libct/cg: add Resources=nil unit test

Cgroup controllers should never panic, and yet sometimes they do.

Add a unit test to check that controllers never panic when called with
nil arguments and/or resources, and fix a few found cases.

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

libcontainer/cgroups/fs2/fs2.go

@@ -129,6 +129,9 @@ func (m *manager) GetStats() (*cgroups.Stats, error) {
 }
 
 func (m *manager) Freeze(state configs.FreezerState) error {
+	if m.config.Resources == nil {
+		return errors.New("cannot toggle freezer: cgroups not configured for container")
+	}
 	if err := setFreezer(m.dirPath, state); err != nil {
 		return err
 	}
@@ -145,6 +148,9 @@ func (m *manager) Path(_ string) string {
 }
 
 func (m *manager) Set(r *configs.Resources) error {
+	if r == nil {
+		return nil
+	}
 	if err := m.getControllers(); err != nil {
 		return err
 	}

libcontainer/cgroups/manager/manager_test.go

@@ -0,0 +1,45 @@
+package manager
+
+import (
+	"testing"
+
+	"github.com/opencontainers/runc/libcontainer/configs"
+)
+
+// TestNilResources checks that a cgroup manager do not panic when
+// config.Resources is nil. While it does not make sense to use a
+// manager with no resources, it should not result in a panic.
+//
+// This tests either v1 or v2 managers (both fs and systemd)
+// depending on what cgroup version is available on the host.
+
+func TestNilResources(t *testing.T) {
+	for _, sd := range []bool{false, true} {
+		cg := &configs.Cgroup{} // .Resources is nil
+		cg.Systemd = sd
+		mgr, err := New(cg)
+		if err != nil {
+			// Some managers require non-nil Resources during
+			// instantiation -- provide and retry. In such case
+			// we're mostly testing Set(nil) below.
+			cg.Resources = &configs.Resources{}
+			mgr, err = New(cg)
+			if err != nil {
+				t.Error(err)
+				continue
+			}
+		}
+		_ = mgr.Apply(-1)
+		_ = mgr.Set(nil)
+		_ = mgr.Freeze(configs.Thawed)
+		_ = mgr.Exists()
+		_, _ = mgr.GetAllPids()
+		_, _ = mgr.GetCgroups()
+		_, _ = mgr.GetFreezerState()
+		_ = mgr.Path("")
+		_ = mgr.GetPaths()
+		_, _ = mgr.GetStats()
+		_, _ = mgr.OOMKillCount()
+		_ = mgr.Destroy()
+	}
+}

libcontainer/cgroups/systemd/v1.go

@@ -389,6 +389,9 @@ func (m *legacyManager) freezeBeforeSet(unitName string, r *configs.Resources) (
 }
 
 func (m *legacyManager) Set(r *configs.Resources) error {
+	if r == nil {
+		return nil
+	}
 	if r.Unified != nil {
 		return cgroups.ErrV1NoUnified
 	}

libcontainer/cgroups/systemd/v2.go

@@ -384,6 +384,9 @@ func (m *unifiedManager) GetStats() (*cgroups.Stats, error) {
 }
 
 func (m *unifiedManager) Set(r *configs.Resources) error {
+	if r == nil {
+		return nil
+	}
 	properties, err := genV2ResourcesProperties(r, m.dbus)
 	if err != nil {
 		return err