Libcontainer fs.GetStats() will not look for cgroups relative to the cgroup of init process if the parent

in cgroups.Cgroup is absolute. This is required to get stats of other containers while running inside a docker container.

Docker-DCO-1.1-Signed-off-by: Vishnu Kannan <vishnuk@google.com> (github: vishh)

cgroups/fs/apply_raw.go

@@ -150,6 +150,10 @@ func (raw *data) parent(subsystem string) (string, error) {
 }
 
 func (raw *data) path(subsystem string) (string, error) {
+	// If the cgroup name/path is absolute do not look relative to the cgroup of the init process.
+	if filepath.IsAbs(raw.cgroup) {
+		return filepath.Join(raw.root, subsystem, raw.cgroup), nil
+	}
 	parent, err := raw.parent(subsystem)
 	if err != nil {
 		return "", err

cgroups/fs/utils_test.go

@@ -5,6 +5,8 @@ import (
 	"os"
 	"path/filepath"
 	"testing"
+
+	"github.com/docker/libcontainer/cgroups"
 )
 
 const (
@@ -66,3 +68,20 @@ func TestGetCgroupParamsInt(t *testing.T) {
 		t.Fatal("Expecting error, got none")
 	}
 }
+
+func TestAbsolutePathHandling(t *testing.T) {
+	testCgroup := cgroups.Cgroup{
+		Name:   "bar",
+		Parent: "/foo",
+	}
+	cgroupData := data{
+		root:   "/sys/fs/cgroup",
+		cgroup: "/foo/bar",
+		c:      &testCgroup,
+		pid:    1,
+	}
+	expectedPath := filepath.Join(cgroupData.root, "cpu", testCgroup.Parent, testCgroup.Name)
+	if path, err := cgroupData.path("cpu"); path != expectedPath || err != nil {
+		t.Fatalf("expected path %s but got %s %s", expectedPath, path, err)
+	}
+}