implement lazyregexp package

Based on the "lazyregexp" package in golang.org/x/mod;
https://cs.opensource.google/go/x/mod/+/refs/tags/v0.19.0:internal/lazyregexp/lazyre.go;l=66-78

This package allows defining regular expressions that should not be
compiled until used, but still providing validation to prevent
invalid regular expressions from producing a panic at runtime.

The lazyregexp package provides a subset of the methods provided
by "regexp" and only implements the methods used in the codebase.
Additional methods can be added when needed.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

internal/lazyregexp/lazyregexp.go

@@ -0,0 +1,86 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+// Copyright 2018 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+// Code below was largely copied from golang.org/x/mod@v0.22;
+// https://github.com/golang/mod/blob/v0.22.0/internal/lazyregexp/lazyre.go
+// with some additional methods added.
+
+// Package lazyregexp is a thin wrapper over regexp, allowing the use of global
+// regexp variables without forcing them to be compiled at init.
+package lazyregexp
+
+import (
+	"os"
+	"regexp"
+	"strings"
+	"sync"
+)
+
+// Regexp is a wrapper around [regexp.Regexp], where the underlying regexp will be
+// compiled the first time it is needed.
+type Regexp struct {
+	str  string
+	once sync.Once
+	rx   *regexp.Regexp
+}
+
+func (re *Regexp) re() *regexp.Regexp {
+	re.once.Do(re.build)
+	return re.rx
+}
+
+func (re *Regexp) build() {
+	re.rx = regexp.MustCompile(re.str)
+	re.str = ""
+}
+
+func (re *Regexp) FindStringIndex(s string) (loc []int) {
+	return re.re().FindStringIndex(s)
+}
+
+func (re *Regexp) FindStringSubmatch(s string) []string {
+	return re.re().FindStringSubmatch(s)
+}
+
+func (re *Regexp) MatchString(s string) bool {
+	return re.re().MatchString(s)
+}
+
+func (re *Regexp) ReplaceAll(src, repl []byte) []byte {
+	return re.re().ReplaceAll(src, repl)
+}
+
+func (re *Regexp) String() string {
+	return re.re().String()
+}
+
+var inTest = len(os.Args) > 0 && strings.HasSuffix(strings.TrimSuffix(os.Args[0], ".exe"), ".test")
+
+// New creates a new lazy regexp, delaying the compiling work until it is first
+// needed. If the code is being run as part of tests, the regexp compiling will
+// happen immediately.
+func New(str string) *Regexp {
+	lr := &Regexp{str: str}
+	if inTest {
+		// In tests, always compile the regexps early.
+		lr.re()
+	}
+	return lr
+}

internal/lazyregexp/lazyregexp_test.go

@@ -0,0 +1,39 @@
+/*
+   Copyright The containerd Authors.
+
+   Licensed under the Apache License, Version 2.0 (the "License");
+   you may not use this file except in compliance with the License.
+   You may obtain a copy of the License at
+
+       http://www.apache.org/licenses/LICENSE-2.0
+
+   Unless required by applicable law or agreed to in writing, software
+   distributed under the License is distributed on an "AS IS" BASIS,
+   WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+   See the License for the specific language governing permissions and
+   limitations under the License.
+*/
+
+package lazyregexp
+
+import (
+	"testing"
+)
+
+func TestCompileOnce(t *testing.T) {
+	t.Run("invalid regexp", func(t *testing.T) {
+		defer func() {
+			if r := recover(); r == nil {
+				t.Errorf("expected a panic")
+			}
+		}()
+		_ = New("[")
+	})
+	t.Run("valid regexp", func(t *testing.T) {
+		re := New("[a-z]")
+		ok := re.MatchString("hello")
+		if !ok {
+			t.Errorf("expected a match")
+		}
+	})
+}