Widget preview leaks processes when terminated

[DanTup]

Steps to reproduce

Not sure if this is Windows specific.

• Create or cd to a sample Flutter project
• Run flutter widget-preview start --web-server
• Wait for "Done loading previews" to appear
• Press Ctrl+C to terminate the process
• Repeat some number of times

Here's a PowerShell script that does this in a loop 10 times:

for ($i = 1; $i -le 10; $i++) {
    $tempLog = Join-Path -Path $env:TEMP -ChildPath "flut_$i.txt"
    # Removed RedirectStandardError and the delay between processes
    $p = Start-Process -FilePath "flutter" -ArgumentList "widget-preview", "start", "--web-server" -PassThru -NoNewWindow -RedirectStandardOutput $tempLog
    $start = Get-Date
    while (-not ((Get-Content $tempLog -Raw -ErrorAction SilentlyContinue) -match "Done loading previews") -and ((Get-Date) - $start).TotalSeconds -lt 300) {
        Start-Sleep -Milliseconds 500
    }
    Stop-Process -Id $p.Id -Force -ErrorAction SilentlyContinue
}

cc @bkonyi

Expected results

No dart processes should be left running in the background.

Actual results

Lots of dart, dartaotruntime and dartvm processes are left running (some of these are DevTools and DTD).

Code sample

N/A

Screenshots or Video

Logs

No useful output is collected (even in with -v) when pressing Ctrl+C:

[        ] <- accept
[        ] Caching compiled dill
[  +23 ms] lib\main.dart is being served at http://localhost:53696
[        ] The web-server device requires the Dart Debug Chrome extension for debugging. Consider using the Chrome or Edge devices for an improved
development
           workflow.
[   +2 ms] Done loading previews.
^CTerminate batch job (Y/N)? y
PS D:\Dev\Test Projects\flutter_counter>

Flutter Doctor output

Doctor output

[!] Flutter (Channel master, 3.33.0-1.0.pre-1343, on Microsoft Windows [Version 10.0.26100.4652], locale en-GB) [3.4s]
    • Flutter version 3.33.0-1.0.pre-1343 on channel master at D:\Dev\Google\Flutter\Flutter main
    ! Warning: `dart` on your path resolves to D:\Tools\Dart\Stable\bin\dart.exe, which is not inside your current Flutter SDK checkout at
      D:\Dev\Google\Flutter\Flutter main. Consider adding D:\Dev\Google\Flutter\Flutter main\bin to the front of your path.
    ! Upstream repository git@github.com:DanTup/flutter.git is not a standard remote.
      Set environment variable "FLUTTER_GIT_URL" to git@github.com:DanTup/flutter.git to dismiss this error.
    • Framework revision 1590543f67 (3 days ago), 2025-08-08 23:18:19 -0400
    • Engine revision d77a9629f2
    • Dart version 3.10.0 (build 3.10.0-81.0.dev)
    • DevTools version 2.49.0
    • Feature flags: enable-web, enable-linux-desktop, enable-macos-desktop, enable-windows-desktop, enable-android, enable-ios, cli-animations,
      enable-native-assets, enable-lldb-debugging
    • If those were intentional, you can disregard the above warnings; however it is recommended to use "git" directly to perform update checks and
      upgrades.

[✓] Windows Version (11 Pro 64-bit, 24H2, 2009) [816ms]

[!] Android toolchain - develop for Android devices (Android SDK version 35.0.0) [1,974ms]
    • Android SDK at C:\Users\danny\AppData\Local\Android\sdk
    • Emulator version 35.4.9.0 (build_id 13025442) (CL:N/A)
    • Platform android-36, build-tools 35.0.0
    • Java binary at: C:\Program Files\Android\Android Studio\jbr\bin\java
      This is the JDK bundled with the latest Android Studio installation on this machine.
      To manually set the JDK path, use: `flutter config --jdk-dir="path/to/jdk"`.
    • Java version OpenJDK Runtime Environment (build 21.0.6+-13355223-b631.42)
    ! Some Android licenses not accepted. To resolve this, run: flutter doctor --android-licenses

[✓] Chrome - develop for the web [56ms]
    • Chrome at C:\Program Files\Google\Chrome\Application\chrome.exe

[✓] Visual Studio - develop Windows apps (Visual Studio Community 2022 17.10.5) [55ms]
    • Visual Studio at C:\Program Files\Microsoft Visual Studio\2022\Community
    • Visual Studio Community 2022 version 17.10.35122.118
    • Windows 10 SDK version 10.0.26100.0

[✓] Connected device (3 available) [97ms]
    • Windows (desktop) • windows • windows-x64    • Microsoft Windows [Version 10.0.26100.4652]
    • Chrome (web)      • chrome  • web-javascript • Google Chrome 139.0.7258.66
    • Edge (web)        • edge    • web-javascript • Microsoft Edge 138.0.3351.77

[✓] Network resources [510ms]
    • All expected network resources are available.

! Doctor found issues in 2 categories.

[bkonyi]

I don't think this is specifically an issue with widget-preview, but a general issue with signal watchers on Windows after the recent change making dart.exe an AOT runtime with Dartdev that execs dartvm.exe when we want to run from source. I filed an issue in the SDK: dart-lang/sdk#61399.

[bkonyi]

Should be resolved by dart-lang/sdk@010b696.

[DanTup]

@bkonyi unfortunately I still see this. I'm on a Flutter master from today:

PS D:\Dev\Test Projects\flutter_counter> flutter --version
Flutter 3.36.0-1.0.pre-369 • channel master • git@github.com:DanTup/flutter.git
Framework • revision 1bbbf06 (3 hours ago) • 2025-09-01 08:15:27 -0400
Engine • hash 6bc645a7238287b527f507346b7c32e386386e2c (revision 1bbbf06) (3 hours ago) • 2025-09-01 12:15:27.000Z
Tools • Dart 3.10.0 (build 3.10.0-158.0.dev) • DevTools 2.50.0

This includes Dart 3.10.0-158.0.dev, and the fix linked above is in Dart 3.10.0-144.0.dev. Running the loop from above, I still see all of the processes once complete:

I'm not sure if it's the widget preview or Dart though - should we re-open this one of the other?

[bkonyi]

I'll need to check to see if the shutdown hooks are actually being executed. When I investigated this before, I saw that they weren't.

Can you try running the tool from source using the dartvm binary and see if you can reproduce?

[DanTup]

Can you try running the tool from source using the dartvm binary and see if you can reproduce?

I'm not quite sure what you mean - could you give me a pointer?

BTW, I'm not sure if it's just bad naming, but the change in dart-lang/sdk@010b696 specifically mentions "ConsoleCtrlHandler".. Is this just any SIGINT signal, or something specific to console/Ctrl+C? (because my script just just sending SIGINT)

[bkonyi]

So I'm not able to reproduce at the latest revision on main, at least not with flutter widget-preview start. Can I get you to share that Powershell script here for me to try out?

I'm not quite sure what you mean - could you give me a pointer?

Try running something like this: $env:FLUTTER_ROOT\bin\cache\dart-sdk\bin\dartvm.exe $env:FLUTTER_ROOT\packages\flutter_tools\bin\flutter_tools.dart widget-preview start

BTW, I'm not sure if it's just bad naming, but the change in dart-lang/sdk@010b696 specifically mentions "ConsoleCtrlHandler".. Is this just any SIGINT signal, or something specific to console/Ctrl+C? (because my script just just sending SIGINT)

Yeah, this should be for any SIGINT as far as I know.

[DanTup]

So I'm not able to reproduce at the latest revision on main, at least not with flutter widget-preview start. Can I get you to share that Powershell script here for me to try out?

It's the same one in the top comment from this issue. It just starts flutter (from PATH) with the args shown and writes the output to a log waiting for Done loading previews, then kills and repeated 10 times (probably they could be spawned concurrently, since this is a bit slow 😄).

Try running something like this: $env:FLUTTER_ROOT\bin\cache\dart-sdk\bin\dartvm.exe $env:FLUTTER_ROOT\packages\flutter_tools\bin\flutter_tools.dart widget-preview start

Got it, I'll do some testing and post back shortly.