idna: *labelIter.next() to skip last label only if it is empty and verifyDNSLength is false.

The existing implementation always skips the empty last label regardless of what verifyDNSLength is set to, which causes pure-ASCII domains ending with a single empty root label to be wrongly accepted when verifyDNSLength is true.

This behavior is described in the Unicode Technical Standard 46 at https://unicode.org/reports/tr46/\#ToASCII

Fixes golang/go#47182

Author: elliotwutingfeng

internal/export/idna/idna10.0.0.go

@@ -351,7 +351,7 @@ func (p *Profile) process(s string, toASCII bool) (string, error) {
 	if err == nil && p.verifyDNSLength && s == "" {
 		err = &labelError{s, "A4"}
 	}
-	labels := labelIter{orig: s}
+	labels := labelIter{orig: s, verifyDNSLength: p.verifyDNSLength}
 	for ; !labels.done(); labels.next() {
 		label := labels.label()
 		if label == "" {
@@ -538,11 +538,12 @@ func validateAndMap(p *Profile, s string) (vm string, bidi bool, err error) {
 
 // A labelIter allows iterating over domain name labels.
 type labelIter struct {
-	orig     string
-	slice    []string
-	curStart int
-	curEnd   int
-	i        int
+	orig            string
+	slice           []string
+	curStart        int
+	curEnd          int
+	i               int
+	verifyDNSLength bool
 }
 
 func (l *labelIter) reset() {
@@ -574,7 +575,8 @@ func (l *labelIter) label() string {
 	return l.orig[l.curStart:l.curEnd]
 }
 
-// next sets the value to the next label. It skips the last label if it is empty.
+// next sets the value to the next label. It skips the last label if it is empty
+// and l.verifyDNSLength is false.
 func (l *labelIter) next() {
 	l.i++
 	if l.slice != nil {
@@ -583,7 +585,7 @@ func (l *labelIter) next() {
 		}
 	} else {
 		l.curStart = l.curEnd + 1
-		if l.curStart == len(l.orig)-1 && l.orig[l.curStart] == '.' {
+		if !l.verifyDNSLength && l.curStart == len(l.orig)-1 && l.orig[l.curStart] == '.' {
 			l.curStart = len(l.orig)
 		}
 	}

internal/export/idna/idna10.0.0_test.go

@@ -67,7 +67,8 @@ func TestLabelErrors(t *testing.T) {
 		{lengthA, ".b", ".b", "A4"},
 		{lengthA, "\u3002b", ".b", "A4"},
 		{lengthA, "..b", "..b", "A4"},
-		{lengthA, "b..", "b..", ""},
+		{lengthA, "b..", "b..", "A4"},
+		{lengthA, "ƀ..", "xn--lha..", "A4"},
 
 		// Sharpened Bidi rules for Unicode 10.0.0. Apply for ALL labels in ANY
 		// of the labels is RTL.
@@ -81,6 +82,7 @@ func TestLabelErrors(t *testing.T) {
 		{resolve, "\u3002b", ".b", ""},
 		{resolve, "..b", "..b", ""},
 		{resolve, "b..", "b..", ""},
+		{resolve, "ƀ..", "xn--lha..", ""},
 		{resolve, "\xed", "", "P1"},
 
 		// Raw punycode

internal/export/idna/idna9.0.0.go

@@ -351,7 +351,7 @@ func (p *Profile) process(s string, toASCII bool) (string, error) {
 	if err == nil && p.verifyDNSLength && s == "" {
 		err = &labelError{s, "A4"}
 	}
-	labels := labelIter{orig: s}
+	labels := labelIter{orig: s, verifyDNSLength: p.verifyDNSLength}
 	for ; !labels.done(); labels.next() {
 		label := labels.label()
 		if label == "" {
@@ -500,11 +500,12 @@ func validateAndMap(p *Profile, s string) (string, error) {
 
 // A labelIter allows iterating over domain name labels.
 type labelIter struct {
-	orig     string
-	slice    []string
-	curStart int
-	curEnd   int
-	i        int
+	orig            string
+	slice           []string
+	curStart        int
+	curEnd          int
+	i               int
+	verifyDNSLength bool
 }
 
 func (l *labelIter) reset() {
@@ -536,7 +537,8 @@ func (l *labelIter) label() string {
 	return l.orig[l.curStart:l.curEnd]
 }
 
-// next sets the value to the next label. It skips the last label if it is empty.
+// next sets the value to the next label. It skips the last label if it is empty
+// and l.verifyDNSLength is false.
 func (l *labelIter) next() {
 	l.i++
 	if l.slice != nil {
@@ -545,7 +547,7 @@ func (l *labelIter) next() {
 		}
 	} else {
 		l.curStart = l.curEnd + 1
-		if l.curStart == len(l.orig)-1 && l.orig[l.curStart] == '.' {
+		if !l.verifyDNSLength && l.curStart == len(l.orig)-1 && l.orig[l.curStart] == '.' {
 			l.curStart = len(l.orig)
 		}
 	}

internal/export/idna/idna9.0.0_test.go

@@ -71,13 +71,15 @@ func TestLabelErrors(t *testing.T) {
 		{lengthA, ".b", "b", ""},
 		{lengthA, "\u3002b", "b", ""},
 		{lengthA, "..b", "b", ""},
-		{lengthA, "b..", "b..", ""},
+		{lengthA, "b..", "b..", "A4"},
+		{lengthA, "ƀ..", "xn--lha..", "A4"},
 
 		{resolve, "a..b", "a..b", ""},
 		{resolve, ".b", "b", ""},
 		{resolve, "\u3002b", "b", ""},
 		{resolve, "..b", "b", ""},
 		{resolve, "b..", "b..", ""},
+		{resolve, "ƀ..", "xn--lha..", ""},
 		{resolve, "\xed", "", "P1"},
 
 		// Raw punycode