groups tests: Add groups mutex and print message when tests skipped

test/test.rs

@@ -43,6 +43,9 @@ lazy_static! {
     /// Any test that changes the process's current working directory must grab
     /// this mutex
     pub static ref CWD_MTX: Mutex<()> = Mutex::new(());
+    /// Any test that changes the process's supplementary groups must grab this
+    /// mutex
+    pub static ref GROUPS_MTX: Mutex<()> = Mutex::new(());
     /// Any test that creates child processes must grab this mutex, regardless
     /// of what it does with those children.
     pub static ref FORK_MTX: Mutex<()> = Mutex::new(());

test/test_unistd.rs

@@ -110,9 +110,16 @@ mod linux_android {
 fn test_setgroups() {
     // Skip this test when not run as root as `setgroups()` requires root.
     if !Uid::current().is_root() {
+        use std::io;
+        let stderr = io::stderr();
+        let mut handle = stderr.lock();
+        writeln!(handle, "test_setgroups requires root privileges. Skipping test.").unwrap();
         return
     }
 
+    #[allow(unused_variables)]
+    let m = ::GROUPS_MTX.lock().expect("Mutex got poisoned by another test");
+
     // Save the existing groups
     let old_groups = getgroups().unwrap();
 
@@ -134,9 +141,16 @@ fn test_initgroups() {
     // Skip this test when not run as root as `initgroups()` and `setgroups()`
     // require root.
     if !Uid::current().is_root() {
+        use std::io;
+        let stderr = io::stderr();
+        let mut handle = stderr.lock();
+        writeln!(handle, "test_initgroups requires root privileges. Skipping test.").unwrap();
         return
     }
 
+    #[allow(unused_variables)]
+    let m = ::GROUPS_MUTEX.lock().expect("Mutex got poisoned by another test");
+
     // Save the existing groups
     let old_groups = getgroups().unwrap();