fix: parallel host probing then single connect to winner

Matthias Nott

2026-03-25 0682ae7065a7a6b8dc30ea5da049c916b757a87e

 lib/services/mqtt_service.dart
..
..
@@ -110,47 +110,43 @@
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     final clientId = await _getClientId();
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-    // Connection order: local → cached discovery → Bonjour/scan → VPN → remote
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-    final attempts = <MapEntry<String, int>>[];  // host → timeout ms
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-    if (config.localHost != null && config.localHost!.isNotEmpty) {
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-      attempts.add(MapEntry(config.localHost!, 2500));
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-    }
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-    // Try cached discovered host before scanning again
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-    if (_lastDiscoveredHost != null) {
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-      attempts.add(MapEntry(_lastDiscoveredHost!, 3000));
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-    }
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-    if (config.vpnHost != null && config.vpnHost!.isNotEmpty) {
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-      attempts.add(MapEntry(config.vpnHost!, 3000));
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-    }
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-    if (config.host.isNotEmpty) {
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-      attempts.add(MapEntry(config.host, 5000));
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-    }
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-    _mqttLog('MQTT: attempts=${attempts.map((e) => e.key).join(", ")} port=${config.port}');
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+    // Probe all hosts in parallel to find which one responds, then connect to the winner
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+    final hosts = <String>{};
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+    if (config.localHost != null && config.localHost!.isNotEmpty) hosts.add(config.localHost!);
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+    if (_lastDiscoveredHost != null) hosts.add(_lastDiscoveredHost!);
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+    if (config.vpnHost != null && config.vpnHost!.isNotEmpty) hosts.add(config.vpnHost!);
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+    if (config.host.isNotEmpty) hosts.add(config.host);
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+    _mqttLog('MQTT: probing ${hosts.length} hosts in parallel: ${hosts.join(", ")}');
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-    for (final attempt in attempts) {
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-      if (_intentionalClose) return;
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-      _mqttLog('MQTT: trying ${attempt.key}:${config.port}');
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-      try {
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-        if (await _tryConnect(attempt.key, clientId, timeout: attempt.value)) return;
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-      } catch (e) {
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-        _mqttLog('MQTT: ${attempt.key} error=$e');
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+    // Probe all configured hosts in parallel — first to respond wins
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+    String? winner;
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+    if (hosts.isNotEmpty) {
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+      final probes = hosts.map((h) => _probeHost(h, config.port)).toList();
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+      // Also start discovery in parallel
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+      if (_lastDiscoveredHost == null) {
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+        probes.add(() async {
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+          final discovered = await _discoverViaMdns();
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+          if (discovered != null) {
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+            _lastDiscoveredHost = discovered;
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+            return discovered;
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+          }
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+          return null;
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+        }());
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       }
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+      final results = await Future.wait(probes);
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+      winner = results.firstWhere((r) => r != null, orElse: () => null);
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+    } else if (_lastDiscoveredHost == null) {
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+      // No configured hosts — try discovery only
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+      winner = await _discoverViaMdns();
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+      if (winner != null) _lastDiscoveredHost = winner;
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     }
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-    // All configured hosts failed — try Bonjour/subnet scan (only once, not on retry)
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-    if (_lastDiscoveredHost == null && !_intentionalClose) {
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-      _mqttLog('MQTT: trying Bonjour/subnet discovery...');
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-      final discovered = await _discoverViaMdns();
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-      if (discovered != null && !_intentionalClose) {
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-        _lastDiscoveredHost = discovered;
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-        _mqttLog('MQTT: discovered $discovered, connecting...');
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-        try {
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-          if (await _tryConnect(discovered, clientId, timeout: 3000)) return;
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-        } catch (e) {
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-          _mqttLog('MQTT: discovered host $discovered error=$e');
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-        }
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-      } else {
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-        _mqttLog('MQTT: discovery returned nothing');
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+    if (winner != null && !_intentionalClose) {
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+      _mqttLog('MQTT: probe winner: $winner, connecting...');
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+      try {
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+        if (await _tryConnect(winner, clientId, timeout: 5000)) return;
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+      } catch (e) {
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+        _mqttLog('MQTT: connect to $winner failed: $e');
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       }
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     }
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