Use sync.OnceValue only on go1.21+

sync.OnceValue was introduced in go1.21. Currently validator's go.mod
states minimum go version to be go1.18.

This commit moves the lazy regex initialization into its own file
lazy.go and provides backwards compatibility using buildtags and the
file lazy_compat.go which contains a backported (non-generic) version of
sync.OnceValue.

Signed-off-by: Kimmo Lehto <klehto@mirantis.com>

lazy.go

@@ -0,0 +1,14 @@
+//go:build go1.21
+
+package validator
+
+import (
+	"regexp"
+	"sync"
+)
+
+func lazyRegexCompile(str string) func() *regexp.Regexp {
+	return sync.OnceValue(func() *regexp.Regexp {
+		return regexp.MustCompile(str)
+	})
+}

lazy_compat.go

@@ -0,0 +1,46 @@
+//go:build !go1.21
+
+package validator
+
+import (
+	"regexp"
+	"sync"
+)
+
+// Copied and adapted from go1.21 stdlib's sync.OnceValue for backwards compatibility:
+// OnceValue returns a function that invokes f only once and returns the value
+// returned by f. The returned function may be called concurrently.
+//
+// If f panics, the returned function will panic with the same value on every call.
+func onceValue(f func() *regexp.Regexp) func() *regexp.Regexp {
+	var (
+		once   sync.Once
+		valid  bool
+		p      any
+		result *regexp.Regexp
+	)
+	g := func() {
+		defer func() {
+			p = recover()
+			if !valid {
+				panic(p)
+			}
+		}()
+		result = f()
+		f = nil
+		valid = true
+	}
+	return func() *regexp.Regexp {
+		once.Do(g)
+		if !valid {
+			panic(p)
+		}
+		return result
+	}
+}
+
+func lazyRegexCompile(str string) func() *regexp.Regexp {
+	return onceValue(func() *regexp.Regexp {
+		return regexp.MustCompile(str)
+	})
+}

regexes.go

@@ -1,10 +1,5 @@
 package validator
 
-import (
-	"regexp"
-	"sync"
-)
-
 const (
 	alphaRegexString                 = "^[a-zA-Z]+$"
 	alphaNumericRegexString          = "^[a-zA-Z0-9]+$"
@@ -79,12 +74,6 @@ const (
 	spicedbTypeRegexString           = "^([a-z][a-z0-9_]{1,61}[a-z0-9]/)?[a-z][a-z0-9_]{1,62}[a-z0-9]$"
 )
 
-func lazyRegexCompile(str string) func() *regexp.Regexp {
-	return sync.OnceValue(func() *regexp.Regexp {
-		return regexp.MustCompile(str)
-	})
-}
-
 var (
 	alphaRegex                 = lazyRegexCompile(alphaRegexString)
 	alphaNumericRegex          = lazyRegexCompile(alphaNumericRegexString)