A DESCRIPTION OF THE PROBLEM :
Calling ScheduledExecutorService.schedule with a long delay concurrently with other calls to schedule prevents the other tasks from ever running.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
Concurrently call ScheduledExecutorService.schedule with a long delay and a short delay. When the race condition is encountered, the task with the short delay will never run.

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The task with the short delay always runs.
ACTUAL -
The task with the short delay doesn't always run.

---------- BEGIN SOURCE ----------

import java.util.Collection;
import java.util.concurrent.*;
import java.util.stream.Collectors;
import java.util.stream.Stream;

public class Main
{
    public static void main(String[] args) throws ExecutionException, InterruptedException {
        ScheduledExecutorService scheduledExecutorService = Executors.newSingleThreadScheduledExecutor();

        Runnable task = () -> {};

        Runnable longDelayedTask = () -> scheduledExecutorService.schedule(task, Long.MAX_VALUE, TimeUnit.NANOSECONDS);

        int numberOfConcurrentThreads = 100; // The higher this is, the higher the chance of reproducing the bug

        Collection<Thread> threads =
            Stream.generate(() -> new Thread(longDelayedTask))
                .limit(numberOfConcurrentThreads)
                .collect(Collectors.toList());

        threads.forEach(Thread::start); // Removing this line prevents the next scheduled task from hanging.

        ScheduledFuture<?> scheduledFuture = scheduledExecutorService.schedule(task, 0, TimeUnit.NANOSECONDS);

        scheduledFuture.get(); // waits for the task to finish, but sometimes it never even runs

        scheduledExecutorService.shutdownNow();
    }
}

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CUSTOMER SUBMITTED WORKAROUND :
Synchronize calls to ScheduledExecutorService.schedule or don't use long delays (not sure what the max acceptable delay is).

FREQUENCY : often