exp/cookiejar: eliminate some "."+str garbage.

It's not a big deal, but it's easy to fix.

R=dsymonds
CC=dr.volker.dobler, golang-dev
https://golang.org/cl/7425043

src/pkg/exp/cookiejar/jar.go

@@ -121,7 +121,7 @@ func (e *entry) domainMatch(host string) bool {
 	if e.Domain == host {
 		return true
 	}
-	return !e.HostOnly && strings.HasSuffix(host, "."+e.Domain)
+	return !e.HostOnly && hasDotSuffix(host, e.Domain)
 }
 
 // pathMatch implements "path-match" according to RFC 6265 section 5.1.4.
@@ -139,6 +139,11 @@ func (e *entry) pathMatch(requestPath string) bool {
 	return false
 }
 
+// hasDotSuffix returns whether s ends in "."+suffix.
+func hasDotSuffix(s, suffix string) bool {
+	return len(s) > len(suffix) && s[len(s)-len(suffix)-1] == '.' && s[len(s)-len(suffix):] == suffix
+}
+
 // byPathLength is a []entry sort.Interface that sorts according to RFC 6265
 // section 5.4 point 2: by longest path and then by earliest creation time.
 type byPathLength []entry
@@ -469,7 +474,7 @@ func (j *Jar) domainAndType(host, domain string) (string, bool, error) {
 
 	// See RFC 6265 section 5.3 #5.
 	if j.psList != nil {
-		if ps := j.psList.PublicSuffix(domain); ps != "" && !strings.HasSuffix(domain, "."+ps) {
+		if ps := j.psList.PublicSuffix(domain); ps != "" && !hasDotSuffix(domain, ps) {
 			if host == domain {
 				// This is the one exception in which a cookie
 				// with a domain attribute is a host cookie.
@@ -481,7 +486,7 @@ func (j *Jar) domainAndType(host, domain string) (string, bool, error) {
 
 	// The domain must domain-match host: www.mycompany.com cannot
 	// set cookies for .ourcompetitors.com.
-	if host != domain && !strings.HasSuffix(host, "."+domain) {
+	if host != domain && !hasDotSuffix(host, domain) {
 		return "", false, errIllegalDomain
 	}
 

src/pkg/exp/cookiejar/jar_test.go

@@ -40,6 +40,80 @@ func newTestJar() *Jar {
 	return jar
 }
 
+var hasDotSuffixTests = [...]struct {
+	s, suffix string
+}{
+	{"", ""},
+	{"", "."},
+	{"", "x"},
+	{".", ""},
+	{".", "."},
+	{".", ".."},
+	{".", "x"},
+	{".", "x."},
+	{".", ".x"},
+	{".", ".x."},
+	{"x", ""},
+	{"x", "."},
+	{"x", ".."},
+	{"x", "x"},
+	{"x", "x."},
+	{"x", ".x"},
+	{"x", ".x."},
+	{".x", ""},
+	{".x", "."},
+	{".x", ".."},
+	{".x", "x"},
+	{".x", "x."},
+	{".x", ".x"},
+	{".x", ".x."},
+	{"x.", ""},
+	{"x.", "."},
+	{"x.", ".."},
+	{"x.", "x"},
+	{"x.", "x."},
+	{"x.", ".x"},
+	{"x.", ".x."},
+	{"com", ""},
+	{"com", "m"},
+	{"com", "om"},
+	{"com", "com"},
+	{"com", ".com"},
+	{"com", "x.com"},
+	{"com", "xcom"},
+	{"com", "xorg"},
+	{"com", "org"},
+	{"com", "rg"},
+	{"foo.com", ""},
+	{"foo.com", "m"},
+	{"foo.com", "om"},
+	{"foo.com", "com"},
+	{"foo.com", ".com"},
+	{"foo.com", "o.com"},
+	{"foo.com", "oo.com"},
+	{"foo.com", "foo.com"},
+	{"foo.com", ".foo.com"},
+	{"foo.com", "x.foo.com"},
+	{"foo.com", "xfoo.com"},
+	{"foo.com", "xfoo.org"},
+	{"foo.com", "foo.org"},
+	{"foo.com", "oo.org"},
+	{"foo.com", "o.org"},
+	{"foo.com", ".org"},
+	{"foo.com", "org"},
+	{"foo.com", "rg"},
+}
+
+func TestHasDotSuffix(t *testing.T) {
+	for _, tc := range hasDotSuffixTests {
+		got := hasDotSuffix(tc.s, tc.suffix)
+		want := strings.HasSuffix(tc.s, "."+tc.suffix)
+		if got != want {
+			t.Errorf("s=%q, suffix=%q: got %v, want %v", tc.s, tc.suffix, got, want)
+		}
+	}
+}
+
 var canonicalHostTests = map[string]string{
 	"www.example.com":         "www.example.com",
 	"WWW.EXAMPLE.COM":         "www.example.com",