[ECO-5514] fix: improve WebSocket transport lifecycle and activity management

* Force-cancel hanging socket connections
* Replace synchronized blocks with fine-grained locking using `activityTimerMonitor` to avoid deadlocks.
* Refactor `close()` for safer access to shared fields and handle uninitialized cases gracefully.
* Simplify activity timer logic and ensure consistent disposal of `WebSocketClient` and `WebSocketHandler`.

Author: ttypic

lib/src/main/java/io/ably/lib/transport/WebSocketTransport.java

@@ -52,6 +52,7 @@ public class WebSocketTransport implements ITransport {
     private ConnectListener connectListener;
     private WebSocketClient webSocketClient;
     private final WebSocketEngine webSocketEngine;
+    private WebSocketHandler webSocketHandler;
     private boolean activityCheckTurnedOff = false;
 
     /******************
@@ -94,6 +95,10 @@ private static WebSocketEngine createWebSocketEngine(TransportParams params) {
      * ITransport methods
      ******************/
 
+    /**
+     * Connect is called once when we create transport;
+     * after transport is closed, we never call `connect` again
+     */
     @Override
     public void connect(ConnectListener connectListener) {
         this.connectListener = connectListener;
@@ -107,9 +112,8 @@ public void connect(ConnectListener connectListener) {
                 wsUri = HttpUtils.encodeParams(wsUri, connectParams);
 
             Log.d(TAG, "connect(); wsUri = " + wsUri);
-            synchronized (this) {
-                webSocketClient = this.webSocketEngine.create(wsUri, new WebSocketHandler(this::receive));
-            }
+            webSocketHandler = new WebSocketHandler(this::receive);
+            webSocketClient = this.webSocketEngine.create(wsUri, webSocketHandler);
             webSocketClient.connect();
         } catch (AblyException e) {
             Log.e(TAG, "Unexpected exception attempting connection; wsUri = " + wsUri, e);
@@ -123,11 +127,25 @@ public void connect(ConnectListener connectListener) {
     @Override
     public void close() {
         Log.d(TAG, "close()");
-        synchronized (this) {
-            if (webSocketClient != null) {
-                webSocketClient.close();
-                webSocketClient = null;
-            }
+        // Take local snapshots of the shared references. Callback threads (e.g., onClose)
+        // may concurrently set these fields to null.
+        //
+        // Intentionally avoid synchronizing here:
+        // - The WebSocket library may invoke our WebSocketHandler while holding its own
+        //   internal locks.
+        // - If close() also acquired a lock on WebSocketTransport, we could invert the
+        //   lock order and create a circular wait (deadlock): close() waits for the WS
+        //   library to release its lock, while the WS library waits for a lock on
+        //   WebSocketTransport.
+        final WebSocketClient client = webSocketClient;
+        final WebSocketHandler handler = webSocketHandler;
+        if (client != null && handler != null) {
+            // Record activity so the activity timer remains armed. If a graceful close
+            // stalls, the timer can detect inactivity and force-cancel the socket.
+            handler.flagActivity();
+            client.close();
+        } else {
+            Log.w(TAG, "close() called on uninitialized or already closed transport");
         }
     }
 
@@ -217,10 +235,15 @@ class WebSocketHandler implements WebSocketListener {
          * WsClient private members
          ***************************/
 
-        private Timer timer = new Timer();
+        private final Timer timer = new Timer();
         private TimerTask activityTimerTask = null;
         private long lastActivityTime;
 
+        /**
+         * Monitor for activity timer events
+         */
+        private final Object activityTimerMonitor = new Object();
+
         WebSocketHandler(WebSocketReceiver receiver) {
             this.receiver = receiver;
         }
@@ -305,6 +328,9 @@ public void onClose(final int wsCode, final String wsReason) {
             }
             connectListener.onTransportUnavailable(WebSocketTransport.this, reason);
             dispose();
+
+            webSocketClient = null;
+            webSocketHandler = null;
         }
 
         @Override
@@ -318,66 +344,64 @@ public void onOldJavaVersionDetected(Throwable throwable) {
             Log.w(TAG, "Error when trying to set SSL parameters, most likely due to an old Java API version", throwable);
         }
 
-        private synchronized void dispose() {
-            /* dispose timer */
-            try {
-                timer.cancel();
-                timer = null;
-            } catch (IllegalStateException e) {
-            }
+        private void dispose() {
+            timer.cancel();
         }
 
-        private synchronized void flagActivity() {
+        private void flagActivity() {
             lastActivityTime = System.currentTimeMillis();
             connectionManager.setLastActivity(lastActivityTime);
-            if (activityTimerTask == null && connectionManager.maxIdleInterval != 0 && !activityCheckTurnedOff) {
-                /* No timer currently running because previously there was no
-                 * maxIdleInterval configured, but now there is a
-                 * maxIdleInterval configured.  Call checkActivity so a timer
-                 * gets started.  This happens when flagActivity gets called
-                 * just after processing the connect message that configures
-                 * maxIdleInterval. */
-                checkActivity();
+
+            if (connectionManager.maxIdleInterval == 0) {
+                Log.v(TAG, "checkActivity: turned off because maxIdleInterval is 0");
+                return;
+            }
+
+            if (activityCheckTurnedOff) {
+                Log.v(TAG, "checkActivity: turned off for test purpose");
+                return;
             }
+
+            checkActivity();
         }
 
-        private synchronized void checkActivity() {
+        private void checkActivity() {
             long timeout = getActivityTimeout();
+
             if (timeout == 0) {
                 Log.v(TAG, "checkActivity: infinite timeout");
                 return;
             }
 
-            // Check if timer already running
-            if (activityTimerTask != null) {
-                return;
+            synchronized (activityTimerMonitor) {
+                // Check if timer already running
+                if (activityTimerTask == null) {
+                    // Start the activity timer task
+                    startActivityTimer(timeout + 100);
+                }
             }
-
-            // Start the activity timer task
-            startActivityTimer(timeout + 100);
         }
 
-        private synchronized void startActivityTimer(long timeout) {
-            if (activityTimerTask == null) {
-                schedule((activityTimerTask = new TimerTask() {
-                    public void run() {
-                        try {
-                            onActivityTimerExpiry();
-                        } catch (Throwable t) {
-                            Log.e(TAG, "Unexpected exception in activity timer handler", t);
-                        }
+        private void startActivityTimer(long timeout) {
+            activityTimerTask = new TimerTask() {
+                public void run() {
+                    try {
+                        onActivityTimerExpiry();
+                    } catch (Throwable t) {
+                        Log.e(TAG, "Unexpected exception in activity timer handler", t);
+                        webSocketClient.cancel(ABNORMAL_CLOSE, "Activity timer closed unexpectedly");
                     }
-                }), timeout);
-            }
+                }
+            };
+            schedule(activityTimerTask, timeout);
         }
 
-        private synchronized void schedule(TimerTask task, long delay) {
-            if (timer != null) {
-                try {
-                    timer.schedule(task, delay);
-                } catch (IllegalStateException ise) {
-                    Log.e(TAG, "Unexpected exception scheduling activity timer", ise);
-                }
+        private void schedule(TimerTask task, long delay) {
+            try {
+                timer.schedule(task, delay);
+            } catch (IllegalStateException ise) {
+                Log.e(TAG, "Unexpected exception scheduling activity timer", ise);
+                webSocketClient.cancel(ABNORMAL_CLOSE, "Activity timer closed unexpectedly");
             }
         }
 
@@ -392,7 +416,7 @@ private void onActivityTimerExpiry() {
                 return;
             }
 
-            synchronized (this) {
+            synchronized (activityTimerMonitor) {
                 activityTimerTask = null;
                 // Otherwise, we've had some activity, restart the timer for the next timeout
                 Log.v(TAG, "onActivityTimerExpiry: ok");