reverseproxy: Implement cookie hash selection policy (#3809)

* add CookieHashSelection for session affinity

* add CookieHashSelection for session affinity

* register module

* reverse_proxy: Add and fix cookie lb_policy

* reverse_proxy: Manage hmac.write error on cookie hash selection

* reverse_proxy: fix some comments

* reverse_proxy: variable `cookieValue` is inside the else block

* reverse_proxy: Abstract duplicate nuanced logic of reservoir sampling into a function

* reverse_proxy: Set a default secret is indeed useless

* reverse_proxy: add configuration syntax for cookie lb_policy

* reverse_proxy: doc typo and improvement

Co-authored-by: utick <123liuqingdong@163.com>

3 files changed, 208 insertions, 63 deletions

diff --git a/modules/caddyhttp/reverseproxy/reverseproxy.go b/modules/caddyhttp/reverseproxy/reverseproxy.go
index 305a0b3..7fc61ae 100644
--- a/modules/caddyhttp/reverseproxy/reverseproxy.go
+++ b/modules/caddyhttp/reverseproxy/reverseproxy.go
@@ -368,7 +368,7 @@ func (h *Handler) ServeHTTP(w http.ResponseWriter, r *http.Request, next caddyht
 	var proxyErr error
 	for {
 		// choose an available upstream
-		upstream := h.LoadBalancing.SelectionPolicy.Select(h.Upstreams, r)
+		upstream := h.LoadBalancing.SelectionPolicy.Select(h.Upstreams, r, w)
 		if upstream == nil {
 			if proxyErr == nil {
 				proxyErr = fmt.Errorf("no upstreams available")
@@ -816,7 +816,7 @@ type LoadBalancing struct {
 
 // Selector selects an available upstream from the pool.
 type Selector interface {
-	Select(UpstreamPool, *http.Request) *Upstream
+	Select(UpstreamPool, *http.Request, http.ResponseWriter) *Upstream
 }
 
 // Hop-by-hop headers. These are removed when sent to the backend.
diff --git a/modules/caddyhttp/reverseproxy/selectionpolicies.go b/modules/caddyhttp/reverseproxy/selectionpolicies.go
index 2aef63d..a1010f4 100644
--- a/modules/caddyhttp/reverseproxy/selectionpolicies.go
+++ b/modules/caddyhttp/reverseproxy/selectionpolicies.go
@@ -15,6 +15,9 @@
 package reverseproxy
 
 import (
+	"crypto/hmac"
+	"crypto/sha256"
+	"encoding/hex"
 	"fmt"
 	"hash/fnv"
 	weakrand "math/rand"
@@ -37,6 +40,7 @@ func init() {
 	caddy.RegisterModule(IPHashSelection{})
 	caddy.RegisterModule(URIHashSelection{})
 	caddy.RegisterModule(HeaderHashSelection{})
+	caddy.RegisterModule(CookieHashSelection{})
 
 	weakrand.Seed(time.Now().UTC().UnixNano())
 }
@@ -54,24 +58,8 @@ func (RandomSelection) CaddyModule() caddy.ModuleInfo {
 }
 
 // Select returns an available host, if any.
-func (r RandomSelection) Select(pool UpstreamPool, request *http.Request) *Upstream {
-	// use reservoir sampling because the number of available
-	// hosts isn't known: https://en.wikipedia.org/wiki/Reservoir_sampling
-	var randomHost *Upstream
-	var count int
-	for _, upstream := range pool {
-		if !upstream.Available() {
-			continue
-		}
-		// (n % 1 == 0) holds for all n, therefore a
-		// upstream will always be chosen if there is at
-		// least one available
-		count++
-		if (weakrand.Int() % count) == 0 {
-			randomHost = upstream
-		}
-	}
-	return randomHost
+func (r RandomSelection) Select(pool UpstreamPool, request *http.Request, _ http.ResponseWriter) *Upstream {
+	return selectRandomHost(pool)
 }
 
 // UnmarshalCaddyfile sets up the module from Caddyfile tokens.
@@ -134,7 +122,7 @@ func (r RandomChoiceSelection) Validate() error {
 }
 
 // Select returns an available host, if any.
-func (r RandomChoiceSelection) Select(pool UpstreamPool, _ *http.Request) *Upstream {
+func (r RandomChoiceSelection) Select(pool UpstreamPool, _ *http.Request, _ http.ResponseWriter) *Upstream {
 	k := r.Choose
 	if k > len(pool) {
 		k = len(pool)
@@ -174,7 +162,7 @@ func (LeastConnSelection) CaddyModule() caddy.ModuleInfo {
 // Select selects the up host with the least number of connections in the
 // pool. If more than one host has the same least number of connections,
 // one of the hosts is chosen at random.
-func (LeastConnSelection) Select(pool UpstreamPool, _ *http.Request) *Upstream {
+func (LeastConnSelection) Select(pool UpstreamPool, _ *http.Request, _ http.ResponseWriter) *Upstream {
 	var bestHost *Upstream
 	var count int
 	leastReqs := -1
@@ -227,7 +215,7 @@ func (RoundRobinSelection) CaddyModule() caddy.ModuleInfo {
 }
 
 // Select returns an available host, if any.
-func (r *RoundRobinSelection) Select(pool UpstreamPool, _ *http.Request) *Upstream {
+func (r *RoundRobinSelection) Select(pool UpstreamPool, _ *http.Request, _ http.ResponseWriter) *Upstream {
 	n := uint32(len(pool))
 	if n == 0 {
 		return nil
@@ -265,7 +253,7 @@ func (FirstSelection) CaddyModule() caddy.ModuleInfo {
 }
 
 // Select returns an available host, if any.
-func (FirstSelection) Select(pool UpstreamPool, _ *http.Request) *Upstream {
+func (FirstSelection) Select(pool UpstreamPool, _ *http.Request, _ http.ResponseWriter) *Upstream {
 	for _, host := range pool {
 		if host.Available() {
 			return host
@@ -297,7 +285,7 @@ func (IPHashSelection) CaddyModule() caddy.ModuleInfo {
 }
 
 // Select returns an available host, if any.
-func (IPHashSelection) Select(pool UpstreamPool, req *http.Request) *Upstream {
+func (IPHashSelection) Select(pool UpstreamPool, req *http.Request, _ http.ResponseWriter) *Upstream {
 	clientIP, _, err := net.SplitHostPort(req.RemoteAddr)
 	if err != nil {
 		clientIP = req.RemoteAddr
@@ -328,7 +316,7 @@ func (URIHashSelection) CaddyModule() caddy.ModuleInfo {
 }
 
 // Select returns an available host, if any.
-func (URIHashSelection) Select(pool UpstreamPool, req *http.Request) *Upstream {
+func (URIHashSelection) Select(pool UpstreamPool, req *http.Request, _ http.ResponseWriter) *Upstream {
 	return hostByHashing(pool, req.RequestURI)
 }
 
@@ -358,7 +346,7 @@ func (HeaderHashSelection) CaddyModule() caddy.ModuleInfo {
 }
 
 // Select returns an available host, if any.
-func (s HeaderHashSelection) Select(pool UpstreamPool, req *http.Request) *Upstream {
+func (s HeaderHashSelection) Select(pool UpstreamPool, req *http.Request, _ http.ResponseWriter) *Upstream {
 	if s.Field == "" {
 		return nil
 	}
@@ -371,7 +359,7 @@ func (s HeaderHashSelection) Select(pool UpstreamPool, req *http.Request) *Upstr
 
 	val := req.Header.Get(s.Field)
 	if val == "" {
-		return RandomSelection{}.Select(pool, req)
+		return RandomSelection{}.Select(pool, req, nil)
 	}
 	return hostByHashing(pool, val)
 }
@@ -387,6 +375,114 @@ func (s *HeaderHashSelection) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
 	return nil
 }
 
+// CookieHashSelection is a policy that selects
+// a host based on a given cookie name.
+type CookieHashSelection struct {
+	// The HTTP cookie name whose value is to be hashed and used for upstream selection.
+	Name string `json:"name,omitempty"`
+	// Secret to hash (Hmac256) chosen upstream in cookie
+	Secret string `json:"secret,omitempty"`
+}
+
+// CaddyModule returns the Caddy module information.
+func (CookieHashSelection) CaddyModule() caddy.ModuleInfo {
+	return caddy.ModuleInfo{
+		ID:  "http.reverse_proxy.selection_policies.cookie",
+		New: func() caddy.Module { return new(CookieHashSelection) },
+	}
+}
+
+// Select returns an available host, if any.
+func (s CookieHashSelection) Select(pool UpstreamPool, req *http.Request, w http.ResponseWriter) *Upstream {
+	if s.Name == "" {
+		s.Name = "lb"
+	}
+	cookie, err := req.Cookie(s.Name)
+	// If there's no cookie, select new random host
+	if err != nil || cookie == nil {
+		return selectNewHostWithCookieHashSelection(pool, w, s.Secret, s.Name)
+	} else {
+		// If the cookie is present, loop over the available upstreams until we find a match
+		cookieValue := cookie.Value
+		for _, upstream := range pool {
+			if !upstream.Available() {
+				continue
+			}
+			sha, err := hashCookie(s.Secret, upstream.Dial)
+			if err == nil && sha == cookieValue {
+				return upstream
+			}
+		}
+	}
+	// If there is no matching host, select new random host
+	return selectNewHostWithCookieHashSelection(pool, w, s.Secret, s.Name)
+}
+
+// UnmarshalCaddyfile sets up the module from Caddyfile tokens. Syntax:
+//     lb_policy cookie [<name> [<secret>]]
+//
+// By default name is `lb`
+func (s *CookieHashSelection) UnmarshalCaddyfile(d *caddyfile.Dispenser) error {
+	args := d.RemainingArgs()
+	switch len(args) {
+	case 1:
+	case 2:
+		s.Name = args[1]
+	case 3:
+		s.Name = args[1]
+		s.Secret = args[2]
+	default:
+		return d.ArgErr()
+	}
+	return nil
+}
+
+// Select a new Host randomly and add a sticky session cookie
+func selectNewHostWithCookieHashSelection(pool []*Upstream, w http.ResponseWriter, cookieSecret string, cookieName string) *Upstream {
+	randomHost := selectRandomHost(pool)
+
+	if randomHost != nil {
+		// Hash (HMAC with some key for privacy) the upstream.Dial string as the cookie value
+		sha, err := hashCookie(cookieSecret, randomHost.Dial)
+		if err == nil {
+			// write the cookie.
+			http.SetCookie(w, &http.Cookie{Name: cookieName, Value: sha, Secure: false})
+		}
+	}
+	return randomHost
+}
+
+// hashCookie hashes (HMAC 256) some data with the secret
+func hashCookie(secret string, data string) (string, error) {
+	h := hmac.New(sha256.New, []byte(secret))
+	_, err := h.Write([]byte(data))
+	if err != nil {
+		return "", err
+	}
+	return hex.EncodeToString(h.Sum(nil)), nil
+}
+
+// selectRandomHost returns a random available host
+func selectRandomHost(pool []*Upstream) *Upstream {
+	// use reservoir sampling because the number of available
+	// hosts isn't known: https://en.wikipedia.org/wiki/Reservoir_sampling
+	var randomHost *Upstream
+	var count int
+	for _, upstream := range pool {
+		if !upstream.Available() {
+			continue
+		}
+		// (n % 1 == 0) holds for all n, therefore a
+		// upstream will always be chosen if there is at
+		// least one available
+		count++
+		if (weakrand.Int() % count) == 0 {
+			randomHost = upstream
+		}
+	}
+	return randomHost
+}
+
 // leastRequests returns the host with the
 // least number of active requests to it.
 // If more than one host has the same
@@ -454,6 +550,7 @@ var (
 	_ Selector = (*IPHashSelection)(nil)
 	_ Selector = (*URIHashSelection)(nil)
 	_ Selector = (*HeaderHashSelection)(nil)
+	_ Selector = (*CookieHashSelection)(nil)
 
 	_ caddy.Validator   = (*RandomChoiceSelection)(nil)
 	_ caddy.Provisioner = (*RandomChoiceSelection)(nil)
diff --git a/modules/caddyhttp/reverseproxy/selectionpolicies_test.go b/modules/caddyhttp/reverseproxy/selectionpolicies_test.go
index 49585da..5368a1a 100644
--- a/modules/caddyhttp/reverseproxy/selectionpolicies_test.go
+++ b/modules/caddyhttp/reverseproxy/selectionpolicies_test.go
@@ -33,37 +33,37 @@ func TestRoundRobinPolicy(t *testing.T) {
 	rrPolicy := new(RoundRobinSelection)
 	req, _ := http.NewRequest("GET", "/", nil)
 
-	h := rrPolicy.Select(pool, req)
+	h := rrPolicy.Select(pool, req, nil)
 	// First selected host is 1, because counter starts at 0
 	// and increments before host is selected
 	if h != pool[1] {
 		t.Error("Expected first round robin host to be second host in the pool.")
 	}
-	h = rrPolicy.Select(pool, req)
+	h = rrPolicy.Select(pool, req, nil)
 	if h != pool[2] {
 		t.Error("Expected second round robin host to be third host in the pool.")
 	}
-	h = rrPolicy.Select(pool, req)
+	h = rrPolicy.Select(pool, req, nil)
 	if h != pool[0] {
 		t.Error("Expected third round robin host to be first host in the pool.")
 	}
 	// mark host as down
 	pool[1].SetHealthy(false)
-	h = rrPolicy.Select(pool, req)
+	h = rrPolicy.Select(pool, req, nil)
 	if h != pool[2] {
 		t.Error("Expected to skip down host.")
 	}
 	// mark host as up
 	pool[1].SetHealthy(true)
 
-	h = rrPolicy.Select(pool, req)
+	h = rrPolicy.Select(pool, req, nil)
 	if h == pool[2] {
 		t.Error("Expected to balance evenly among healthy hosts")
 	}
 	// mark host as full
 	pool[1].CountRequest(1)
 	pool[1].MaxRequests = 1
-	h = rrPolicy.Select(pool, req)
+	h = rrPolicy.Select(pool, req, nil)
 	if h != pool[2] {
 		t.Error("Expected to skip full host.")
 	}
@@ -76,12 +76,12 @@ func TestLeastConnPolicy(t *testing.T) {
 
 	pool[0].CountRequest(10)
 	pool[1].CountRequest(10)
-	h := lcPolicy.Select(pool, req)
+	h := lcPolicy.Select(pool, req, nil)
 	if h != pool[2] {
 		t.Error("Expected least connection host to be third host.")
 	}
 	pool[2].CountRequest(100)
-	h = lcPolicy.Select(pool, req)
+	h = lcPolicy.Select(pool, req, nil)
 	if h != pool[0] && h != pool[1] {
 		t.Error("Expected least connection host to be first or second host.")
 	}
@@ -94,44 +94,44 @@ func TestIPHashPolicy(t *testing.T) {
 
 	// We should be able to predict where every request is routed.
 	req.RemoteAddr = "172.0.0.1:80"
-	h := ipHash.Select(pool, req)
+	h := ipHash.Select(pool, req, nil)
 	if h != pool[1] {
 		t.Error("Expected ip hash policy host to be the second host.")
 	}
 	req.RemoteAddr = "172.0.0.2:80"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[1] {
 		t.Error("Expected ip hash policy host to be the second host.")
 	}
 	req.RemoteAddr = "172.0.0.3:80"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[2] {
 		t.Error("Expected ip hash policy host to be the third host.")
 	}
 	req.RemoteAddr = "172.0.0.4:80"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[1] {
 		t.Error("Expected ip hash policy host to be the second host.")
 	}
 
 	// we should get the same results without a port
 	req.RemoteAddr = "172.0.0.1"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[1] {
 		t.Error("Expected ip hash policy host to be the second host.")
 	}
 	req.RemoteAddr = "172.0.0.2"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[1] {
 		t.Error("Expected ip hash policy host to be the second host.")
 	}
 	req.RemoteAddr = "172.0.0.3"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[2] {
 		t.Error("Expected ip hash policy host to be the third host.")
 	}
 	req.RemoteAddr = "172.0.0.4"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[1] {
 		t.Error("Expected ip hash policy host to be the second host.")
 	}
@@ -140,13 +140,13 @@ func TestIPHashPolicy(t *testing.T) {
 	// healthy host is available
 	req.RemoteAddr = "172.0.0.1"
 	pool[1].SetHealthy(false)
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[2] {
 		t.Error("Expected ip hash policy host to be the third host.")
 	}
 
 	req.RemoteAddr = "172.0.0.2"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[2] {
 		t.Error("Expected ip hash policy host to be the third host.")
 	}
@@ -154,12 +154,12 @@ func TestIPHashPolicy(t *testing.T) {
 
 	req.RemoteAddr = "172.0.0.3"
 	pool[2].SetHealthy(false)
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[0] {
 		t.Error("Expected ip hash policy host to be the first host.")
 	}
 	req.RemoteAddr = "172.0.0.4"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[1] {
 		t.Error("Expected ip hash policy host to be the second host.")
 	}
@@ -171,22 +171,22 @@ func TestIPHashPolicy(t *testing.T) {
 		{Host: new(upstreamHost)},
 	}
 	req.RemoteAddr = "172.0.0.1:80"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[0] {
 		t.Error("Expected ip hash policy host to be the first host.")
 	}
 	req.RemoteAddr = "172.0.0.2:80"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[1] {
 		t.Error("Expected ip hash policy host to be the second host.")
 	}
 	req.RemoteAddr = "172.0.0.3:80"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[0] {
 		t.Error("Expected ip hash policy host to be the first host.")
 	}
 	req.RemoteAddr = "172.0.0.4:80"
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != pool[1] {
 		t.Error("Expected ip hash policy host to be the second host.")
 	}
@@ -194,7 +194,7 @@ func TestIPHashPolicy(t *testing.T) {
 	// We should get nil when there are no healthy hosts
 	pool[0].SetHealthy(false)
 	pool[1].SetHealthy(false)
-	h = ipHash.Select(pool, req)
+	h = ipHash.Select(pool, req, nil)
 	if h != nil {
 		t.Error("Expected ip hash policy host to be nil.")
 	}
@@ -205,13 +205,13 @@ func TestFirstPolicy(t *testing.T) {
 	firstPolicy := new(FirstSelection)
 	req := httptest.NewRequest(http.MethodGet, "/", nil)
 
-	h := firstPolicy.Select(pool, req)
+	h := firstPolicy.Select(pool, req, nil)
 	if h != pool[0] {
 		t.Error("Expected first policy host to be the first host.")
 	}
 
 	pool[0].SetHealthy(false)
-	h = firstPolicy.Select(pool, req)
+	h = firstPolicy.Select(pool, req, nil)
 	if h != pool[1] {
 		t.Error("Expected first policy host to be the second host.")
 	}
@@ -222,19 +222,19 @@ func TestURIHashPolicy(t *testing.T) {
 	uriPolicy := new(URIHashSelection)
 
 	request := httptest.NewRequest(http.MethodGet, "/test", nil)
-	h := uriPolicy.Select(pool, request)
+	h := uriPolicy.Select(pool, request, nil)
 	if h != pool[0] {
 		t.Error("Expected uri policy host to be the first host.")
 	}
 
 	pool[0].SetHealthy(false)
-	h = uriPolicy.Select(pool, request)
+	h = uriPolicy.Select(pool, request, nil)
 	if h != pool[1] {
 		t.Error("Expected uri policy host to be the first host.")
 	}
 
 	request = httptest.NewRequest(http.MethodGet, "/test_2", nil)
-	h = uriPolicy.Select(pool, request)
+	h = uriPolicy.Select(pool, request, nil)
 	if h != pool[1] {
 		t.Error("Expected uri policy host to be the second host.")
 	}
@@ -247,26 +247,26 @@ func TestURIHashPolicy(t *testing.T) {
 	}
 
 	request = httptest.NewRequest(http.MethodGet, "/test", nil)
-	h = uriPolicy.Select(pool, request)
+	h = uriPolicy.Select(pool, request, nil)
 	if h != pool[0] {
 		t.Error("Expected uri policy host to be the first host.")
 	}
 
 	pool[0].SetHealthy(false)
-	h = uriPolicy.Select(pool, request)
+	h = uriPolicy.Select(pool, request, nil)
 	if h != pool[1] {
 		t.Error("Expected uri policy host to be the first host.")
 	}
 
 	request = httptest.NewRequest(http.MethodGet, "/test_2", nil)
-	h = uriPolicy.Select(pool, request)
+	h = uriPolicy.Select(pool, request, nil)
 	if h != pool[1] {
 		t.Error("Expected uri policy host to be the second host.")
 	}
 
 	pool[0].SetHealthy(false)
 	pool[1].SetHealthy(false)
-	h = uriPolicy.Select(pool, request)
+	h = uriPolicy.Select(pool, request, nil)
 	if h != nil {
 		t.Error("Expected uri policy policy host to be nil.")
 	}
@@ -311,7 +311,7 @@ func TestRandomChoicePolicy(t *testing.T) {
 	randomChoicePolicy := new(RandomChoiceSelection)
 	randomChoicePolicy.Choose = 2
 
-	h := randomChoicePolicy.Select(pool, request)
+	h := randomChoicePolicy.Select(pool, request, nil)
 
 	if h == nil {
 		t.Error("RandomChoicePolicy should not return nil")
@@ -322,3 +322,51 @@ func TestRandomChoicePolicy(t *testing.T) {
 	}
 
 }
+
+func TestCookieHashPolicy(t *testing.T) {
+	pool := testPool()
+	pool[0].Dial = "localhost:8080"
+	pool[1].Dial = "localhost:8081"
+	pool[2].Dial = "localhost:8082"
+	pool[0].SetHealthy(true)
+	pool[1].SetHealthy(false)
+	pool[2].SetHealthy(false)
+	request := httptest.NewRequest(http.MethodGet, "/test", nil)
+	w := httptest.NewRecorder()
+	cookieHashPolicy := new(CookieHashSelection)
+	h := cookieHashPolicy.Select(pool, request, w)
+	cookie_server1 := w.Result().Cookies()[0]
+	if cookie_server1 == nil {
+		t.Error("cookieHashPolicy should set a cookie")
+	}
+	if cookie_server1.Name != "lb" {
+		t.Error("cookieHashPolicy should set a cookie with name lb")
+	}
+	if h != pool[0] {
+		t.Error("Expected cookieHashPolicy host to be the first only available host.")
+	}
+	pool[1].SetHealthy(true)
+	pool[2].SetHealthy(true)
+	request = httptest.NewRequest(http.MethodGet, "/test", nil)
+	w = httptest.NewRecorder()
+	request.AddCookie(cookie_server1)
+	h = cookieHashPolicy.Select(pool, request, w)
+	if h != pool[0] {
+		t.Error("Expected cookieHashPolicy host to stick to the first host (matching cookie).")
+	}
+	s := w.Result().Cookies()
+	if len(s) != 0 {
+		t.Error("Expected cookieHashPolicy to not set a new cookie.")
+	}
+	pool[0].SetHealthy(false)
+	request = httptest.NewRequest(http.MethodGet, "/test", nil)
+	w = httptest.NewRecorder()
+	request.AddCookie(cookie_server1)
+	h = cookieHashPolicy.Select(pool, request, w)
+	if h == pool[0] {
+		t.Error("Expected cookieHashPolicy to select a new host.")
+	}
+	if w.Result().Cookies() == nil {
+		t.Error("Expected cookieHashPolicy to set a new cookie.")
+	}
+}