libct/cgroup: prep to rm GetClosestMountpointAncestor

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

Prepare to remove it by replacing with the implementation based on
the parser from github.com/moby/sys/mountinfo parser.

This commit is here to make sure the proposed replacement passes the
unit test.

Funny, but the unit test need to be slightly modified since it
supplies the wrong mountinfo (space as the first character, empty line
at the end).

Validated by

 $ go test -v -run Ance
 === RUN   TestGetClosestMountpointAncestor
 --- PASS: TestGetClosestMountpointAncestor (0.00s)
 PASS
 ok  	github.com/opencontainers/runc/libcontainer/cgroups	0.002s

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

libcontainer/cgroups/fs/cpuset.go

@@ -62,7 +62,11 @@ func (s *CpusetGroup) ApplyDir(dir string, cgroup *configs.Cgroup, pid int) erro
 	if err != nil {
 		return err
 	}
-	root := filepath.Dir(cgroups.GetClosestMountpointAncestor(dir, string(mountInfo)))
+	root, err := cgroups.GetClosestMountpointAncestor(dir, string(mountInfo))
+	if err != nil {
+		return err
+	}
+	root = filepath.Dir(root)
 	// 'ensureParent' start with parent because we don't want to
 	// explicitly inherit from parent, it could conflict with
 	// 'cpuset.cpu_exclusive'.

libcontainer/cgroups/utils.go

@@ -16,6 +16,7 @@ import (
 	"time"
 
 	units "github.com/docker/go-units"
+	"github.com/moby/sys/mountinfo"
 	"golang.org/x/sys/unix"
 )
 
@@ -126,19 +127,25 @@ func isSubsystemAvailable(subsystem string) bool {
 	return avail
 }
 
-func GetClosestMountpointAncestor(dir, mountinfo string) string {
-	deepestMountPoint := ""
-	for _, mountInfoEntry := range strings.Split(mountinfo, "\n") {
-		mountInfoParts := strings.Fields(mountInfoEntry)
-		if len(mountInfoParts) < 5 {
-			continue
+func GetClosestMountpointAncestor(dir, data string) (string, error) {
+	mi, err := mountinfo.GetMountsFromReader(strings.NewReader(data), mountinfo.ParentsFilter(dir))
+	if err != nil {
+		return "", err
 	}
-		mountPoint := mountInfoParts[4]
-		if strings.HasPrefix(mountPoint, deepestMountPoint) && strings.HasPrefix(dir, mountPoint) {
-			deepestMountPoint = mountPoint
+	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 deepestMountPoint
+
+	return mi[idx].Mountpoint, nil
 }
 
 func FindCgroupMountpointDir() (string, error) {

libcontainer/cgroups/utils_test.go

@@ -374,7 +374,7 @@ func TestIgnoreCgroup2Mount(t *testing.T) {
 }
 
 func TestGetClosestMountpointAncestor(t *testing.T) {
-	fakeMountInfo := ` 18 24 0:17 / /sys rw,nosuid,nodev,noexec,relatime - sysfs sysfs rw
+	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
@@ -382,8 +382,8 @@ func TestGetClosestMountpointAncestor(t *testing.T) {
 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
-`
+100 99 1:31 / / rw,relatime - fake fake rw,fake`
+
 	testCases := []struct {
 		input  string
 		output string
@@ -395,7 +395,10 @@ func TestGetClosestMountpointAncestor(t *testing.T) {
 	}
 
 	for _, c := range testCases {
-		mountpoint := GetClosestMountpointAncestor(c.input, fakeMountInfo)
+		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)
 		}