libct/cg/fscommon: introduce and use ParseError

1. Introduce ParseError type as a way to unify error messages related to
   file parsing. Use it from GetCgroup* functions.

2. Do not discard the error from strconv.Parse{Int,Uint} -- it contains
   the value being parsed, and the details about the error.

2. As the error above already contains the value, drop it from format.

[v2: use path.Join in Error]

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

libcontainer/cgroups/fscommon/utils.go

@@ -5,6 +5,7 @@ package fscommon
 import (
 	"fmt"
 	"math"
+	"path"
 	"strconv"
 	"strings"
 
@@ -20,6 +21,19 @@ var (
 	WriteFile = cgroups.WriteFile
 )
 
+// ParseError records a parse error details, including the file path.
+type ParseError struct {
+	Path string
+	File string
+	Err  error
+}
+
+func (e *ParseError) Error() string {
+	return "unable to parse " + path.Join(e.Path, e.File) + ": " + e.Err.Error()
+}
+
+func (e *ParseError) Unwrap() error { return e.Err }
+
 // ParseUint converts a string to an uint64 integer.
 // Negative values are returned at zero as, due to kernel bugs,
 // some of the memory cgroup stats can be negative.
@@ -72,7 +86,11 @@ func GetValueByKey(path, file, key string) (uint64, error) {
 	for _, line := range lines {
 		arr := strings.Split(line, " ")
 		if len(arr) == 2 && arr[0] == key {
-			return ParseUint(arr[1], 10, 64)
+			val, err := ParseUint(arr[1], 10, 64)
+			if err != nil {
+				err = &ParseError{Path: path, File: file, Err: err}
+			}
+			return val, err
 		}
 	}
 
@@ -93,7 +111,7 @@ func GetCgroupParamUint(path, file string) (uint64, error) {
 
 	res, err := ParseUint(contents, 10, 64)
 	if err != nil {
-		return res, fmt.Errorf("unable to parse file %q", path+"/"+file)
+		return res, &ParseError{Path: path, File: file, Err: err}
 	}
 	return res, nil
 }
@@ -112,7 +130,7 @@ func GetCgroupParamInt(path, file string) (int64, error) {
 
 	res, err := strconv.ParseInt(contents, 10, 64)
 	if err != nil {
-		return res, fmt.Errorf("unable to parse %q as a int from Cgroup file %q", contents, path+"/"+file)
+		return res, &ParseError{Path: path, File: file, Err: err}
 	}
 	return res, nil
 }