libct/cgroups: rm GetClosestMountpointAncestor

The function GetClosestMountpointAncestor is not very efficient,
does not really belong to cgroup package, and is only used once
(from fs/cpuset.go).

Remove it, replacing with the implementation based on moby/sys/mountinfo
parser.

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

libcontainer/cgroups/fs/cpuset.go

@@ -9,6 +9,7 @@ import (
 	"os"
 	"path/filepath"
 
+	"github.com/moby/sys/mountinfo"
 	"github.com/opencontainers/runc/libcontainer/cgroups"
 	"github.com/opencontainers/runc/libcontainer/cgroups/fscommon"
 	"github.com/opencontainers/runc/libcontainer/configs"
@@ -52,17 +53,35 @@ func (s *CpusetGroup) GetStats(path string, stats *cgroups.Stats) error {
 	return nil
 }
 
+// Get the source mount point of directory passed in as argument.
+func getMount(dir string) (string, error) {
+	mi, err := mountinfo.GetMounts(mountinfo.ParentsFilter(dir))
+	if err != nil {
+		return "", err
+	}
+	if len(mi) < 1 {
+		return "", fmt.Errorf("Can't find mount point of %s", dir)
+	}
+
+	// find the longest mount point
+	var idx, maxlen int
+	for i := range mi {
+		if len(mi[i].Mountpoint) > maxlen {
+			maxlen = len(mi[i].Mountpoint)
+			idx = i
+		}
+	}
+
+	return mi[idx].Mountpoint, nil
+}
+
 func (s *CpusetGroup) ApplyDir(dir string, cgroup *configs.Cgroup, pid int) error {
 	// This might happen if we have no cpuset cgroup mounted.
 	// Just do nothing and don't fail.
 	if dir == "" {
 		return nil
 	}
-	mountInfo, err := ioutil.ReadFile("/proc/self/mountinfo")
+	root, err := getMount(dir)
-	if err != nil {
-		return err
-	}
-	root, err := cgroups.GetClosestMountpointAncestor(dir, string(mountInfo))
 	if err != nil {
 		return err
 	}

libcontainer/cgroups/utils.go

@@ -16,7 +16,6 @@ import (
 	"time"
 
 	units "github.com/docker/go-units"
-	"github.com/moby/sys/mountinfo"
 	"golang.org/x/sys/unix"
 )
 
@@ -127,27 +126,6 @@ func isSubsystemAvailable(subsystem string) bool {
 	return avail
 }
 
-func GetClosestMountpointAncestor(dir, data string) (string, error) {
-	mi, err := mountinfo.GetMountsFromReader(strings.NewReader(data), mountinfo.ParentsFilter(dir))
-	if err != nil {
-		return "", err
-	}
-	if len(mi) < 1 {
-		return "", err
-	}
-
-	// find the longest mount point
-	var idx, maxlen int
-	for i := range mi {
-		if len(mi[i].Mountpoint) > maxlen {
-			maxlen = len(mi[i].Mountpoint)
-			idx = i
-		}
-	}
-
-	return mi[idx].Mountpoint, nil
-}
-
 func FindCgroupMountpointDir() (string, error) {
 	f, err := os.Open("/proc/self/mountinfo")
 	if err != nil {

libcontainer/cgroups/utils_test.go

@@ -373,38 +373,6 @@ func TestIgnoreCgroup2Mount(t *testing.T) {
 	}
 }
 
-func TestGetClosestMountpointAncestor(t *testing.T) {
-	const fakeMountInfo = `18 24 0:17 / /sys rw,nosuid,nodev,noexec,relatime - sysfs sysfs rw
-100 99 1:31 / /foo/bar rw,relatime - fake fake rw,fake
-100 99 1:31 / /foo/bar/baz2 rw,relatime - fake fake rw,fake
-100 99 1:31 / /foo/bar/baz rw,relatime - fake fake rw,fake
-100 99 1:31 / /foo/bar/bazza rw,relatime - fake fake rw,fake
-100 99 1:31 / /foo/bar/baz3 rw,relatime - fake fake rw,fake
-100 99 1:31 / /foo rw,relatime - fake fake rw,fake
-100 99 1:31 / /unrelated rw,relatime - fake fake rw,fake
-100 99 1:31 / / rw,relatime - fake fake rw,fake`
-
-	testCases := []struct {
-		input  string
-		output string
-	}{
-		{input: "/foo/bar/baz/a/b/c", output: "/foo/bar/baz"},
-		{input: "/foo/bar/baz", output: "/foo/bar/baz"},
-		{input: "/foo/bar/bazza", output: "/foo/bar/bazza"},
-		{input: "/a/b/c/d", output: "/"},
-	}
-
-	for _, c := range testCases {
-		mountpoint, err := GetClosestMountpointAncestor(c.input, fakeMountInfo)
-		if err != nil {
-			t.Fatal(err)
-		}
-		if mountpoint != c.output {
-			t.Errorf("expected %s, got %s", c.output, mountpoint)
-		}
-	}
-}
-
 func TestFindCgroupMountpointAndRoot(t *testing.T) {
 	fakeMountInfo := `
 35 27 0:29 / /foo rw,nosuid,nodev,noexec,relatime shared:18 - cgroup cgroup rw,devices