Causality is lost when Task.Run is used

[davidfowl]

Causality is lost when Task.Run is used. This makes sense but I wonder if it's possible to stitch the async stacks in this case. Here's the code:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Hosting;

namespace WebApplication352
{
    public class Startup
    {
        // This method gets called by the runtime. Use this method to add services to the container.
        // For more information on how to configure your application, visit https://go.microsoft.com/fwlink/?LinkID=398940
        public void ConfigureServices(IServiceCollection services)
        {
        }

        // This method gets called by the runtime. Use this method to configure the HTTP request pipeline.
        public void Configure(IApplicationBuilder app, IWebHostEnvironment env)
        {
            if (env.IsDevelopment())
            {
                app.UseDeveloperExceptionPage();
            }

            app.UseRouting();

            app.UseEndpoints(endpoints =>
            {
                endpoints.MapGet("/", async context =>
                {
                    await Task.Run(async () =>
                    {
                        await Task.Delay(10000);

                        await context.Response.WriteAsync("Hello World!");
                    });
                });
            });
        }
    }
}

This is what dumpasync -stacks looks like:

0000021214e64c18 00007ff8325691c0       96          0 WebApplication352.Startup+<>c__DisplayClass1_0+<<Configure>b__2>d
Async "stack":
.0000021114e82640 System.Threading.Tasks.UnwrapPromise`1[[System.Threading.Tasks.VoidTaskResult, System.Private.CoreLib]]
--------------------------------------------------------------------------------

0000021114e82688 00007ff832568770       96          0 WebApplication352.Startup+<>c+<<Configure>b__1_1>d
Async "stack":
.0000021114e82728 (0) Microsoft.AspNetCore.Routing.EndpointMiddleware+<<Invoke>g__AwaitRequestTask|6_0>d
..0000021114e827b0 (0) Microsoft.AspNetCore.Diagnostics.DeveloperExceptionPageMiddleware+<Invoke>d__9
...0000021114e82840 (1) Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol+<ProcessRequests>d__217`1[[System.__Canon, System.Private.CoreLib]]  
....0000021114e82900 (0) Microsoft.AspNetCore.Server.Kestrel.Core.Internal.Http.HttpProtocol+<ProcessRequestsAsync>d__216`1[[System.__Canon, System.Private.CoreLib]]

cc @stephentoub

[davidfowl]

Simpler example:

async Task StateMachine1()
{
    await Task.Run(StateMachine2);
}

async Task StateMachine2()
{
    await Task.Delay(5000);
}

0000016a796d1890 00007ff82fa127b8       96          0 WebApplication352.Program+<StateMachine2>d__3
Async "stack":
.0000016bf9725180 System.Threading.Tasks.UnwrapPromise`1[[System.Threading.Tasks.VoidTaskResult, System.Private.CoreLib]]
--------------------------------------------------------------------------------
0000016bf9725288 00007ff82f9feda8       96          0 WebApplication352.Program+<StateMachine1>d__2
--------------------------------------------------------------------------------

[stephentoub]

I've rewritten dumpasync in C# and I've addressed this as well (or at least the simpler repro successfully connects the stacks... I didn't try the original).