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

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

[TeknoVenus]

Hi.

Re-tested with latest versions (preview 6 and 7) using the same setup as outlined previously (in Vagrant VM).

Behaviour has changed, but unfortunately hot reload does not work.

Currently seeing the following behaviour in preview 7:

• Start dotnet watch
• Edit .razor page in Client/Pages directory
• dotnet watch detects a change has been made

watch : File changed: /home/vagrant/hotreloadtest2/Client/Pages/Index.razor.

• No hot reload occurs in browser
• Editing a different page results in no File changed log message printed to console
• Pressing Ctrl+C to terminate dotnet watch results in the following exception

watch : Shutdown requested. Press Ctrl+C again to force exit.
watch : 
Reading hot reload capabilities failed. Using default capabilities.
watch : Process id 8333 ran for 159438ms
watch : System.Threading.Tasks.TaskCanceledException: A task was canceled.
   at Microsoft.DotNet.Watcher.Tools.BlazorWebAssemblyDeltaApplier.<>c__DisplayClass6_0.<<GetApplyUpdateCapabilitiesAsync>g__GetApplyUpdateCapabilitiesCoreAsync|0>d.MoveNext() in dotnet-watch.dll:token 0x60002a9+0x9c
--- End of stack trace from previous location ---
   at Microsoft.DotNet.Watcher.Tools.BlazorWebAssemblyHostedDeltaApplier.GetApplyUpdateCapabilitiesAsync(DotNetWatchContext context, CancellationToken cancellationToken) in dotnet-watch.dll:token 0x6000189+0xa1
   at Microsoft.DotNet.Watcher.Tools.CompilationWorkspaceProvider.GetHotReloadCapabilitiesAsync(Task`1 hotReloadCapabilitiesTask, IReporter reporter) in dotnet-watch.dll:token 0x6000193+0x64
Unhandled exception. System.Threading.Tasks.TaskCanceledException: A task was canceled.
   at Microsoft.DotNet.Watcher.Tools.BlazorWebAssemblyDeltaApplier.<>c__DisplayClass6_0.<<GetApplyUpdateCapabilitiesAsync>g__GetApplyUpdateCapabilitiesCoreAsync|0>d.MoveNext() in dotnet-watch.dll:token 0x60002a9+0x9c
--- End of stack trace from previous location ---
   at Microsoft.DotNet.Watcher.Tools.BlazorWebAssemblyHostedDeltaApplier.GetApplyUpdateCapabilitiesAsync(DotNetWatchContext context, CancellationToken cancellationToken) in dotnet-watch.dll:token 0x6000189+0xa1
   at Microsoft.DotNet.Watcher.Tools.CompilationWorkspaceProvider.CreateProject(TaskCompletionSource`1 taskCompletionSource, Task`1 hotReloadCapabilitiesTask, String projectPath, IReporter reporter, CancellationToken cancellationToken) in dotnet-watch.dll:token 0x6000192+0x1e7
   at System.Threading.Tasks.Task.<>c.<ThrowAsync>b__127_1(Object state) in System.Private.CoreLib.dll:token 0x600323f+0x0
   at System.Threading.QueueUserWorkItemCallbackDefaultContext.Execute() in System.Private.CoreLib.dll:token 0x6002f67+0x14
   at System.Threading.ThreadPoolWorkQueue.Dispatch() in System.Private.CoreLib.dll:token 0x6002f41+0x110
   at System.Threading.PortableThreadPool.WorkerThread.WorkerThreadStart() in System.Private.CoreLib.dll:token 0x600303b+0x67
   at System.Threading.Thread.StartCallback() in System.Private.CoreLib.dll:token 0x6002bb9+0xe

No high CPU usage is observed this time.

dotnet info from latest test:

.NET SDK (reflecting any global.json):
 Version:   6.0.100-preview.7.21379.14
 Commit:    22d70b47bc

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.7.21379.14/

Host (useful for support):
  Version: 6.0.0-preview.7.21377.19
  Commit:  91ba01788d

.NET SDKs installed:
  6.0.100-preview.7.21379.14 [/home/vagrant/dotnet/sdk]

.NET runtimes installed:
  Microsoft.AspNetCore.App 6.0.0-preview.7.21378.6 [/home/vagrant/dotnet/shared/Microsoft.AspNetCore.App]
  Microsoft.NETCore.App 6.0.0-preview.7.21377.19 [/home/vagrant/dotnet/shared/Microsoft.NETCore.App]

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

[msftbot[bot]]

This issue has been automatically marked as stale because it has been marked as requiring author feedback but has not had any activity for 4 days. It will be closed if no further activity occurs within 3 days of this comment. If it is closed, feel free to comment when you are able to provide the additional information and we will re-investigate.

See our Issue Management Policies for more information.