util: throw if file has !digit characters

Throw if file has non-numeric data, in addition to hinting the path that
caused the error.

Fixes: prometheus#304, prometheus/node_exporter#1710
Signed-off-by: Pranshu Srivastava <rexagod@gmail.com>

internal/util/parse.go

@@ -14,9 +14,11 @@
 package util
 
 import (
+	"fmt"
 	"io/ioutil"
 	"strconv"
 	"strings"
+	"unicode"
 )
 
 // ParseUint32s parses a slice of strings into a slice of uint32s.
@@ -70,6 +72,11 @@ func ReadUintFromFile(path string) (uint64, error) {
 	if err != nil {
 		return 0, err
 	}
+	for _, c := range data {
+		if !unicode.IsDigit(rune(c)) {
+			return 0, fmt.Errorf("%w: %s", strconv.ErrSyntax, path)
+		}
+	}
 	return strconv.ParseUint(strings.TrimSpace(string(data)), 10, 64)
 }