Blazor wasm: dotnet watch hot reload 100% CPU on Ubuntu

[TeknoVenus]

Describe the bug

When running a hosted blazor wasm app, dotnet watch hangs with 100% CPU usage after a file is changed on Ubuntu 16.04 and 20.04 (tested platforms, could affect others). It detects the file has changed and then hangs. Does not exit when Ctrl+C is pressed and must be killed with SIGTERM.

To Reproduce

• Create blazor server app: dotnet new blazorwasm --hosted -o TestingHotReloadWasm --no-https
• Start app: cd Server && dotnet watch
• Make change to Client/Pages/Index.razor - edit <h1> text. Save file
• dotnet watch will detect file change but will hang and use 100% CPU. Pressing Ctrl+C will not quit the command

❯ dotnet watch --verbose
watch : Found HotReloadProfile=blazorwasmhosted. Watching with hot-reload
watch : Hot reload enabled. For a list of supported edits, see https://aka.ms/dotnet/hot-reload. Press "Ctrl + R" to restart.
watch : Evaluating dotnet-watch file set.
watch : Running MSBuild target 'GenerateWatchList' on '/home/stephen.foulds/Documents/tmp/TestingHotReloadWasm/Server/TestingHotReloadWasm.Server.csproj'
watch : Started '/snap/dotnet-sdk/127/dotnet' '' with process id 12860
watch : Process id 12860 ran for 678ms
watch : Watching 36 file(s) for changes
watch : Building...
watch : Started '/snap/dotnet-sdk/127/dotnet' '' with process id 12919
  Determining projects to restore...
  All projects are up-to-date for restore.
  You are using a preview version of .NET. See: https://aka.ms/dotnet-core-preview
  You are using a preview version of .NET. See: https://aka.ms/dotnet-core-preview
  TestingHotReloadWasm.Shared -> /home/stephen.foulds/Documents/tmp/TestingHotReloadWasm/Shared/bin/Debug/net6.0/TestingHotReloadWasm.Shared.dll
  TestingHotReloadWasm.Client -> /home/stephen.foulds/Documents/tmp/TestingHotReloadWasm/Client/bin/Debug/net6.0/TestingHotReloadWasm.Client.dll
  TestingHotReloadWasm.Client (Blazor output) -> /home/stephen.foulds/Documents/tmp/TestingHotReloadWasm/Client/bin/Debug/net6.0/wwwroot
  TestingHotReloadWasm.Server -> /home/stephen.foulds/Documents/tmp/TestingHotReloadWasm/Server/bin/Debug/net6.0/TestingHotReloadWasm.Server.dll
watch : Process id 12919 ran for 1602ms
watch : dotnet-watch is configured to launch a browser on ASP.NET Core application startup.
watch : Refresh server running at wss://localhost:33783.
watch : Using MSBuild at '/snap/dotnet-sdk/127/sdk/6.0.100-preview.4.21255.9/' to load projects.
watch : Started '/home/stephen.foulds/Documents/tmp/TestingHotReloadWasm/Server/bin/Debug/net6.0/TestingHotReloadWasm.Server' '' with process id 13033
watch : Running TestingHotReloadWasm with the following arguments: run
watch : Started
info: Microsoft.Hosting.Lifetime[14]
      Now listening on: http://localhost:5000
watch : Launching browser.
info: Microsoft.Hosting.Lifetime[0]
      Application started. Press Ctrl+C to shut down.
info: Microsoft.Hosting.Lifetime[0]
      Hosting environment: Development
info: Microsoft.Hosting.Lifetime[0]
      Content root path: /home/stephen.foulds/Documents/tmp/TestingHotReloadWasm/Server
watch : File changed: /home/stephen.foulds/Documents/tmp/TestingHotReloadWasm/Client/Pages/Index.razor.

Exceptions (if any)

Further technical details

Reproduced in the following environments. One installed from snap, the other installed manually by downloading tar.gz and unpacking to $HOME/dotnet.

❯ dotnet --info
.NET SDK (reflecting any global.json):
 Version:   6.0.100-preview.4.21255.9
 Commit:    950e4949a7

Runtime Environment:
 OS Name:     ubuntu
 OS Version:  16.04
 OS Platform: Linux
 RID:         ubuntu.16.04-x64
 Base Path:   /snap/dotnet-sdk/127/sdk/6.0.100-preview.4.21255.9/

Host (useful for support):
  Version: 6.0.0-preview.4.21253.7
  Commit:  bfd6048a60

.NET SDKs installed:
  6.0.100-preview.4.21255.9 [/snap/dotnet-sdk/127/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.0-preview.4.21253.5 [/snap/dotnet-sdk/127/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.0-preview.4.21253.7 [/snap/dotnet-sdk/127/shared/Microsoft.NETCore.App]

To install additional .NET runtimes or SDKs:
  https://aka.ms/dotnet-download

vagrant@ubuntu-focal:~$ dotnet --info
.NET SDK (reflecting any global.json):
 Version:   6.0.100-preview.4.21255.9
 Commit:    950e4949a7

Runtime Environment:
 OS Name:     ubuntu
 OS Version:  20.04
 OS Platform: Linux
 RID:         ubuntu.20.04-x64
 Base Path:   /home/vagrant/dotnet/sdk/6.0.100-preview.4.21255.9/

Host (useful for support):
  Version: 6.0.0-preview.4.21253.7
  Commit:  bfd6048a60

.NET SDKs installed:
  6.0.100-preview.4.21255.9 [/home/vagrant/dotnet/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.0-preview.4.21253.5 [/home/vagrant/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.0-preview.4.21253.7 [/home/vagrant/dotnet/shared/Microsoft.NETCore.App]

To install additional .NET runtimes or SDKs:
  https://aka.ms/dotnet-download

[pranavkm]

Thanks for the issue report @TeknoVenus. Are you seeing this with the blazorwasm hosted template? I ask because I was recently diagnosing issues with watch on a WSL2 Ubuntu image, and didn't see the issue on an app created from the template.

[TeknoVenus]

Yes I was seeing this with the hosted template, but I also see it with the non-hosted version as well.

Hot reload works perfectly for the blazorserver template on the same machine, so seems to be limited to wasm.

[mkArtakMSFT]

Thanks for contacting us, @TeknoVenus.
We think this may have been already fixed in preview 6 builds. Can you try the nightly build and confirm?

[TeknoVenus]

Tested with the latest Preview 5 build and can still reproduce the issue:

❯ dotnet --info
.NET SDK (reflecting any global.json):
 Version:   6.0.100-preview.5.21302.13
 Commit:    d6380bcae7

Runtime Environment:
 OS Name:     ubuntu
 OS Version:  16.04
 OS Platform: Linux
 RID:         ubuntu.16.04-x64
 Base Path:   /snap/dotnet-sdk/131/sdk/6.0.100-preview.5.21302.13/

Host (useful for support):
  Version: 6.0.0-preview.5.21301.5
  Commit:  ec3e0b276b

.NET SDKs installed:
  6.0.100-preview.5.21302.13 [/snap/dotnet-sdk/131/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.0-preview.5.21301.17 [/snap/dotnet-sdk/131/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.0-preview.5.21301.5 [/snap/dotnet-sdk/131/shared/Microsoft.NETCore.App]

To install additional .NET runtimes or SDKs:
  https://aka.ms/dotnet-download

Also tested in a Vagrant VM and can reproduce the same result.

• Use the Vagrant file below to create a new Vagrant VM (vagrant up). This will create a bare Ubuntu 20 install then download/unpack the preview 5 SDK
• Once VM provisioned, ssh in with vagrant ssh
• Create new project dotnet new blazorwasm --hosted -o hotreloadtest --no-https
• Start app (cd Server && dotnet watch)
• Open new ssh instance and edit a razor page. dotnet watch will detect a change has occured then hang and use 100% cpu

    PID USER      PR  NI    VIRT    RES    SHR S  %CPU  %MEM     TIME+ COMMAND
   7450 vagrant   20   0 3628904 232972  82328 S 113.3  11.4   0:58.20 dotnet
   7383 vagrant   20   0 3566236 138588  64216 S   0.0   6.8   0:05.03 dotnet
   7436 vagrant   20   0 3558864  62060  42848 S   0.0   3.0   0:00.55 dotnet
   7524 vagrant   20   0   12.0g 152304  61612 S   0.0   7.5   0:05.96 dotnet
   7616 vagrant   20   0   12.0g  98868  51584 S   0.0   4.9   0:00.76 dotnet

Vagrant.configure("2") do |config|
  config.vm.network "public_network"
  config.vm.box = "ubuntu/focal64"
  config.vm.provider "virtualbox" do |vb|
    vb.memory = "2048"
  end

  config.vm.provision "shell", inline: <<-SHELL
    wget https://packages.microsoft.com/config/ubuntu/20.10/packages-microsoft-prod.deb -O packages-microsoft-prod.deb
    dpkg -i packages-microsoft-prod.deb
    apt-get update
    apt-get install -y apt-transport-https
    apt-get update
  SHELL

  config.vm.provision "shell", privileged: false, inline: <<-SHELL
      rm -rf $HOME/dotnet
      rm -rf ./dotnet-sdk-6.0.100-preview.5.21302.13-linux-x64.tar.gz
      wget -q https://download.visualstudio.microsoft.com/download/pr/919880af-ab5a-4b58-8058-7baaea4a09d1/4fe186d747cf416cbdc83fd8354e15ea/dotnet-sdk-6.0.100-preview.5.21302.13-linux-x64.tar.gz
      mkdir -p $HOME/dotnet && tar zxf dotnet-sdk-6.0.100-preview.5.21302.13-linux-x64.tar.gz -C $HOME/dotnet
      echo "export DOTNET_ROOT=$HOME/dotnet" >> ~/.bashrc
      echo "path=$path:$home/DOTNET" >> ~/.bashrc
  SHELL
end