Detect whether Delegate is available on both slices and scopes

Starting with systemd 237, in preparation for cgroup v2, delegation is
only now available for scopes, not slices.

Update libcontainer code to detect whether delegation is available on
both and use that information when creating new slices.

Signed-off-by: Filipe Brandenburger <filbranden@google.com>

libcontainer/cgroups/systemd/apply_systemd.go

@@ -75,7 +75,8 @@ var (
 	hasStartTransientUnit           bool
 	hasStartTransientSliceUnit      bool
 	hasTransientDefaultDependencies bool
-	hasDelegate                     bool
+	hasDelegateScope                bool
+	hasDelegateSlice                bool
 )
 
 func newProp(name string, units interface{}) systemdDbus.Property {
@@ -150,12 +151,12 @@ func UseSystemd() bool {
 		theConn.StopUnit(scope, "replace", nil)
 
 		// Assume StartTransientUnit on a scope allows Delegate
-		hasDelegate = true
-		dl := newProp("Delegate", true)
-		if _, err := theConn.StartTransientUnit(scope, "replace", []systemdDbus.Property{dl}, nil); err != nil {
+		hasDelegateScope = true
+		dlScope := newProp("Delegate", true)
+		if _, err := theConn.StartTransientUnit(scope, "replace", []systemdDbus.Property{dlScope}, nil); err != nil {
 			if dbusError, ok := err.(dbus.Error); ok {
 				if strings.Contains(dbusError.Name, "org.freedesktop.DBus.Error.PropertyReadOnly") {
-					hasDelegate = false
+					hasDelegateScope = false
 				}
 			}
 		}
@@ -187,6 +188,22 @@ func UseSystemd() bool {
 			time.Sleep(time.Millisecond)
 		}
 
+		// Not critical because of the stop unit logic above.
+		theConn.StopUnit(slice, "replace", nil)
+
+		// Assume StartTransientUnit on a slice allows Delegate
+		hasDelegateSlice = true
+		dlSlice := newProp("Delegate", true)
+		if _, err := theConn.StartTransientUnit(slice, "replace", []systemdDbus.Property{dlSlice}, nil); err != nil {
+			if dbusError, ok := err.(dbus.Error); ok {
+				// Starting with systemd v237, Delegate is not even a property of slices anymore,
+				// so the D-Bus call fails with "InvalidArgs" error.
+				if strings.Contains(dbusError.Name, "org.freedesktop.DBus.Error.PropertyReadOnly") || strings.Contains(dbusError.Name, "org.freedesktop.DBus.Error.InvalidArgs") {
+					hasDelegateSlice = false
+				}
+			}
+		}
+
 		// Not critical because of the stop unit logic above.
 		theConn.StopUnit(scope, "replace", nil)
 		theConn.StopUnit(slice, "replace", nil)
@@ -242,9 +259,16 @@ func (m *Manager) Apply(pid int) error {
 		properties = append(properties, newProp("PIDs", []uint32{uint32(pid)}))
 	}
 
-	if hasDelegate {
-		// This is only supported on systemd versions 218 and above.
-		properties = append(properties, newProp("Delegate", true))
+	// Check if we can delegate. This is only supported on systemd versions 218 and above.
+	if strings.HasSuffix(unitName, ".slice") {
+		if hasDelegateSlice {
+			// systemd 237 and above no longer allows delegation on a slice
+			properties = append(properties, newProp("Delegate", true))
+		}
+	} else {
+		if hasDelegateScope {
+			properties = append(properties, newProp("Delegate", true))
+		}
 	}
 
 	// Always enable accounting, this gets us the same behaviour as the fs implementation,