Reactor plugin will cause app error

[will-zdu]

Describe the bug
A clear and concise description of what the bug is.
ound error
Caused by: java.lang.IllegalStateException: block()/blockFirst()/blockLast() are blocking, which is not supported in thread single-1
at reactor.core.publisher.BlockingSingleSubscriber.blockingGet(BlockingSingleSubscriber.java:83)
at reactor.core.publisher.Mono.block(Mono.java:1707)
at com.example.examplereactor.TestHook1.lambda$main$0(TestHook1.java:46)
at reactor.core.publisher.FluxStream.subscribe(FluxStream.java:48)
at reactor.core.publisher.InternalFluxOperator.subscribe(InternalFluxOperator.java:62)
at reactor.core.publisher.FluxSubscribeOn$SubscribeOnSubscriber.run(FluxSubscribeOn.java:194)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:84)
at reactor.core.scheduler.WorkerTask.call(WorkerTask.java:37)
at java.util.concurrent.FutureTask.run(FutureTask.java:266)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.access$201(ScheduledThreadPoolExecutor.java:180)
at java.util.concurrent.ScheduledThreadPoolExecutor$ScheduledFutureTask.run(ScheduledThreadPoolExecutor.java:293)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624)
at java.lang.Thread.run(Thread.java:748)

Steps to reproduce
If possible, provide a recipe for reproducing the error.

package app.controller;

import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
import reactor.core.CoreSubscriber;
import reactor.core.Scannable;
import reactor.core.publisher.Hooks;
import reactor.core.publisher.Mono;
import reactor.core.publisher.Operators;
import reactor.core.scheduler.Schedulers;

import java.util.concurrent.Callable;
import java.util.concurrent.TimeUnit;
import java.util.function.BiFunction;

public class TestHook {

    private static final Logger logger = LoggerFactory.getLogger(TestHook.class);

    static{
        try{
            Hooks.onEachOperator("testHook", Operators.lift(new Lifter<>()));
        }catch (Throwable ignored){
        }
    }


    public static class Lifter<T>
            implements BiFunction<Scannable, CoreSubscriber<? super T>, CoreSubscriber<? super T>> {

        /** Holds reference to strategy to prevent it from being collected. */

        @Override
        public CoreSubscriber<? super T> apply(Scannable publisher, CoreSubscriber<? super T> sub) {
            return sub;
        }
    }

    public static void main(String[] args) throws InterruptedException {
        Callable<String> callable = () -> {
            logger.info("Operation enter call1: ");
            Mono.just("test1").block();
            return "call1";
        };


        Mono.defer(() -> Mono.fromCallable(callable)
                             .publishOn(Schedulers.elastic())
                             .flatMap(source -> {
                                 logger.info("Again enter map :{} ", source);
                                 return Mono.just(source);
                             })).subscribeOn(Schedulers.single()).subscribe();


        TimeUnit.SECONDS.sleep(3);
    }
}

What did you expect to see?
A clear and concise description of what you expected to see.
no error found
What did you see instead?
A clear and concise description of what you saw instead.
reactor/reactor-core#3450
reactor/reactor-core#3452
What version are you using?
(e.g., v0.9.0, 393e4a2, etc)
anyone will be reported it ,it's a bug of reactor
Environment
Compiler: (e.g., "AdoptOpenJDK 11.0.6")
OS: (e.g., "Ubuntu 20.04")
Runtime (if different from JDK above): (e.g., "Oracle JRE 8u251")
OS (if different from OS compiled on): (e.g., "Windows Server 2019")

Additional context
Add any other context about the problem here.
I have add a bug to reactor ,but they just suggest me to change my code ,but as javaagent developer ,we can not control app's code

[mateuszrzeszutek]

Hey @will-zdu ,

While I changed our instrumentation to be slightly less impactful in #8571, the Reactor maintainers are right and the Mono.just(...).block() call in your Callable should not really be there in the first place. Consider changing your application code and fixing that.