lnd+lnrpc: enable WebSocket ping/pong messages

Fixes #4497 by sending out ping messages in a regular interval to make
sure the connection is still alive.

lnd.go

@@ -1266,7 +1266,8 @@ func startRestProxy(cfg *Config, rpcServer *rpcServer, restDialOpts []grpc.DialO
 
 	// Wrap the default grpc-gateway handler with the WebSocket handler.
 	restHandler := lnrpc.NewWebSocketProxy(
-		mux, rpcsLog, lnrpc.LndClientStreamingURIs,
+		mux, rpcsLog, lnrpc.DefaultPingInterval, lnrpc.DefaultPongWait,
+		lnrpc.LndClientStreamingURIs,
 	)
 
 	// Use a WaitGroup so we can be sure the instructions on how to input the

lnrpc/websocket_proxy.go

@@ -10,6 +10,7 @@ import (
 	"net/textproto"
 	"regexp"
 	"strings"
+	"time"
 
 	"github.com/btcsuite/btclog"
 	"github.com/gorilla/websocket"
@@ -50,15 +51,26 @@ var (
 	defaultProtocolsToAllow = map[string]bool{
 		"Grpc-Metadata-Macaroon": true,
 	}
+
+	// DefaultPingInterval is the default number of seconds to wait between
+	// sending ping requests.
+	DefaultPingInterval = time.Second * 30
+
+	// DefaultPongWait is the maximum duration we wait for a pong response
+	// to a ping we sent before we assume the connection died.
+	DefaultPongWait = time.Second * 5
 )
 
 // NewWebSocketProxy attempts to expose the underlying handler as a response-
 // streaming WebSocket stream with newline-delimited JSON as the content
-// encoding. The clientStreamingURIs parameter can hold a list of all patterns
+// encoding. If pingInterval is a non-zero duration, a ping message will be
+// sent out periodically and a pong response message is expected from the
+// client. The clientStreamingURIs parameter can hold a list of all patterns
 // for URIs that are mapped to client-streaming RPC methods. We need to keep
 // track of those to make sure we initialize the request body correctly for the
 // underlying grpc-gateway library.
 func NewWebSocketProxy(h http.Handler, logger btclog.Logger,
+	pingInterval, pongWait time.Duration,
 	clientStreamingURIs []*regexp.Regexp) http.Handler {
 
 	p := &WebsocketProxy{
@@ -73,6 +85,12 @@ func NewWebSocketProxy(h http.Handler, logger btclog.Logger,
 		},
 		clientStreamingURIs: clientStreamingURIs,
 	}
+
+	if pingInterval > 0 && pongWait > 0 {
+		p.pingInterval = pingInterval
+		p.pongWait = pongWait
+	}
+
 	return p
 }
 
@@ -87,6 +105,15 @@ type WebsocketProxy struct {
 	// those to make sure we initialize the request body correctly for the
 	// underlying grpc-gateway library.
 	clientStreamingURIs []*regexp.Regexp
+
+	pingInterval time.Duration
+	pongWait     time.Duration
+}
+
+// pingPongEnabled returns true if a ping interval is set to enable sending and
+// expecting regular ping/pong messages.
+func (p *WebsocketProxy) pingPongEnabled() bool {
+	return p.pingInterval > 0 && p.pongWait > 0
 }
 
 // ServeHTTP handles the incoming HTTP request. If the request is an
@@ -200,6 +227,58 @@ func (p *WebsocketProxy) upgradeToWebSocketProxy(w http.ResponseWriter,
 		}
 	}()
 
+	// Ping write loop: Send a ping message regularly if ping/pong is
+	// enabled.
+	if p.pingPongEnabled() {
+		// We'll send out our first ping in pingInterval. So the initial
+		// deadline is that interval plus the time we allow for a
+		// response to be sent.
+		initialDeadline := time.Now().Add(p.pingInterval + p.pongWait)
+		_ = conn.SetReadDeadline(initialDeadline)
+
+		// Whenever a pong message comes in, we extend the deadline
+		// until the next read is expected by the interval plus pong
+		// wait time.
+		conn.SetPongHandler(func(appData string) error {
+			nextDeadline := time.Now().Add(
+				p.pingInterval + p.pongWait,
+			)
+			_ = conn.SetReadDeadline(nextDeadline)
+			return nil
+		})
+		go func() {
+			ticker := time.NewTicker(p.pingInterval)
+			defer ticker.Stop()
+
+			for {
+				select {
+				case <-ctx.Done():
+					p.logger.Debug("WS: ping loop done")
+					return
+
+				case <-ticker.C:
+					// Writing the ping shouldn't take any
+					// longer than we'll wait for a response
+					// in the first place.
+					writeDeadline := time.Now().Add(
+						p.pongWait,
+					)
+					_ = conn.SetWriteDeadline(writeDeadline)
+
+					err := conn.WriteMessage(
+						websocket.PingMessage, nil,
+					)
+					if err != nil {
+						p.logger.Warnf("WS: could not "+
+							"send ping message: %v",
+							err)
+						return
+					}
+				}
+			}
+		}()
+	}
+
 	// Write loop: Take messages from the response forwarder and write them
 	// to the WebSocket.
 	for responseForwarder.Scan() {