net/http/httptest: allow creation of Server manually

The Server struct has exported fields, which allows users to manually
create a Server object without using using NewServer or NewTLSServer
and directly call Start or StartTLS on their object.

In order to ensure that manual creation of Server works, the
NewUnstartedServer function should not initialize Server in any way
that the user was not able to do themselves. For example, the setting
of a unexported filed, client, is not something a user can do.
Thus, rather than setting the client field in NewUnstartedServer,
we lazily initialize it when Start or StartTLS is called.

Otherwise, the Server logic can nil panic later when it assumes that this
field has been initialized.

Fixes #20871

Change-Id: I65c6a9f893ea963b0fbad0990b33af08007c1140
Reviewed-on: https://go-review.googlesource.com/47353
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>

2 files changed, 59 insertions, 16 deletions

diff --git a/src/net/http/httptest/server.go b/src/net/http/httptest/server.go
index 1baec23d57..6075397a26 100644
--- a/src/net/http/httptest/server.go
+++ b/src/net/http/httptest/server.go
@@ -93,9 +93,6 @@ func NewUnstartedServer(handler http.Handler) *Server {
 	return &Server{
 		Listener: newLocalListener(),
 		Config:   &http.Server{Handler: handler},
-		client: &http.Client{
-			Transport: &http.Transport{},
-		},
 	}
 }
 
@@ -104,6 +101,9 @@ func (s *Server) Start() {
 	if s.URL != "" {
 		panic("Server already started")
 	}
+	if s.client == nil {
+		s.client = &http.Client{Transport: &http.Transport{}}
+	}
 	s.URL = "http://" + s.Listener.Addr().String()
 	s.wrap()
 	s.goServe()
@@ -118,6 +118,9 @@ func (s *Server) StartTLS() {
 	if s.URL != "" {
 		panic("Server already started")
 	}
+	if s.client == nil {
+		s.client = &http.Client{Transport: &http.Transport{}}
+	}
 	cert, err := tls.X509KeyPair(internal.LocalhostCert, internal.LocalhostKey)
 	if err != nil {
 		panic(fmt.Sprintf("httptest: NewTLSServer: %v", err))
diff --git a/src/net/http/httptest/server_test.go b/src/net/http/httptest/server_test.go
index d97cec5fdd..8ab50cdb0a 100644
--- a/src/net/http/httptest/server_test.go
+++ b/src/net/http/httptest/server_test.go
@@ -12,8 +12,48 @@ import (
 	"testing"
 )
 
+type newServerFunc func(http.Handler) *Server
+
+var newServers = map[string]newServerFunc{
+	"NewServer":    NewServer,
+	"NewTLSServer": NewTLSServer,
+
+	// The manual variants of newServer create a Server manually by only filling
+	// in the exported fields of Server.
+	"NewServerManual": func(h http.Handler) *Server {
+		ts := &Server{Listener: newLocalListener(), Config: &http.Server{Handler: h}}
+		ts.Start()
+		return ts
+	},
+	"NewTLSServerManual": func(h http.Handler) *Server {
+		ts := &Server{Listener: newLocalListener(), Config: &http.Server{Handler: h}}
+		ts.StartTLS()
+		return ts
+	},
+}
+
 func TestServer(t *testing.T) {
-	ts := NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	for _, name := range []string{"NewServer", "NewServerManual"} {
+		t.Run(name, func(t *testing.T) {
+			newServer := newServers[name]
+			t.Run("Server", func(t *testing.T) { testServer(t, newServer) })
+			t.Run("GetAfterClose", func(t *testing.T) { testGetAfterClose(t, newServer) })
+			t.Run("ServerCloseBlocking", func(t *testing.T) { testServerCloseBlocking(t, newServer) })
+			t.Run("ServerCloseClientConnections", func(t *testing.T) { testServerCloseClientConnections(t, newServer) })
+			t.Run("ServerClientTransportType", func(t *testing.T) { testServerClientTransportType(t, newServer) })
+		})
+	}
+	for _, name := range []string{"NewTLSServer", "NewTLSServerManual"} {
+		t.Run(name, func(t *testing.T) {
+			newServer := newServers[name]
+			t.Run("ServerClient", func(t *testing.T) { testServerClient(t, newServer) })
+			t.Run("TLSServerClientTransportType", func(t *testing.T) { testTLSServerClientTransportType(t, newServer) })
+		})
+	}
+}
+
+func testServer(t *testing.T, newServer newServerFunc) {
+	ts := newServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Write([]byte("hello"))
 	}))
 	defer ts.Close()
@@ -32,8 +72,8 @@ func TestServer(t *testing.T) {
 }
 
 // Issue 12781
-func TestGetAfterClose(t *testing.T) {
-	ts := NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+func testGetAfterClose(t *testing.T, newServer newServerFunc) {
+	ts := newServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Write([]byte("hello"))
 	}))
 
@@ -58,8 +98,8 @@ func TestGetAfterClose(t *testing.T) {
 	}
 }
 
-func TestServerCloseBlocking(t *testing.T) {
-	ts := NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+func testServerCloseBlocking(t *testing.T, newServer newServerFunc) {
+	ts := newServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Write([]byte("hello"))
 	}))
 	dial := func() net.Conn {
@@ -87,9 +127,9 @@ func TestServerCloseBlocking(t *testing.T) {
 }
 
 // Issue 14290
-func TestServerCloseClientConnections(t *testing.T) {
+func testServerCloseClientConnections(t *testing.T, newServer newServerFunc) {
 	var s *Server
-	s = NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+	s = newServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		s.CloseClientConnections()
 	}))
 	defer s.Close()
@@ -102,8 +142,8 @@ func TestServerCloseClientConnections(t *testing.T) {
 
 // Tests that the Server.Client method works and returns an http.Client that can hit
 // NewTLSServer without cert warnings.
-func TestServerClient(t *testing.T) {
-	ts := NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+func testServerClient(t *testing.T, newTLSServer newServerFunc) {
+	ts := newTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 		w.Write([]byte("hello"))
 	}))
 	defer ts.Close()
@@ -124,8 +164,8 @@ func TestServerClient(t *testing.T) {
 
 // Tests that the Server.Client.Transport interface is implemented
 // by a *http.Transport.
-func TestServerClientTransportType(t *testing.T) {
-	ts := NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+func testServerClientTransportType(t *testing.T, newServer newServerFunc) {
+	ts := newServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 	}))
 	defer ts.Close()
 	client := ts.Client()
@@ -136,8 +176,8 @@ func TestServerClientTransportType(t *testing.T) {
 
 // Tests that the TLS Server.Client.Transport interface is implemented
 // by a *http.Transport.
-func TestTLSServerClientTransportType(t *testing.T) {
-	ts := NewTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+func testTLSServerClientTransportType(t *testing.T, newTLSServer newServerFunc) {
+	ts := newTLSServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
 	}))
 	defer ts.Close()
 	client := ts.Client()