NullPointerException in TaskRestartCallback

[rfkm]

Hello. We are encountering the problem of some tasks getting stuck on extremely rare occasions. The thread dump did not reveal any obviously blocked threads in our code base. However, we found that an uncaught error reporter reported the following NPE:

java.lang.NullPointerException
        at monix.eval.internal.TaskRestartCallback.run(TaskRestartCallback.scala:65)
        at monix.execution.internal.Trampoline.monix$execution$internal$Trampoline$$immediateLoop(Trampoline.scala:66)
        at monix.execution.internal.Trampoline$ResumeRun$1.run(Trampoline.scala:51)
        at monix.execution.schedulers.TracingRunnable.run(TracingRunnable.scala:33)
        at java.base/java.util.concurrent.ForkJoinTask$RunnableExecuteAction.exec(ForkJoinTask.java:1426)
        at java.base/java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:290)
        at java.base/java.util.concurrent.ForkJoinPool$WorkQueue.topLevelExec(ForkJoinPool.java:1020)
        at java.base/java.util.concurrent.ForkJoinPool.scan(ForkJoinPool.java:1656)
        at java.base/java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1594)
        at java.base/java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:177)

I'm not sure but I suspect that the scheduled TaskRestartCallback was executed multiple times.

monix/monix-eval/shared/src/main/scala/monix/eval/internal/TaskRestartCallback.scala

Lines 63 to 67 in de87984

	final def run(): Unit = {
	val fn = this.register
	this.register = null
	fn(context, this)
	}

I have not yet been able to create a minimal reproduction environment, but there is a place in our code where we are using scala.concurrent.blocking and it may have something to do with it.

monix/monix-execution/shared/src/main/scala/monix/execution/internal/Trampoline.scala

Lines 46 to 61 in de87984

	protected final def forkTheRest(ec: ExecutionContext): Unit = {
	final class ResumeRun(head: Runnable, rest: ChunkedArrayQueue[Runnable]) extends Runnable {
	def run(): Unit = {
	immediateQueue.enqueueAll(rest)
	immediateLoop(head, ec)
	}
	}
	val head = immediateQueue.dequeue()
	if (head ne null) {
	val rest = immediateQueue
	immediateQueue = makeQueue()
	ec.execute(new ResumeRun(head, rest))
	}
	}

The blocking context may cause forkTheRest to be invoked. Is this a thread-safe implementation?
Since ChunkedArrayQueue does not appear to be thread-safe, isn't it dangerous to potentially reference it from other threads?
I am not very familiar with the code inside monix, so it is very possible that I am misunderstanding something, but in any case, I am sure that something strange is going on and I would appreciate your help in finding out the cause.