net/http: add Cookie.Valid method

The (*http.Cookie).String method used by SetCookie will silently discard
or sanitize any fields it deems invalid, making it difficult to tell
whether a cookie will be sent as expected.

This change introduces a new (*http.Cookie).Valid method which may be
used to check if any cookie fields will be discarded or sanitized prior
to calling (*http.Cookie).String.

Fixes #46370

Change-Id: I2db80078de190d267a9c675a9717c8be8acc8704
Reviewed-on: https://go-review.googlesource.com/c/go/+/338590
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Trust: Cherry Mui <cherryyz@google.com>
Trust: Damien Neil <dneil@google.com>
Reviewed-by: Damien Neil <dneil@google.com>

api/next.txt

@@ -142,3 +142,4 @@ pkg testing, type FuzzResult struct, T time.Duration
 pkg testing, type InternalFuzzTarget struct
 pkg testing, type InternalFuzzTarget struct, Fn func(*F)
 pkg testing, type InternalFuzzTarget struct, Name string
+pkg net/http, method (*Cookie) Valid() error

src/net/http/cookie.go

@@ -5,6 +5,8 @@
 package http
 
 import (
+	"errors"
+	"fmt"
 	"log"
 	"net"
 	"net/http/internal/ascii"
@@ -236,6 +238,37 @@ func (c *Cookie) String() string {
 	return b.String()
 }
 
+// Valid reports whether the cookie is valid.
+func (c *Cookie) Valid() error {
+	if c == nil {
+		return errors.New("http: nil Cookie")
+	}
+	if !isCookieNameValid(c.Name) {
+		return errors.New("http: invalid Cookie.Name")
+	}
+	if !validCookieExpires(c.Expires) {
+		return errors.New("http: invalid Cookie.Expires")
+	}
+	for i := 0; i < len(c.Value); i++ {
+		if !validCookieValueByte(c.Value[i]) {
+			return fmt.Errorf("http: invalid byte %q in Cookie.Value", c.Value[i])
+		}
+	}
+	if len(c.Path) > 0 {
+		for i := 0; i < len(c.Path); i++ {
+			if !validCookiePathByte(c.Path[i]) {
+				return fmt.Errorf("http: invalid byte %q in Cookie.Path", c.Path[i])
+			}
+		}
+	}
+	if len(c.Domain) > 0 {
+		if !validCookieDomain(c.Domain) {
+			return errors.New("http: invalid Cookie.Domain")
+		}
+	}
+	return nil
+}
+
 // readCookies parses all "Cookie" values from the header h and
 // returns the successfully parsed Cookies.
 //

src/net/http/cookie_test.go

@@ -529,6 +529,31 @@ func TestCookieSanitizePath(t *testing.T) {
 	}
 }
 
+func TestCookieValid(t *testing.T) {
+	tests := []struct {
+		cookie *Cookie
+		valid  bool
+	}{
+		{nil, false},
+		{&Cookie{Name: ""}, false},
+		{&Cookie{Name: "invalid-expires"}, false},
+		{&Cookie{Name: "invalid-value", Value: "foo\"bar"}, false},
+		{&Cookie{Name: "invalid-path", Path: "/foo;bar/"}, false},
+		{&Cookie{Name: "invalid-domain", Domain: "example.com:80"}, false},
+		{&Cookie{Name: "valid", Value: "foo", Path: "/bar", Domain: "example.com", Expires: time.Unix(0, 0)}, true},
+	}
+
+	for _, tt := range tests {
+		err := tt.cookie.Valid()
+		if err != nil && tt.valid {
+			t.Errorf("%#v.Valid() returned error %v; want nil", tt.cookie, err)
+		}
+		if err == nil && !tt.valid {
+			t.Errorf("%#v.Valid() returned nil; want error", tt.cookie)
+		}
+	}
+}
+
 func BenchmarkCookieString(b *testing.B) {
 	const wantCookieString = `cookie-9=i3e01nf61b6t23bvfmplnanol3; Path=/restricted/; Domain=example.com; Expires=Tue, 10 Nov 2009 23:00:00 GMT; Max-Age=3600`
 	c := &Cookie{