cpuset: fail if trying to enable memory pressure on a non-root cg

Signed-off-by: Levente Kurusa <lkurusa@acm.org>

tests/cpuset.rs

@@ -0,0 +1,19 @@
+extern crate cgroups;
+
+use cgroups::{Cgroup, CgroupError};
+use cgroups::cpuset::CpuSetController;
+
+#[test]
+fn test_cpuset_memory_pressure_root_cg() {
+    let hier = cgroups::hierarchies::V1::new();
+    let cg = Cgroup::new(&hier, String::from("test_cpuset_memory_pressure_root_cg"));
+    {
+        let cpuset: &CpuSetController = cg.controller_of().unwrap();
+
+        // This is not a root control group, so it should fail via InvalidOperation.
+        let res = cpuset.set_enable_memory_pressure(true);
+        assert!(res.is_err());
+        assert_eq!(res.unwrap_err(), CgroupError::InvalidOperation);
+    }
+    cg.delete();
+}