Make BackgroundService run ExecuteAsync as task

[ericstj]

Fixes#36063

That issue requested that we move the entire BackgroundService.ExecuteAsync to the async part of the method rather than give the service a chance to do synchronous work before it's first await. In this way BackgroundServices will not hold up the start of the application and folks are free to implement ExecuteAsync as a completely blocking method.

This is a breaking change, but one that was deemed acceptable.

[Copilot]

Pull Request Overview

This PR ensures that all of BackgroundService.ExecuteAsync runs on a background thread to avoid blocking startup, and updates tests to validate the new asynchronous behavior, including support for fully synchronous ExecuteAsync implementations.

• Wraps ExecuteAsync in Task.Run within StartAsync to push all execution off the caller thread.
• Updates existing tests to expect TaskCanceledException on immediate cancellation and renames/refactors them.
• Adds new tests covering synchronous ExecuteAsync implementations and their cancellation/stop behavior.

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
src/libraries/Microsoft.Extensions.Hosting.Abstractions/src/BackgroundService.cs	Wraps ExecuteAsync call in Task.Run, capturing the returned task.
src/libraries/Microsoft.Extensions.Hosting/tests/UnitTests/BackgroundServiceTests.cs	Renames/refactors tests and adds new tests for sync execution and stop.

[WeihanLi]

Should we expose a config workaround for this breaking change

[ericstj]

Should we expose a config workaround for this breaking change

That was specifically covered in #36063 and the API proposal around it. The API to control this was rejected and as a rule we don't proactively add AppContext compat switches until we have a strong signal that we need them. The belief here is that most folks will prefer that their BackgroundTask has the entirety of its ExecuteAsync method run on the background thread. Folks already have the workaround to override StartAsync and do some work before calling base.StartAsync, implement IHostedLifecycleService on their BackGroundService, or forgo BackgroundService and implement their own IHostedService

[lonix1]

Thank you for finally addressing this! I assume it will be part of v10 to be released in November.

Given the number of years this was open, the large number of people involved, and the potentially breaking or complex changes required on working systems, please reopen the original issue. It was closed by a bot.

There will probably be a need by some to discuss it after upgrading, and it will be very hard to find the correct place; it took me a long time to find that thread and understand the context and underlying issue. You will have many new threads and confusion because of it. Just keep the original one open, please.