libct/spec: replace isValidName regex with a function

Also, add a simple test and a benchmark (just out of sheer curiosity).

Benchmark results:

name           old time/op  new time/op  delta
IsValidName-4   540ns ± 3%    45ns ± 1%  -91.76%  (p=0.008 n=5+5)

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

libcontainer/specconv/spec_linux.go

@@ -7,7 +7,6 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"regexp"
 	"strings"
 	"sync"
 	"time"
@@ -411,8 +410,21 @@ func createLibcontainerMount(cwd string, m specs.Mount) (*configs.Mount, error)
 	return mnt, nil
 }
 
-// systemd property name check: latin letters only, at least 3 of them
-var isValidName = regexp.MustCompile(`^[a-zA-Z]{3,}$`).MatchString
+// isValidName checks if systemd property name is valid. It should consists
+// of latin letters only, and have least 3 of them.
+func isValidName(s string) bool {
+	if len(s) < 3 {
+		return false
+	}
+	// Check ASCII characters rather than Unicode runes.
+	for _, ch := range s {
+		if (ch >= 'A' && ch <= 'Z') || (ch >= 'a' && ch <= 'z') {
+			continue
+		}
+		return false
+	}
+	return true
+}
 
 // Some systemd properties are documented as having "Sec" suffix
 // (e.g. TimeoutStopSec) but are expected to have "USec" suffix

libcontainer/specconv/spec_linux_test.go

@@ -762,6 +762,39 @@ func TestInitSystemdProps(t *testing.T) {
 	}
 }
 
+func TestIsValidName(t *testing.T) {
+	testCases := []struct {
+		in    string
+		valid bool
+	}{
+		{"", false},   // too short
+		{"xx", false}, // too short
+		{"xxx", true},
+		{"someValidName", true},
+		{"A name", false},  // space
+		{"3335", false},    // numbers
+		{"Name1", false},   // numbers
+		{"Кир", false},     // non-ascii
+		{"მადლობა", false}, // non-ascii
+		{"合い言葉", false},    // non-ascii
+	}
+
+	for _, tc := range testCases {
+		valid := isValidName(tc.in)
+		if valid != tc.valid {
+			t.Errorf("case %q: expected %v, got %v", tc.in, tc.valid, valid)
+		}
+	}
+}
+
+func BenchmarkIsValidName(b *testing.B) {
+	for i := 0; i < b.N; i++ {
+		for _, s := range []string{"", "xx", "xxx", "someValidName", "A name", "Кир", "მადლობა", "合い言葉"} {
+			_ = isValidName(s)
+		}
+	}
+}
+
 func TestNullProcess(t *testing.T) {
 	spec := Example()
 	spec.Process = nil