From 7b71a4f2fc11dea0bf792bfc902cde0ae44bfc4d Mon Sep 17 00:00:00 2001
From: hailin <hailin.zhao@gdzx.xyz>
Date: Wed, 25 Feb 2026 03:45:43 -0800
Subject: [PATCH] fix: properly close WebSocket with subscription cancel +
 fire-and-forget

Root cause: IOWebSocketChannel.sink.close() can hang indefinitely
(dart-lang/web_socket_channel#185). Previous fix used unawaited close
but didn't cancel the stream subscription, so the old listener could
still push events to _messageController.

Fix: Extract _closeCurrentConnection() that:
1. Cancels StreamSubscription first (stops duplicate events immediately)
2. Fire-and-forget sink.close(goingAway) (frees underlying socket)

This follows the workaround recommended in the official issue tracker.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 .../lib/core/network/websocket_client.dart    | 44 +++++++++++++------
 1 file changed, 31 insertions(+), 13 deletions(-)

diff --git a/it0_app/lib/core/network/websocket_client.dart b/it0_app/lib/core/network/websocket_client.dart
index 3596e79..552bf8f 100644
--- a/it0_app/lib/core/network/websocket_client.dart
+++ b/it0_app/lib/core/network/websocket_client.dart
@@ -3,11 +3,13 @@ import 'dart:convert';
 import 'dart:math';
 import 'package:web_socket_channel/io.dart';
 import 'package:web_socket_channel/web_socket_channel.dart';
+import 'package:web_socket_channel/status.dart' as status;
 import 'package:flutter_riverpod/flutter_riverpod.dart';
 import '../config/app_config.dart';
 
 class WebSocketClient {
   WebSocketChannel? _channel;
+  StreamSubscription? _streamSubscription;
   final String baseUrl;
   Timer? _heartbeatTimer;
   Timer? _reconnectTimer;
@@ -30,16 +32,13 @@ class WebSocketClient {
     _lastPath = path;
     _lastToken = token;
 
-    // Close previous connection to prevent duplicate event delivery.
-    // Fire-and-forget: don't await close() as it can hang waiting for
-    // the server to acknowledge the close handshake.
-    if (_channel != null) {
-      _heartbeatTimer?.cancel();
-      final old = _channel!;
-      _channel = null;
-      _isConnected = false;
-      unawaited(old.sink.close().catchError((_) {}));
-    }
+    // Tear down previous connection to prevent duplicate event delivery.
+    // 1) Cancel the stream subscription first — stops events from reaching
+    //    _messageController immediately (no more duplicates).
+    // 2) Fire-and-forget sink.close() — don't await because
+    //    IOWebSocketChannel.sink.close() can hang indefinitely
+    //    (known issue: https://github.com/dart-lang/web_socket_channel/issues/185).
+    await _closeCurrentConnection();
 
     final uri = Uri.parse('$baseUrl$path');
     try {
@@ -50,7 +49,7 @@ class WebSocketClient {
       _isConnected = true;
       _reconnectAttempts = 0;
 
-      _channel!.stream.listen(
+      _streamSubscription = _channel!.stream.listen(
         (data) {
           final decoded = jsonDecode(data as String) as Map<String, dynamic>;
           _messageController.add(decoded);
@@ -122,9 +121,28 @@ class WebSocketClient {
   Future<void> disconnect() async {
     _heartbeatTimer?.cancel();
     _reconnectTimer?.cancel();
-    _isConnected = false;
     _reconnectAttempts = _maxReconnectAttempts; // Prevent auto-reconnect
-    await _channel?.sink.close();
+    await _closeCurrentConnection();
+  }
+
+  /// Cancel stream subscription, then fire-and-forget sink.close().
+  Future<void> _closeCurrentConnection() async {
+    _heartbeatTimer?.cancel();
+    _isConnected = false;
+
+    // Cancel subscription first — this immediately stops events from
+    // the old channel reaching _messageController.
+    if (_streamSubscription != null) {
+      await _streamSubscription!.cancel();
+      _streamSubscription = null;
+    }
+
+    // Close the underlying WebSocket. Don't await — sink.close() can hang.
+    if (_channel != null) {
+      final old = _channel!;
+      _channel = null;
+      unawaited(old.sink.close(status.goingAway).catchError((_) {}));
+    }
   }
 
   void dispose() {