libct/cg/devices: stop using regex

Looking into data generated by setting

	GODEBUG="inittrace=1"

I have noticed this line:

init github.com/opencontainers/runc/libcontainer/cgroups/devices @1.2 ms, 0.020 ms clock, 10512 bytes, 133 allocs

This is the leader for both bytes and allocs among the packages from
this repo, and all of it is caused by a single regex:

> var devicesListRegexp = regexp.MustCompile(`^([abc])\s+(\d+|\*):(\d+|\*)\s+([rwm]+)$`)

It seems that the same parsing can be done without relying on
a regular expression, no decrease in readability, and 2x faster
(according to the benchmark added), and also makes runc start
slightly faster and leaner.

Before:
    BenchmarkParseLine-4   	  176240	      6768 ns/op	    6576 B/op	      64 allocs/op

After:
    BenchmarkParseLine-4   	  322441	      3535 ns/op	    5520 B/op	      53 allocs/op

[v2: single split with SplitFunc; fix a typo in error message]
[v3: rebase after 3159 merge; re-ran benchmarks (results are similar)]

Co-authored-by: Sebastiaan van Stijn <[email protected]>
Signed-off-by: Kir Kolyshkin <[email protected]>

Author: kolyshkin

libcontainer/cgroups/devices/devices_emulator.go

@@ -22,12 +22,11 @@ package devices
 
 import (
 	"bufio"
-	"errors"
 	"fmt"
 	"io"
-	"regexp"
 	"sort"
 	"strconv"
+	"strings"
 
 	"github.com/opencontainers/runc/libcontainer/devices"
 )
@@ -79,19 +78,21 @@ func (e *Emulator) IsAllowAll() bool {
 	return e.IsBlacklist() && len(e.rules) == 0
 }
 
-var devicesListRegexp = regexp.MustCompile(`^([abc])\s+(\d+|\*):(\d+|\*)\s+([rwm]+)$`)
-
 func parseLine(line string) (*deviceRule, error) {
-	matches := devicesListRegexp.FindStringSubmatch(line)
-	if matches == nil {
-		return nil, errors.New("line doesn't match devices.list format")
+	// Input: node major:minor perms.
+	fields := strings.FieldsFunc(line, func(r rune) bool {
+		return r == ' ' || r == ':'
+	})
+	if len(fields) != 4 {
+		return nil, fmt.Errorf("malformed devices.list rule %s", line)
 	}
+
 	var (
 		rule  deviceRule
-		node  = devices.Type(matches[1])
-		major = matches[2]
-		minor = matches[3]
-		perms = matches[4]
+		node  = devices.Type(fields[0])
+		major = fields[1]
+		minor = fields[2]
+		perms = fields[3]
 	)
 
 	// Parse the node type.
@@ -105,7 +106,6 @@ func parseLine(line string) (*deviceRule, error) {
 	case devices.BlockDevice, devices.CharDevice:
 		rule.meta.node = node
 	default:
-		// Should never happen!
 		return nil, fmt.Errorf("unknown device type %q", node)
 	}
 
@@ -134,7 +134,6 @@ func parseLine(line string) (*deviceRule, error) {
 	// Parse the access permissions.
 	rule.perms = devices.Permissions(perms)
 	if !rule.perms.IsValid() || rule.perms.IsEmpty() {
-		// Should never happen!
 		return nil, fmt.Errorf("parse access mode: contained unknown modes or is empty: %q", perms)
 	}
 

libcontainer/cgroups/devices/devices_emulator_test.go

@@ -19,8 +19,10 @@
 package devices
 
 import (
+	"bufio"
 	"bytes"
 	"reflect"
+	"strings"
 	"testing"
 
 	"github.com/opencontainers/runc/libcontainer/devices"
@@ -1112,3 +1114,31 @@ func TestDeviceEmulatorTransitionFromBlacklist(t *testing.T) {
 func TestDeviceEmulatorTransitionFromWhitelist(t *testing.T) {
 	testDeviceEmulatorTransition(t, false)
 }
+
+func BenchmarkParseLine(b *testing.B) {
+	list := `c *:* m
+b *:* m
+c 1:3 rwm
+c 1:5 rwm
+c 1:7 rwm
+c 1:8 rwm
+c 1:9 rwm
+c 5:0 rwm
+c 5:2 rwm
+c 136:* rwm
+c 10:200 rwm`
+
+	var r *deviceRule
+	var err error
+	for i := 0; i < b.N; i++ {
+		s := bufio.NewScanner(strings.NewReader(list))
+		for s.Scan() {
+			line := s.Text()
+			r, err = parseLine(line)
+		}
+		if err := s.Err(); err != nil {
+			b.Fatal(err)
+		}
+	}
+	b.Logf("rule: %v, err: %v", r, err)
+}