virtcontainers: add function to identify systemd cgroup path

Add function to identify if the given cgroup path is a systemd
cgroup path.
We need to parse the cgroup path to know which cgroup manager we have to use,
since some container engines do not use `--systemd-cgroup` runtime option.

Signed-off-by: Julio Montes <julio.montes@intel.com>

virtcontainers/cgroups.go

@@ -11,6 +11,7 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
+	"regexp"
 	"strings"
 
 	"github.com/containerd/cgroups"
@@ -182,3 +183,13 @@ func renameCgroupPath(path string) (string, error) {
 	return filepath.Join(cgroupPathDir, cgroupPathName), nil
 
 }
+
+func isSystemdCgroup(cgroupPath string) bool {
+	// systemd cgroup path: slice:prefix:name
+	re := regexp.MustCompile(`([[:alnum:]]|\.)+:([[:alnum:]]|\.)+:([[:alnum:]]|\.)+`)
+	found := re.FindStringIndex(cgroupPath)
+
+	// if found string is equal to cgroupPath then
+	// it's a correct systemd cgroup path.
+	return found != nil && cgroupPath[found[0]:found[1]] == cgroupPath
+}

virtcontainers/cgroups_test.go

@@ -197,3 +197,30 @@ func TestUpdateCgroups(t *testing.T) {
 	err = s.cgroupsDelete()
 	assert.NoError(err)
 }
+
+func TestIsSystemdCgroup(t *testing.T) {
+	assert := assert.New(t)
+
+	tests := []struct {
+		path     string
+		expected bool
+	}{
+		{"slice:kata:afhts2e5d4g5s", true},
+		{"slice.system:kata:afhts2e5d4g5s", true},
+		{"/kata/afhts2e5d4g5s", false},
+		{"a:b:c:d", false},
+		{":::", false},
+		{"", false},
+		{":", false},
+		{"::", false},
+		{":::", false},
+		{"a:b", false},
+		{"a:b:", false},
+		{":a:b", false},
+		{"@:@:@", false},
+	}
+
+	for _, t := range tests {
+		assert.Equal(t.expected, isSystemdCgroup(t.path), "invalid systemd cgroup path: %v", t.path)
+	}
+}