libct/cg/fs.getCgroupRoot: reuse (cached) cgroup mountinfo

Drop the custom mountinfo parser, reuse the cgroups.GetCgroupMounts()
to get the cgroup root. Faster, and much less code.

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

libcontainer/cgroups/fs/fs.go

@@ -3,11 +3,9 @@
 package fs
 
 import (
-	"bufio"
 	"fmt"
 	"os"
 	"path/filepath"
-	"strings"
 	"sync"
 
 	"github.com/opencontainers/runc/libcontainer/cgroups"
@@ -133,46 +131,19 @@ func getCgroupRoot() (string, error) {
 		return cgroupRoot, nil
 	}
 
-	// slow path: parse mountinfo, find the first mount where fs=cgroup
-	// (e.g. "/sys/fs/cgroup/memory"), use its parent.
-	f, err := os.Open("/proc/self/mountinfo")
+	// slow path: parse mountinfo
+	mi, err := cgroups.GetCgroupMounts(false)
 	if err != nil {
 		return "", err
 	}
-	defer f.Close()
-
-	var root string
-	scanner := bufio.NewScanner(f)
-	for scanner.Scan() {
-		text := scanner.Text()
-		fields := strings.Split(text, " ")
-		// Safe as mountinfo encodes mountpoints with spaces as \040.
-		index := strings.Index(text, " - ")
-		postSeparatorFields := strings.Fields(text[index+3:])
-		numPostFields := len(postSeparatorFields)
-
-		// This is an error as we can't detect if the mount is for "cgroup"
-		if numPostFields == 0 {
-			return "", fmt.Errorf("mountinfo: found no fields post '-' in %q", text)
-		}
-
-		if postSeparatorFields[0] == "cgroup" {
-			// Check that the mount is properly formatted.
-			if numPostFields < 3 {
-				return "", fmt.Errorf("Error found less than 3 fields post '-' in %q", text)
-			}
-
-			root = filepath.Dir(fields[4])
-			break
-		}
-	}
-	if err := scanner.Err(); err != nil {
-		return "", err
-	}
-	if root == "" {
+	if len(mi) < 1 {
 		return "", errors.New("no cgroup mount found in mountinfo")
 	}
 
+	// Get the first cgroup mount (e.g. "/sys/fs/cgroup/memory"),
+	// use its parent directory.
+	root := filepath.Dir(mi[0].Mountpoint)
+
 	if _, err := os.Stat(root); err != nil {
 		return "", err
 	}