Stuck on "Flutter: Hot restarting..."

[simplenotezy]

Steps to reproduce

• Attempt to start a (larger ?) app in debug mode.
• Make some code changes
• Click "Restart" - sometimes a few times

Expected results

It should restart every time without issues

Actual results

It's often stuck in "Flutter: Hot restarting..." when making changes and then restarting.

It happens several times a day, seriously hindering developer productivity - and has done so for at least 12 months - on different (mainly ARM) setups, both to me and colleagues.

Since upgrading from 3.13.1 to 3.16.9 I also started noticing that the debugger is detached from VSCode but still running on the phone as well as in XCode. I also notice that the console logs in vscode remain empty, even on restarts. Restarting the debug session usually make content reappear in console logs.

Related: #4958, flutter/flutter#139651

Code sample

I am willing to share our code-base with a flutter contributer, but apart from that I don't think theres anything concrete I can share to reproduce this issue.

Screenshots or Video

Screenshots / Video demonstration

hot.restarting.mov

Logs

It's run through vs code - not sure how I can share the logs

Flutter Doctor output

Doctor output

❯ flutter doctor -v
❯ flutter doctor -v
[✓] Flutter (Channel stable, 3.16.9, on macOS 14.1.1 23B81 darwin-arm64, locale en-DK)
    • Flutter version 3.16.9 on channel stable at /Users/mf/fvm/versions/3.16.9
    • Upstream repository https://github.com/flutter/flutter.git
    • Framework revision 41456452f2 (2 weeks ago), 2024-01-25 10:06:23 -0800
    • Engine revision f40e976bed
    • Dart version 3.2.6
    • DevTools version 2.28.5

[!] Android toolchain - develop for Android devices (Android SDK version 34.0.0)
    • Android SDK at /Users/mf/Library/Android/sdk
    ✗ cmdline-tools component is missing
      Run `path/to/sdkmanager --install "cmdline-tools;latest"`
      See https://developer.android.com/studio/command-line for more details.
    ✗ Android license status unknown.
      Run `flutter doctor --android-licenses` to accept the SDK licenses.
      See https://flutter.dev/docs/get-started/install/macos#android-setup for more details.

[✓] Xcode - develop for iOS and macOS (Xcode 15.0.1)
    • Xcode at /Applications/Xcode.app/Contents/Developer
    • Build 15A507
    • CocoaPods version 1.13.0

[✓] Chrome - develop for the web
    • Chrome at /Applications/Google Chrome.app/Contents/MacOS/Google Chrome

[✓] Android Studio (version 2022.3)
    • Android Studio at /Applications/Android Studio.app/Contents
    • Flutter plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/9212-flutter
    • Dart plugin can be installed from:
      🔨 https://plugins.jetbrains.com/plugin/6351-dart
    • Java version OpenJDK Runtime Environment (build 17.0.6+0-17.0.6b829.9-10027231)

[✓] VS Code (version 1.86.0)
    • VS Code at /Applications/Visual Studio Code.app/Contents
    • Flutter extension version 3.82.0

[DanTup]

Thanks, I was able to repro this.

It seems that overlapped hot restarts don't complete correctly:

// Restart #1
[12:33:18 PM] [DAP] [Info] ==> {"command":"restart","arguments":{"arguments":{"dartCodeDebugSessionID":"session-9008d","type":"dart","request":"launch","program":"C:\\Dev\\Test Projects\\dartcode_4980\\lib\\main.dart","projectRootPath":"C:\\Dev\\Test Projects\\dartcode_4980","cwd":"C:\\Dev\\Test Projects\\dartcode_4980","name":"Flutter (Windows)","deviceId":"windows","deviceName":"Windows (windows-x64)","toolEnv":{"FLUTTER_HOST":"VSCode","PUB_ENVIRONMENT":"vscode.dart-code","FLUTTER_ROOT":"C:\\Dev\\Google\\flutter"},"sendLogsToClient":false,"sendCustomProgressEvents":true,"allowAnsiColorOutput":true,"additionalProjectPaths":["C:\\Dev\\Test Projects\\dartcode_4980"],"args":[],"vmAdditionalArgs":[],"toolArgs":["-d","windows","--devtools-server-address","http://127.0.0.1:9100/"],"vmServicePort":0,"dartSdkPath":"C:\\Dev\\Google\\flutter\\bin\\cache\\dart-sdk","vmServiceLogFile":"C:\\Dev\\Logs\\observatory.flutter_windows_.txt","webDaemonLogFile":"C:\\Dev\\Logs\\web_daemon.flutter_windows_.txt","maxLogLineLength":99999999,"debugSdkLibraries":false,"debugExternalPackageLibraries":false,"showDartDeveloperLogs":true,"evaluateGettersInDebugViews":true,"showGettersInDebugViews":true,"evaluateToStringInDebugViews":true,"flutterSdkPath":"C:\\Dev\\Google\\flutter","useInspectorNotificationsForWidgetErrors":true,"flutterRunLogFile":"C:\\Dev\\Logs\\flutter_run.flutter_windows_.txt","flutterTestLogFile":"C:\\Dev\\Logs\\flutter_test.flutter_windows_.txt","showMemoryUsage":false,"debuggerType":2,"__sessionId":"16366636-2b95-4206-9f63-5d72281075af"}},"type":"request","seq":8}
[12:33:18 PM] [DAP] [Info] <== {"seq":88,"type":"event","body":{"message":"Hot restarting…","progressId":"hotRestart_2","title":"Flutter"},"event":"dart.progressStart"}
[12:33:18 PM] [DAP] [Info] <== {"seq":91,"type":"event","body":{"category":"stdout","output":"Restarted application in 168ms.\n"},"event":"output"}
[12:33:18 PM] [DAP] [Info] <== {"seq":92,"type":"event","body":{"progressId":"hotRestart_2"},"event":"dart.progressEnd"}
[12:33:18 PM] [DAP] [Info] <== {"seq":93,"type":"response","command":"restart","request_seq":8,"success":true}


// Restart 2 + 3 overlap
[12:33:18 PM] [DAP] [Info] ==> {"command":"restart","arguments":{"arguments":{"dartCodeDebugSessionID":"session-9008d","type":"dart","request":"launch","program":"C:\\Dev\\Test Projects\\dartcode_4980\\lib\\main.dart","projectRootPath":"C:\\Dev\\Test Projects\\dartcode_4980","cwd":"C:\\Dev\\Test Projects\\dartcode_4980","name":"Flutter (Windows)","deviceId":"windows","deviceName":"Windows (windows-x64)","toolEnv":{"FLUTTER_HOST":"VSCode","PUB_ENVIRONMENT":"vscode.dart-code","FLUTTER_ROOT":"C:\\Dev\\Google\\flutter"},"sendLogsToClient":false,"sendCustomProgressEvents":true,"allowAnsiColorOutput":true,"additionalProjectPaths":["C:\\Dev\\Test Projects\\dartcode_4980"],"args":[],"vmAdditionalArgs":[],"toolArgs":["-d","windows","--devtools-server-address","http://127.0.0.1:9100/"],"vmServicePort":0,"dartSdkPath":"C:\\Dev\\Google\\flutter\\bin\\cache\\dart-sdk","vmServiceLogFile":"C:\\Dev\\Logs\\observatory.flutter_windows_.txt","webDaemonLogFile":"C:\\Dev\\Logs\\web_daemon.flutter_windows_.txt","maxLogLineLength":99999999,"debugSdkLibraries":false,"debugExternalPackageLibraries":false,"showDartDeveloperLogs":true,"evaluateGettersInDebugViews":true,"showGettersInDebugViews":true,"evaluateToStringInDebugViews":true,"flutterSdkPath":"C:\\Dev\\Google\\flutter","useInspectorNotificationsForWidgetErrors":true,"flutterRunLogFile":"C:\\Dev\\Logs\\flutter_run.flutter_windows_.txt","flutterTestLogFile":"C:\\Dev\\Logs\\flutter_test.flutter_windows_.txt","showMemoryUsage":false,"debuggerType":2,"__sessionId":"16366636-2b95-4206-9f63-5d72281075af"}},"type":"request","seq":10}
[12:33:18 PM] [DAP] [Info] <== {"seq":95,"type":"event","body":{"message":"Hot restarting…","progressId":"hotRestart_3","title":"Flutter"},"event":"dart.progressStart"}
[12:33:19 PM] [DAP] [Info] ==> {"command":"restart","arguments":{"arguments":{"dartCodeDebugSessionID":"session-9008d","type":"dart","request":"launch","program":"C:\\Dev\\Test Projects\\dartcode_4980\\lib\\main.dart","projectRootPath":"C:\\Dev\\Test Projects\\dartcode_4980","cwd":"C:\\Dev\\Test Projects\\dartcode_4980","name":"Flutter (Windows)","deviceId":"windows","deviceName":"Windows (windows-x64)","toolEnv":{"FLUTTER_HOST":"VSCode","PUB_ENVIRONMENT":"vscode.dart-code","FLUTTER_ROOT":"C:\\Dev\\Google\\flutter"},"sendLogsToClient":false,"sendCustomProgressEvents":true,"allowAnsiColorOutput":true,"additionalProjectPaths":["C:\\Dev\\Test Projects\\dartcode_4980"],"args":[],"vmAdditionalArgs":[],"toolArgs":["-d","windows","--devtools-server-address","http://127.0.0.1:9100/"],"vmServicePort":0,"dartSdkPath":"C:\\Dev\\Google\\flutter\\bin\\cache\\dart-sdk","vmServiceLogFile":"C:\\Dev\\Logs\\observatory.flutter_windows_.txt","webDaemonLogFile":"C:\\Dev\\Logs\\web_daemon.flutter_windows_.txt","maxLogLineLength":99999999,"debugSdkLibraries":false,"debugExternalPackageLibraries":false,"showDartDeveloperLogs":true,"evaluateGettersInDebugViews":true,"showGettersInDebugViews":true,"evaluateToStringInDebugViews":true,"flutterSdkPath":"C:\\Dev\\Google\\flutter","useInspectorNotificationsForWidgetErrors":true,"flutterRunLogFile":"C:\\Dev\\Logs\\flutter_run.flutter_windows_.txt","flutterTestLogFile":"C:\\Dev\\Logs\\flutter_test.flutter_windows_.txt","showMemoryUsage":false,"debuggerType":2,"__sessionId":"16366636-2b95-4206-9f63-5d72281075af"}},"type":"request","seq":12}
[12:33:19 PM] [DAP] [Info] <== {"seq":167,"type":"event","body":{"message":"Hot restarting…","progressId":"hotRestart_4","title":"Flutter"},"event":"dart.progressStart"}
[12:33:19 PM] [DAP] [Info] ==> {"command":"restart","arguments":{"arguments":{"dartCodeDebugSessionID":"session-9008d","type":"dart","request":"launch","program":"C:\\Dev\\Test Projects\\dartcode_4980\\lib\\main.dart","projectRootPath":"C:\\Dev\\Test Projects\\dartcode_4980","cwd":"C:\\Dev\\Test Projects\\dartcode_4980","name":"Flutter (Windows)","deviceId":"windows","deviceName":"Windows (windows-x64)","toolEnv":{"FLUTTER_HOST":"VSCode","PUB_ENVIRONMENT":"vscode.dart-code","FLUTTER_ROOT":"C:\\Dev\\Google\\flutter"},"sendLogsToClient":false,"sendCustomProgressEvents":true,"allowAnsiColorOutput":true,"additionalProjectPaths":["C:\\Dev\\Test Projects\\dartcode_4980"],"args":[],"vmAdditionalArgs":[],"toolArgs":["-d","windows","--devtools-server-address","http://127.0.0.1:9100/"],"vmServicePort":0,"dartSdkPath":"C:\\Dev\\Google\\flutter\\bin\\cache\\dart-sdk","vmServiceLogFile":"C:\\Dev\\Logs\\observatory.flutter_windows_.txt","webDaemonLogFile":"C:\\Dev\\Logs\\web_daemon.flutter_windows_.txt","maxLogLineLength":99999999,"debugSdkLibraries":false,"debugExternalPackageLibraries":false,"showDartDeveloperLogs":true,"evaluateGettersInDebugViews":true,"showGettersInDebugViews":true,"evaluateToStringInDebugViews":true,"flutterSdkPath":"C:\\Dev\\Google\\flutter","useInspectorNotificationsForWidgetErrors":true,"flutterRunLogFile":"C:\\Dev\\Logs\\flutter_run.flutter_windows_.txt","flutterTestLogFile":"C:\\Dev\\Logs\\flutter_test.flutter_windows_.txt","showMemoryUsage":false,"debuggerType":2,"__sessionId":"16366636-2b95-4206-9f63-5d72281075af"}},"type":"request","seq":13}
[12:33:19 PM] [DAP] [Info] <== {"seq":168,"type":"event","body":{"message":"Hot restarting…","progressId":"hotRestart_5","title":"Flutter"},"event":"dart.progressStart"}
[12:33:19 PM] [DAP] [Info] <== {"seq":174,"type":"event","body":{"category":"stdout","output":"Restarted application in 306ms.\n"},"event":"output"}

// Only first restart completes
[12:33:19 PM] [DAP] [Info] <== {"seq":175,"type":"event","body":{"progressId":"hotRestart_3"},"event":"dart.progressEnd"}
[12:33:19 PM] [DAP] [Info] <== {"seq":176,"type":"response","command":"restart","request_seq":10,"success":true}

[DanTup]

I think the issue here is occurring in Flutter. I've written up some notes at flutter/flutter#143371 and will continue to investigate.

[simplenotezy]

Well spotted; thanks @DanTup - let us know if we can help debug in any way.