Schedule commands in current thread context

[ywelsch]

Changes ThreadPool's schedule method to run the schedule task in the context of the thread that scheduled the task.

This is the more sensible default for this method, and eliminates a range of bugs where the current thread context is mistakenly dropped.

Closes #17143

[elasticmachine]

Pinging @elastic/es-core-infra (:Core/Infra/Core)

[nik9000]

+1

It has been a while since I looked at this! The reason I never made this change when I worked on this before was that I thought that some calls to schedule intentionally didn't keep the thread context. I wonder if it'd be best to explicitly throw away the context in the places where we don't want to preserve it. It might be as simple as adding a boolean to schedule or something like that. I hate to change the signature but that'd force you to think about whether or not you want to preserve the context every time you call schedule.

[jimczi]

LGTM, it's hard to say if there are usages today that rely on the current default. Although scratching the context can make more harm than preserving it so I think we're on the right side with this change.

[ywelsch]

It might be as simple as adding a boolean to schedule or something like that. I hate to change the signature but that'd force you to think about whether or not you want to preserve the context every time you call schedule

I thought about this initially, and looked at a lot of callers, coming to the conclusion that I couldn't find any place where not preserving the calling context would make sense. Adding a boolean would i think add more confusion, and can still be introduced if we have an actual case where we need it.

[nik9000]

LGTM then!