Letting a task complete while redirecting?

We are developing a monolithic web application – very stateful. It handles both HTTP requests and long lived SignalR connections. (In ASP.NET Core 3.1 – we will upgrade to .NET 5 later.)

We do a redirect from a login page to our “main page”. The main page takes a while to load and initialize, after that it connects with SignalR. We also have a lot of work to do at the server side. Doing the server work in the login request (before redirecting to the main page) would slow down the login.

“Oh, let’s use a Task then!”, I thought. That is, put the server work in a Task, save that in the user state, and let it execute in parallel with the loading of the main page. Something like this (simplified):

public static async Task ServerSideInit()
    // do a lot of init work

// at the end of the controller handling the login page POST:
UserState.BackgroundTask = ServerSideInit();

// when the main page connects via SignalR:
try {
    await UserState.BackgroundTask;
catch {
    // handle errors in the init work

This would really speed things up. It won’t matter if the page loading or the init work finishes first – we await the Task. And the work in ServerSideInit() isn’t critical. If something happens and the main page never connects, the UserState (and the Task) will be destroyed after a timeout – and that’s perfectly OK. (There are some caveats. We would e.g. have to use IServiceProvider to create/dispose a scope in ServerSideInit(), so we get a scoped DbContext outside of the controller. But that’s OK.)

But then I read that there is a risk the ASP.NET Core framework shuts down the Task when wrapping up the POST request! (Do you have to await async methods?) The simple HostingEnvironment.QueueBackgroundWorkItem isn’t available any longer. There is a new BackgroundService class, though. (https://docs.microsoft.com/en-us/aspnet/core/fundamentals/host/hosted-services?view=aspnetcore-3.1&tabs=visual-studio) But registering a service and queueing jobs seems like a very cumbersome solution… We just want to fire a task that will take a couple of seconds to complete, and let that continue to run after ASP.NET Core has finished handling the POST request.

I’m not very experienced with ASP.NET Core… So I’d be very grateful for some input! Will my simple solution not work? Will the task be terminated by the framework? Is there some easier way to tell the framework “please don’t touch this Task”? Or is BackgroundService the way to go?