     ($test_name:ident, $syscall:ident, $exe: expr) => (
     #[test]