fs: error if cpuacct.usage_all format differs

We can reasonably expect the first three fields of cpuacct.usage_all to
be "cpu user system", and if we see something different, it doesn't make
sense to continue. So do check that the header is as expected.

Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

fs/cpuacct.go

@@ -2,6 +2,7 @@ package fs
 
 import (
 	"bufio"
+	"fmt"
 	"os"
 	"strconv"
 	"strings"
@@ -129,7 +130,15 @@ func getPercpuUsageInModes(path string) ([]uint64, []uint64, error) {
 	defer fd.Close()
 
 	scanner := bufio.NewScanner(fd)
-	scanner.Scan() // skipping header line
+	scanner.Scan() // Read header line.
+	const want = "cpu user system"
+	if !strings.HasPrefix(scanner.Text(), want) {
+		return nil, nil, &parseError{
+			Path: path,
+			File: file,
+			Err:  fmt.Errorf("bad header (want %q, got %q)", want, scanner.Text()),
+		}
+	}
 
 	for scanner.Scan() {
 		// Each line is: cpu user system