Server Side Keep-Alive

src/qz/ws/PrintSocketClient.java

@@ -38,8 +38,7 @@
 import java.util.ArrayList;
 import java.util.HashMap;
 import java.util.Locale;
-import java.util.concurrent.Semaphore;
-import java.util.concurrent.TimeoutException;
+import java.util.concurrent.*;
 
 
 @WebSocket
@@ -66,7 +65,11 @@ public void onConnect(Session session) {
         trayManager.displayInfoMessage("Client connected");
 
         //new connections are unknown until they send a proper certificate
-        openConnections.put(((InetSocketAddress)session.getRemoteAddress()).getPort(), new SocketConnection(Certificate.UNKNOWN));
+        SocketConnection connection =  new SocketConnection(Certificate.UNKNOWN);
+        openConnections.put(((InetSocketAddress)session.getRemoteAddress()).getPort(), connection);
+
+        //Browsers now slow or interrupt JS timers, leading to the websockets keepalive failing. A server-side keepalive ping will qct as a fallback.
+        connection.startKeepAlive(session, (int)(session.getPolicy().getIdleTimeout().toSeconds() / 2));
     }
 
     @OnWebSocketClose

src/qz/ws/SocketConnection.java

@@ -3,6 +3,7 @@
 import jssc.SerialPortException;
 import org.apache.logging.log4j.LogManager;
 import org.apache.logging.log4j.Logger;
+import org.eclipse.jetty.websocket.api.Session;
 import qz.auth.Certificate;
 import qz.communication.*;
 import qz.printer.status.StatusMonitor;
@@ -11,6 +12,10 @@
 import java.io.IOException;
 import java.nio.file.Path;
 import java.util.HashMap;
+import java.util.concurrent.Executors;
+import java.util.concurrent.ScheduledExecutorService;
+import java.util.concurrent.ScheduledFuture;
+import java.util.concurrent.TimeUnit;
 
 public class SocketConnection {
 
@@ -32,6 +37,9 @@ public class SocketConnection {
     // DeviceOptions -> open DeviceIO
     private final HashMap<DeviceOptions,DeviceIO> openDevices = new HashMap<>();
 
+    private static final ScheduledExecutorService keepAliveExecutor = Executors.newScheduledThreadPool(1);
+    private ScheduledFuture<?> keepAlive;
+
 
     public SocketConnection(Certificate cert) {
         certificate = cert;
@@ -95,6 +103,27 @@ public FileIO getFileListener(Path absolute) {
         return openFiles.get(absolute);
     }
 
+    public void startKeepAlive(Session session, int seconds) {
+        if (keepAlive != null && (!keepAlive.isCancelled() || !keepAlive.isDone())) {
+            stopKeepAlive();
+        }
+        //Sane fallback, this should never come up.
+        seconds = Math.max(seconds, 15);
+        keepAlive = keepAliveExecutor.scheduleAtFixedRate(() -> {
+            try {
+                session.getRemote().sendPing(null);
+            }
+            catch(IOException e) {
+                log.warn("Websocket keepalive failed. Stopping", e);
+                this.stopKeepAlive();
+            }
+        }, 0, seconds, TimeUnit.SECONDS);
+    }
+
+    public void stopKeepAlive() {
+        keepAlive.cancel(true);
+    }
+
     public void removeFileListener(Path absolute) {
         openFiles.remove(absolute);
     }
@@ -147,6 +176,7 @@ public synchronized void disconnect() throws SerialPortException, DeviceExceptio
             dio.close();
         }
 
+        stopKeepAlive();
         removeAllFileListeners();
         stopDeviceListening();
         StatusMonitor.stopListening(this);