libct/cg/fscommon.GetCgroupParamUint: improve

1. Use GetCgroupParamString as the initial part of both functions are
   the same and we can reuse it. This also gives us whatever security
   measures GetCgroupParamString has (see previous commit).

2. Fix the error wrapping to not add the value, as it is already a part
   of the error returned by ParseUint.

3. Improve docstring.

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

libcontainer/cgroups/fscommon/utils.go

@@ -5,9 +5,7 @@ package fscommon
 import (
 	"errors"
 	"fmt"
-	"io/ioutil"
 	"math"
-	"path/filepath"
 	"strconv"
 	"strings"
 )
@@ -54,21 +52,21 @@ func GetCgroupParamKeyValue(t string) (string, uint64, error) {
 	}
 }
 
-// Gets a single uint64 value from the specified cgroup file.
-func GetCgroupParamUint(cgroupPath, cgroupFile string) (uint64, error) {
-	fileName := filepath.Join(cgroupPath, cgroupFile)
-	contents, err := ioutil.ReadFile(fileName)
+// GetCgroupParamUint reads a single uint64 value from the specified cgroup file.
+// If the value read is "max", the math.MaxUint64 is returned.
+func GetCgroupParamUint(path, file string) (uint64, error) {
+	contents, err := GetCgroupParamString(path, file)
 	if err != nil {
 		return 0, err
 	}
-	trimmed := strings.TrimSpace(string(contents))
-	if trimmed == "max" {
+	contents = strings.TrimSpace(contents)
+	if contents == "max" {
 		return math.MaxUint64, nil
 	}
 
-	res, err := ParseUint(trimmed, 10, 64)
+	res, err := ParseUint(contents, 10, 64)
 	if err != nil {
-		return res, fmt.Errorf("unable to parse %q as a uint from Cgroup file %q", string(contents), fileName)
+		return res, fmt.Errorf("unable to parse file %q", path+"/"+file)
 	}
 	return res, nil
 }