query SchedMode

[jesse99]

Writing libraries that interface with blocking foreign code is very delicate because it's way too easy for users to call the library with a SchedMode that will wind up blocking multiple tasks. Even worse the failure mode is non-deterministic and depends on the number of tasks running and the number of cores available.

One way to alleviate this is to add some sort of get_sched_mode function to task.rs. Then the library code could assert that it is not being called with a dangerous mode.

[bblum]

This would be a fine holdover solution. I think a real solution would involve simply not exposing such library functions to rust-land at all, or if so, making them scheduler-enabled -- #3367.

[jesse99]

It's a time bomb so I think there should be something in place.

[bblum]

Yes.

[Aatch]

The new scheduler will have work-stealing, so this kind of blocking is less of an issue unless you are only running a single thread.