net/http: fix cookie Expires minimum year to 1601 instead of Epoch year 1970

Following RFC 6265 Section 5.1.1.5, ensure that the minimum
year for which an Expires value is valid and can be included in
the cookie's string, is 1601 instead of the Epoch year 1970.

A detailed specification for parsing the Expiry field is at:
https://tools.ietf.org/html/rfc6265#section-5.2.1

I stumbled across this bug due to this StackOverflow answer
that recommends setting the Expiry to the Epoch:
http://stackoverflow.com/a/5285982

Fixes #17632

Change-Id: I3c1bdf821d369320334a5dc1e4bf22783cbfe9fc
Reviewed-on: https://go-review.googlesource.com/32142Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/net/http/cookie.go

@@ -168,7 +168,7 @@ func (c *Cookie) String() string {
 			log.Printf("net/http: invalid Cookie.Domain %q; dropping domain attribute", c.Domain)
 		}
 	}
-	if c.Expires.Unix() > 0 {
+	if validCookieExpires(c.Expires) {
 		b.WriteString("; Expires=")
 		b2 := b.Bytes()
 		b.Reset()
@@ -246,6 +246,12 @@ func validCookieDomain(v string) bool {
 	return false
 }
 
+// validCookieExpires returns whether v is a valid cookie expires-value.
+func validCookieExpires(t time.Time) bool {
+	// IETF RFC 6265 Section 5.1.1.5, the year must not be less than 1601
+	return t.Year() >= 1601
+}
+
 // isCookieDomainName returns whether s is a valid domain name or a valid
 // domain name with a leading dot '.'.  It is almost a direct copy of
 // package net's isDomainName.

src/net/http/cookie_test.go

@@ -56,6 +56,15 @@ var writeSetCookiesTests = []struct {
 		&Cookie{Name: "cookie-9", Value: "expiring", Expires: time.Unix(1257894000, 0)},
 		"cookie-9=expiring; Expires=Tue, 10 Nov 2009 23:00:00 GMT",
 	},
+	// According to IETF 6265 Section 5.1.1.5, the year cannot be less than 1601
+	{
+		&Cookie{Name: "cookie-10", Value: "expiring-1601", Expires: time.Date(1601, 1, 1, 1, 1, 1, 1, time.UTC)},
+		"cookie-10=expiring-1601; Expires=Mon, 01 Jan 1601 01:01:01 GMT",
+	},
+	{
+		&Cookie{Name: "cookie-11", Value: "invalid-expiry", Expires: time.Date(1600, 1, 1, 1, 1, 1, 1, time.UTC)},
+		"cookie-11=invalid-expiry",
+	},
 	// The "special" cookies have values containing commas or spaces which
 	// are disallowed by RFC 6265 but are common in the wild.
 	{