offers to start and stop IHostedService concurrently

[pedrobsaila]

Fixes #68036

I resumed the work of @jerryk414 on #75894 PR, so shout out to him for most of the work here. I addressed the remaining remarks on that PR.

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Issue Details

---

Fixes #68036

I resumed the work of @jerryk414 on #75894 PR, so shout out to him for most of the work here. I addressed the remaining remarks on that PR.

Author:	pedrobsaila
Assignees:	-
Labels:	new-api-needs-documentation, area-Extensions-Hosting
Milestone:	-

[buyaa-n]

LGTM, thank you!

[eerhardt]

LGTM. Thanks @pedrobsaila and @jerryk414!

[ericstj]

I just happened upon this PR and was discussing with @buyaa-n and @stephentoub. Because we are still running Select over a set of async methods StartAsync|StopAsync the synchronous portion of these methods will still run sequentially on the thread calling Task.WhenAll in order to obtain Tasks to then wait on. If you wanted the synchronous portion of these methods to instead run on a background thread (and concurrently) then you'd want to wrap the call of these async methods in Task.Run.

It's unclear to me if the goal of this feature was to parallelize all of the work done in StartAsync|StopAsync or only the asynchronous portion of those methods. As of now, it only does the latter - and as @buyaa-n pointed out to me, asserts that the synchronous portion still runs in order.

@pedrobsaila @jerryk414 @eerhardt -- what do you think?

[pedrobsaila]

It's unclear to me if the goal of this feature was to parallelize all of the work done in StartAsync|StopAsync or only the asynchronous portion of those methods

I think the goal of this issue was to parallelize all of the work done in StartAsync|StopAsync

the synchronous portion of these methods will still run sequentially on the thread calling Task.WhenAll in order to obtain Tasks to then wait on

You are right, it is sequential. I'll fix it

[pedrobsaila]

please check the PR #85191 fixing the issue.
Thanks @ericstj for spotting this anomaly