NullPointerException when `blockedThreadDetectionEnabled` is set to `true`

[L7R7]

I just tried the 3.5.0-RC1 version in one of our services, mainly to test the thread blocking detection introduced in #3387 and I see the following error in the logs:

Exception in thread "io-compute-1" java.lang.NullPointerException: Cannot invoke "java.util.concurrent.atomic.AtomicBoolean.get()" because the return value of "cats.effect.unsafe.WorkerThread.parked()" is null
 	at cats.effect.unsafe.WorkerThread.run(WorkerThread.scala:430)

I can only guess, but it sounds to me like it could relate to the thread blocking detection

[L7R7]

I don't see the exception with thread blocking detection disabled.
So it's probably this line?

cats-effect/core/jvm/src/main/scala/cats/effect/unsafe/WorkerThread.scala

Line 430 in ed89f22

val parked = thread.parked.get()

[durban]

It's definitely that line. I think this is a race condition when a cached thread returns to the pool, and is not re-initialized yet. (Or it may be, but thread.parked is a racy read, so we don't necessarily see it.)

[durban]

I see 2 options to fix this:

1. We say that this blocking detection is a best-effort thing, and simply check for null (and do nothing if it's null).
2. We get the parked AtomicBoolean from pool.parkedSignals (I think we always see those).