libct/cg: allow Set(nil)

As the cgroup config is already supplied during Apply(), allow Set()
to accept nil -- in which case it uses the previously cached resources
(specified either during Manager creation, or previous call to Set()
with non-nil argument).

While at it, don't forget to save the new resources into the manager's
internal state (previously only fs driver did that).

Signed-off-by: Kir Kolyshkin <kolyshkin@gmail.com>

libcontainer/cgroups/fs/fs.go

@@ -276,7 +276,10 @@ func (m *manager) GetStats() (*cgroups.Stats, error) {
 
 func (m *manager) Set(r *configs.Resources) error {
 	if r == nil {
-		return nil
+		r = m.cgroups.Resources
+		if r == nil {
+			return nil
+		}
 	}
 
 	// If Paths are set, then we are just joining cgroups paths
@@ -308,6 +311,7 @@ func (m *manager) Set(r *configs.Resources) error {
 		}
 	}
 
+	m.cgroups.Resources = r
 	return nil
 }
 

libcontainer/cgroups/fs2/fs2.go

@@ -148,6 +148,12 @@ func (m *manager) Path(_ string) string {
 }
 
 func (m *manager) Set(r *configs.Resources) error {
+	if r == nil {
+		r = m.config.Resources
+		if r == nil {
+			return nil
+		}
+	}
 	if err := m.getControllers(); err != nil {
 		return err
 	}

libcontainer/cgroups/systemd/v1.go

@@ -337,6 +337,12 @@ func (m *legacyManager) Set(r *configs.Resources) error {
 	if m.cgroups.Paths != nil {
 		return nil
 	}
+	if r == nil {
+		r = m.cgroups.Resources
+		if r == nil {
+			return nil
+		}
+	}
 	if r.Unified != nil {
 		return cgroups.ErrV1NoUnified
 	}
@@ -392,6 +398,7 @@ func (m *legacyManager) Set(r *configs.Resources) error {
 		}
 	}
 
+	m.cgroups.Resources = r
 	return nil
 }
 

libcontainer/cgroups/systemd/v2.go

@@ -426,6 +426,12 @@ func (m *unifiedManager) GetStats() (*cgroups.Stats, error) {
 }
 
 func (m *unifiedManager) Set(r *configs.Resources) error {
+	if r == nil {
+		r = m.cgroups.Resources
+		if r == nil {
+			return nil
+		}
+	}
 	dbusConnection, err := getDbusConnection(m.rootless)
 	if err != nil {
 		return err
@@ -471,7 +477,12 @@ func (m *unifiedManager) Set(r *configs.Resources) error {
 	if err != nil {
 		return err
 	}
-	return fsMgr.Set(r)
+	if err := fsMgr.Set(r); err != nil {
+		return err
+	}
+
+	m.cgroups.Resources = r
+	return nil
 }
 
 func (m *unifiedManager) GetPaths() map[string]string {