Add support for the authenticate extension request.

Just like btcd, this commit adds support for the authenticate request
allowing clients unable to set the HTTP Authorization header to
authenticate to use the RPC server.  The rules for the authenticate
request are as follows:

1) Authentication make clients stateful.  Clients may either be flagged
   as authenticated or unauthenticated.

2) Clients may authenticate by exactly one of two possible ways,
   either by setting the Authorization header or by sending a JSON-RPC
   authenticate request as follows:

   {
     "jsonrpc":"1.0",
     "id":0,
     "method":"authenticate",
     "params":["rpcuser", "rpcpass"]
   }

3) When not authenticated by the Authorization header, the first request
   must be an authenticate request.

4) Sending an authenticate request after a client has already
   successfully authenticated (either by the Authorization header or a
   previous authentication request) is invalid.

5) The result used in the response to a successful authenticate request
   is a JSON null.  For any unsuccessful or invalid authenticate
   requests, the connection is terminated.

This change also orders all incoming requests for a client.  This was
required to ensure that any authentication requests are processed
first.

sockets.go

@@ -43,6 +43,10 @@ import (
 )
 
 var (
+	// ErrBadAuth represents an error where a request is denied due to
+	// a missing, incorrect, or duplicate authentication request.
+	ErrBadAuth = errors.New("bad auth")
+
 	// ErrConnRefused represents an error where a connection to another
 	// process cannot be established.
 	ErrConnRefused = errors.New("connection refused")
@@ -68,7 +72,7 @@ type server struct {
 
 type clientContext struct {
 	send chan []byte
-	disconnected chan struct{} // closed on disconnect
+	quit chan struct{} // closed on disconnect
 }
 
 // parseListeners splits the list of listen addresses passed in addrs into
@@ -214,20 +218,48 @@ func ParseRequest(msg []byte) (btcjson.Cmd, *btcjson.Error) {
 
 // ReplyToFrontend responds to a marshaled JSON-RPC request with a
 // marshaled JSON-RPC response for both standard and extension
-// (websocket) clients.
+// (websocket) clients.  The returned error is ErrBadAuth if a
-func ReplyToFrontend(msg []byte, ws bool) []byte {
+// missing, incorrect, or duplicate authentication request is
+// received.
+func (s *server) ReplyToFrontend(msg []byte, ws, authenticated bool) ([]byte, error) {
 	cmd, jsonErr := ParseRequest(msg)
 	var id interface{}
 	if cmd != nil {
 		id = cmd.Id()
 	}
+
+	// If client is not already authenticated, the parsed request must
+	// be for authentication.
+	authCmd, ok := cmd.(*btcws.AuthenticateCmd)
+	if authenticated {
+		if ok {
+			// Duplicate auth request.
+			return nil, ErrBadAuth
+		}
+	} else {
+		if !ok {
+			// The first unauthenticated request must be an auth request.
+			return nil, ErrBadAuth
+		}
+
+		// Check credentials.
+		login := authCmd.Username + ":" + authCmd.Passphrase
+		auth := "Basic " + base64.StdEncoding.EncodeToString([]byte(login))
+		authSha := sha256.Sum256([]byte(auth))
+		cmp := subtle.ConstantTimeCompare(authSha[:], s.authsha[:])
+		if cmp != 1 {
+			return nil, ErrBadAuth
+		}
+		return nil, nil
+	}
+
 	if jsonErr != nil {
 		response := btcjson.Reply{
 			Id:    &id,
 			Error: jsonErr,
 		}
 		mresponse, _ := json.Marshal(response)
-		return mresponse
+		return mresponse, nil
 	}
 
 	cReq := NewClientRequest(cmd, ws)
@@ -248,7 +280,7 @@ func ReplyToFrontend(msg []byte, ws bool) []byte {
 		mresponse, _ = json.Marshal(&response)
 	}
 
-	return mresponse
+	return mresponse, nil
 }
 
 // ServeRPCRequest processes and replies to a JSON-RPC client request.
@@ -258,7 +290,11 @@ func (s *server) ServeRPCRequest(w http.ResponseWriter, r *http.Request) {
 		log.Errorf("RPCS: Error getting JSON message: %v", err)
 	}
 
-	resp := ReplyToFrontend(body, false)
+	resp, err := s.ReplyToFrontend(body, false, true)
+	if err == ErrBadAuth {
+		http.Error(w, "401 Unauthorized.", http.StatusUnauthorized)
+		return
+	}
 	if _, err := w.Write(resp); err != nil {
 		log.Warnf("RPCS: could not respond to RPC request: %v", err)
 	}
@@ -277,7 +313,7 @@ func clientResponseDuplicator() {
 		case n := <-allClients:
 			for cc := range clients {
 				select {
-				case <-cc.disconnected:
+				case <-cc.quit:
 					delete(clients, cc)
 				case cc.send <- n:
 				}
@@ -297,76 +333,176 @@ func NotifyBtcdConnection(reply chan []byte) {
 
 }
 
+// stringQueue manages a queue of strings, reading from in and sending
+// the oldest unsent to out.  This handler closes out and returns after
+// in is closed and any queued items are sent.  Any reads on quit result
+// in immediate shutdown of the handler.
+func stringQueue(in <-chan string, out chan<- string, quit <-chan struct{}) {
+	var q []string
+	var dequeue chan<- string
+	skipQueue := out
+	var next string
+out:
+	for {
+		select {
+		case n, ok := <-in:
+			if !ok {
+				// Sender closed input channel.  Nil channel
+				// and continue so the remaining queued
+				// items may be sent.  If the queue is empty,
+				// break out of the loop.
+				in = nil
+				if dequeue == nil {
+					break out
+				}
+				continue
+			}
+
+			// Either send to out immediately if skipQueue is
+			// non-nil (queue is empty) and reader is ready,
+			// or append to the queue and send later.
+			select {
+			case skipQueue <- n:
+			default:
+				q = append(q, n)
+				dequeue = out
+				skipQueue = nil
+				next = q[0]
+			}
+
+		case dequeue <- next:
+			copy(q, q[1:])
+			q[len(q)-1] = "" // avoid leak
+			q = q[:len(q)-1]
+			if len(q) == 0 {
+				// If the input chan was closed and nil'd,
+				// break out of the loop.
+				if in == nil {
+					break out
+				}
+				dequeue = nil
+				skipQueue = out
+			} else {
+				next = q[0]
+			}
+
+		case <-quit:
+			break out
+		}
+	}
+	close(out)
+}
+
 // WSSendRecv is the handler for websocket client connections.  It loops
 // forever (until disconnected), reading JSON-RPC requests and sending
 // sending responses and notifications.
-func WSSendRecv(ws *websocket.Conn) {
+func (s *server) WSSendRecv(ws *websocket.Conn, authenticated bool) {
 	// Add client context so notifications duplicated to each
 	// client are received by this client.
+	recvQuit := make(chan struct{})
+	sendQuit := make(chan struct{})
 	cc := clientContext{
 		send: make(chan []byte, 1), // buffer size is number of initial notifications
-		disconnected: make(chan struct{}),
+		quit: make(chan struct{}),
 	}
+	go func() {
+		select {
+		case <-recvQuit:
+		case <-sendQuit:
+		}
+		close(cc.quit)
+	}()
 	NotifyBtcdConnection(cc.send) // TODO(jrick): clients should explicitly request this.
 	addClient <- cc
-	defer close(cc.disconnected)
 
 	// received passes all received messages from the currently connected
 	// frontend to the for-select loop.  It is closed when reading a
 	// message from the websocket connection fails (presumably due to
 	// a disconnected client).
-	received := make(chan []byte)
+	recvQueueIn := make(chan string)
 
 	// Receive messages from websocket and send across jsonMsgs until
 	// connection is lost
 	go func() {
 		for {
-			var m []byte
+			var m string
 			if err := websocket.Message.Receive(ws, &m); err != nil {
+				select {
+				case <-sendQuit:
+					// Do not log error.
+
+				default:
 					// Log warning if the client did not disconnect.
 					if err != io.EOF {
 						log.Warnf("Cannot receive client websocket message: %v",
 							err)
 					}
-				close(received)
+				}
+				close(recvQueueIn)
+				close(recvQuit)
 				return
 			}
-			received <- m
+			recvQueueIn <- m
 		}
 	}()
 
+	// Manage queue of received messages for LIFO processing.
+	recvQueueOut := make(chan string)
+	go stringQueue(recvQueueIn, recvQueueOut, cc.quit)
+
+	badAuth := make(chan struct{})
+	go func() {
+		for m := range recvQueueOut {
+			resp, err := s.ReplyToFrontend([]byte(m), true, authenticated)
+			if err == ErrBadAuth {
+				select {
+				case badAuth <- struct{}{}:
+				case <-cc.quit:
+				}
+				return
+			}
+
+			// Authentication passed.
+			authenticated = true
+
+			select {
+			case cc.send <- resp:
+			case <-cc.quit:
+			}
+		}
+		close(cc.send)
+	}()
+
 	const deadline time.Duration = 2 * time.Second
 
+out:
 	for {
 		select {
-		case m, ok := <-received:
+		case <-badAuth:
+			// Bad auth. Disconnect.
+			log.Warnf("Disconnecting improperly authorized websocket client")
+			ws.Close()
+			break out
+
+		case m, ok := <-cc.send:
 			if !ok {
-				// client disconnected.
+				// Nothing left to send.  Return so the handler exits.
-				return
+				break out
 			}
-			// Handle request here.
-			go func(m []byte) {
-				resp := ReplyToFrontend(m, true)
 
-				select {
-				case cc.send <- resp:
-				case <-cc.disconnected:
-				}
-			}(m)
-
-		case m := <-cc.send:
 			err := ws.SetWriteDeadline(time.Now().Add(deadline))
 			if err != nil {
 				log.Errorf("Cannot set write deadline: %v", err)
-				return
+				break out
 			}
-			err = websocket.Message.Send(ws, m)
+			err = websocket.Message.Send(ws, string(m))
 			if err != nil {
 				log.Infof("Cannot complete client websocket send: %v", err)
-				return
+				break out
 			}
 		}
 	}
+	close(sendQuit)
 }
 
 // NotifyNewBlockChainHeight notifies all frontends of a new
@@ -396,17 +532,27 @@ func (s *server) Start() {
 	httpServer := &http.Server{Handler: serveMux}
 	serveMux.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
 		if err := s.checkAuth(r); err != nil {
+			log.Warnf("Unauthorized client connection attempt")
 			http.Error(w, "401 Unauthorized.", http.StatusUnauthorized)
 			return
 		}
 		s.ServeRPCRequest(w, r)
 	})
 	serveMux.HandleFunc("/frontend", func(w http.ResponseWriter, r *http.Request) {
-		if err := s.checkAuth(r); err != nil {
+		authenticated := false
-			http.Error(w, "401 Unauthorized.", http.StatusUnauthorized)
+		if err := s.checkAuth(r); err == nil {
-			return
+			authenticated = true
 		}
-		websocket.Handler(WSSendRecv).ServeHTTP(w, r)
+
+		// A new Server instance is created rather than just creating the
+		// handler closure since the default server will disconnect the
+		// client if the origin is unset.
+		wsServer := websocket.Server{
+			Handler: websocket.Handler(func(ws *websocket.Conn) {
+				s.WSSendRecv(ws, authenticated)
+			}),
+		}
+		wsServer.ServeHTTP(w, r)
 	})
 	for _, listener := range s.listeners {
 		s.wg.Add(1)
@@ -428,15 +574,13 @@ func (s *server) Start() {
 func (s *server) checkAuth(r *http.Request) error {
 	authhdr := r.Header["Authorization"]
 	if len(authhdr) <= 0 {
-		log.Infof("Frontend did not supply authentication.")
+		return ErrBadAuth
-		return errors.New("auth failure")
 	}
 
 	authsha := sha256.Sum256([]byte(authhdr[0]))
 	cmp := subtle.ConstantTimeCompare(authsha[:], s.authsha[:])
 	if cmp != 1 {
-		log.Infof("Frontend did not supply correct authentication.")
+		return ErrBadAuth
-		return errors.New("auth failure")
 	}
 	return nil
 }