Parallel use of mocks with deep stubbing may lead to ConcurrentModificationException

[antonantsyferov]

Java8 / JUnit5
Mockito version: 3.11.2

Test class:

import org.junit.jupiter.api.RepeatedTest;
import org.mockito.Answers;

import java.util.List;
import java.util.stream.IntStream;

import static org.mockito.Mockito.mock;

class ConcurrencyTest {

    @RepeatedTest(value = 1000)
    void test() {
        final Service mock = mock(Service.class, Answers.RETURNS_DEEP_STUBS);
        IntStream.range(1, 100).parallel().forEach(i -> mock.doSomething());
    }

    interface Service {
        List<String> doSomething();
    }
}

Error: 30/5000 iterations fail with java.util.ConcurrentModificationException

Stacktrace:

Caused by: java.util.ConcurrentModificationException
	at java.util.LinkedList$ListItr.checkForComodification(LinkedList.java:966)
	at java.util.LinkedList$ListItr.next(LinkedList.java:888)
	at org.mockito.internal.stubbing.defaultanswers.ReturnsDeepStubs.deepStub(ReturnsDeepStubs.java:85)
	at org.mockito.internal.stubbing.defaultanswers.ReturnsDeepStubs.answer(ReturnsDeepStubs.java:75)
	at org.mockito.Answers.answer(Answers.java:99)
	at org.mockito.internal.handler.MockHandlerImpl.handle(MockHandlerImpl.java:110)
	at org.mockito.internal.handler.NullResultGuardian.handle(NullResultGuardian.java:29)
	at org.mockito.internal.handler.InvocationNotifierHandler.handle(InvocationNotifierHandler.java:34)
	at org.mockito.internal.creation.bytebuddy.MockMethodInterceptor.doIntercept(MockMethodInterceptor.java:82)
	at org.mockito.internal.creation.bytebuddy.MockMethodInterceptor.doIntercept(MockMethodInterceptor.java:56)
	at org.mockito.internal.creation.bytebuddy.MockMethodInterceptor$DispatcherDefaultingToRealMethod.interceptAbstract(MockMethodInterceptor.java:161)
	at ConcurrencyTest$Service$MockitoMock$351714249.doSomething(Unknown Source)
	at ConcurrencyTest.lambda$test$0(ConcurrencyTest.java:14)
	at java.util.stream.ForEachOps$ForEachOp$OfInt.accept(ForEachOps.java:204)
	at java.util.stream.Streams$RangeIntSpliterator.forEachRemaining(Streams.java:110)
	at java.util.Spliterator$OfInt.forEachRemaining(Spliterator.java:693)
	at java.util.stream.AbstractPipeline.copyInto(AbstractPipeline.java:482)
	at java.util.stream.ForEachOps$ForEachTask.compute(ForEachOps.java:290)
	at java.util.concurrent.CountedCompleter.exec(CountedCompleter.java:731)
	at java.util.concurrent.ForkJoinTask.doExec(ForkJoinTask.java:289)
	at java.util.concurrent.ForkJoinPool$WorkQueue.runTask(ForkJoinPool.java:1056)
	at java.util.concurrent.ForkJoinPool.runWorker(ForkJoinPool.java:1692)
	at java.util.concurrent.ForkJoinWorkerThread.run(ForkJoinWorkerThread.java:175)

P.S. If the Answers.RETURNS_DEEP_STUBS is removed, no error occurred.