Improve domain validation function

Now FQDNs are accepted as well (with trailing dot).
Empty strings are not considered valid.
Label length for IDNs is checked using Punycode form as it should.

See #554

framework/address/validation.go

@@ -20,6 +20,8 @@ package address
 
 import (
 	"strings"
+
+	"golang.org/x/net/idna"
 )
 
 /*
@@ -109,17 +111,23 @@ func ValidMailboxName(mbox string) bool {
 
 // ValidDomain checks whether the specified string is a valid DNS domain.
 func ValidDomain(domain string) bool {
-	if len(domain) > 255 {
+	if len(domain) > 255 || len(domain) == 0 {
 		return false
 	}
-	if strings.HasPrefix(domain, ".") || strings.HasSuffix(domain, ".") {
+	if strings.HasPrefix(domain, ".") {
 		return false
 	}
 	if strings.Contains(domain, "..") {
 		return false
 	}
 
-	labels := strings.Split(domain, ".")
+	// Length checks are to be applied to A-labels form.
+	// maddy uses U-labels representation across the code (for lookups, etc).
+	domainASCII, err := idna.ToASCII(domain)
+	if err != nil {
+		return false
+	}
+	labels := strings.Split(domainASCII, ".")
 	for _, label := range labels {
 		if len(label) > 64 {
 			return false

framework/address/validation_test.go

@@ -1,6 +1,7 @@
 package address_test
 
 import (
+	"strings"
 	"testing"
 
 	"github.com/foxcpp/maddy/framework/address"
@@ -11,3 +12,22 @@ func TestValidMailboxName(t *testing.T) {
 		t.Error("caddy.bug should be valid mailbox name")
 	}
 }
+
+func TestValidDomain(t *testing.T) {
+	for _, c := range []struct {
+		Domain string
+		Valid  bool
+	}{
+		{Domain: "maddy.email", Valid: true},
+		{Domain: "", Valid: false},
+		{Domain: "maddy.email.", Valid: true},
+		{Domain: "..", Valid: false},
+		{Domain: strings.Repeat("a", 256), Valid: false},
+		{Domain: "äõäoaõoäaõaäõaoäaoaäõoaäooaoaoiuaiauäõiuüõaõäiauõaaa.tld", Valid: true},            // https://github.com/foxcpp/maddy/issues/554
+		{Domain: "xn--oaoaaaoaoaoaooaoaoiuaiauiuaiauaaa-f1cadccdcmd01eddchqcbe07a.tld", Valid: true}, // https://github.com/foxcpp/maddy/issues/554
+	} {
+		if actual := address.ValidDomain(c.Domain); actual != c.Valid {
+			t.Errorf("expected domain %v to be valid=%v, but got %v", c.Domain, c.Valid, actual)
+		}
+	}
+}