Don't fail if one of the cgroups is not setup

It is fairly common for certain cgroups controllers to
not be enabled on a system.  We should Warn when this happens
versus failing, when doing podman stats command.  This way users
can get information from the other controllers.

Fixes: containers#8588

Signed-off-by: Daniel J Walsh <dwalsh@redhat.com>

pkg/cgroups/cgroups.go

@@ -24,6 +24,7 @@ var (
 	ErrCgroupDeleted = errors.New("cgroup deleted")
 	// ErrCgroupV1Rootless means the cgroup v1 were attempted to be used in rootless environment
 	ErrCgroupV1Rootless = errors.New("no support for CGroups V1 in rootless environments")
+	ErrStatCgroup       = errors.New("no cgroup available for gathering user statistics")
 )
 
 // CgroupControl controls a cgroup hierarchy
@@ -525,10 +526,19 @@ func (c *CgroupControl) AddPid(pid int) error {
 // Stat returns usage statistics for the cgroup
 func (c *CgroupControl) Stat() (*Metrics, error) {
 	m := Metrics{}
+	found := false
 	for _, h := range handlers {
 		if err := h.Stat(c, &m); err != nil {
-			return nil, err
+			if !os.IsNotExist(errors.Cause(err)) {
+				return nil, err
+			}
+			logrus.Warningf("Failed to retrieve cgroup stats: %v", err)
+			continue
 		}
+		found = true
+	}
+	if !found {
+		return nil, ErrStatCgroup
 	}
 	return &m, nil
 }

pkg/cgroups/cgroups_test.go

@@ -0,0 +1,32 @@
+package cgroups
+
+import (
+	"testing"
+
+	"github.com/containers/podman/v2/pkg/rootless"
+	spec "github.com/opencontainers/runtime-spec/specs-go"
+)
+
+func TestCreated(t *testing.T) {
+	// tests only works in rootless mode
+	if rootless.IsRootless() {
+		return
+	}
+
+	var resources spec.LinuxResources
+	cgr, err := New("machine.slice", &resources)
+	if err != nil {
+		t.Error(err)
+	}
+	if err := cgr.Delete(); err != nil {
+		t.Error(err)
+	}
+
+	cgr, err = NewSystemd("machine.slice")
+	if err != nil {
+		t.Error(err)
+	}
+	if err := cgr.Delete(); err != nil {
+		t.Error(err)
+	}
+}