exp/cookiejar: store cookies under TLD+1 on nil public suffix list

The current implementation would store all cookies received from
any .com domain under "com" in the entries map if a nil public
suffix list is used in constructing the Jar. This is inefficient.

This CL uses the TLD+1 of the domain if the public suffix list
is nil which has two advantages:
 - It uses the entries map efficiently.
 - It prevents a host foo.com to set cookies for bar.com.
   (It may set the cookie, but it won't be returned to bar.com.)
A domain like www.british-library.uk may still set a domain
cookie for .british-library.uk in this case.

The behavior for a non-nil public suffix list is unchanged, cookies
are stored under eTLD+1 in this case.

R=nigeltao
CC=golang-dev
https://golang.org/cl/7312105

src/pkg/exp/cookiejar/jar.go

@@ -48,8 +48,8 @@ type Options struct {
 	// an HTTP server can set a cookie for a domain.
 	//
 	// A nil value is valid and may be useful for testing but it is not
-	// secure: it means that the HTTP server for foo.com can set a cookie
-	// for bar.com.
+	// secure: it means that the HTTP server for foo.co.uk can set a cookie
+	// for bar.co.uk.
 	PublicSuffixList PublicSuffixList
 }
 
@@ -333,20 +333,25 @@ func jarKey(host string, psl PublicSuffixList) string {
 	if isIP(host) {
 		return host
 	}
+
+	var i int
 	if psl == nil {
-		// Key cookies under TLD of host.
-		return host[1+strings.LastIndex(host, "."):]
+		i = strings.LastIndex(host, ".")
+		if i == -1 {
+			return host
 		}
+	} else {
 		suffix := psl.PublicSuffix(host)
 		if suffix == host {
 			return host
 		}
-	i := len(host) - len(suffix)
+		i = len(host) - len(suffix)
 		if i <= 0 || host[i-1] != '.' {
 			// The provided public suffix list psl is broken.
 			// Storing cookies under host is a safe stopgap.
 			return host
 		}
+	}
 	prevDot := strings.LastIndex(host[:i-1], ".")
 	return host[prevDot+1:]
 }

src/pkg/exp/cookiejar/jar_test.go

@@ -99,10 +99,25 @@ func TestJarKey(t *testing.T) {
 			t.Errorf("%q: got %q, want %q", host, got, want)
 		}
 	}
+}
 
-	for _, host := range []string{"www.example.com", "example.com", "com"} {
-		if got := jarKey(host, nil); got != "com" {
-			t.Errorf(`%q: got %q, want "com"`, host, got)
+var jarKeyNilPSLTests = map[string]string{
+	"foo.www.example.com": "example.com",
+	"www.example.com":     "example.com",
+	"example.com":         "example.com",
+	"com":                 "com",
+	"foo.www.bbc.co.uk":   "co.uk",
+	"www.bbc.co.uk":       "co.uk",
+	"bbc.co.uk":           "co.uk",
+	"co.uk":               "co.uk",
+	"uk":                  "uk",
+	"192.168.0.5":         "192.168.0.5",
+}
+
+func TestJarKeyNilPSL(t *testing.T) {
+	for host, want := range jarKeyNilPSLTests {
+		if got := jarKey(host, nil); got != want {
+			t.Errorf("%q: got %q, want %q", host, got, want)
 		}
 	}
 }