Sleep in Spock's Mock waits when run by CompletableFuture

When I separately run the runAsyncWithMock test, it waits for 3 seconds until the mock's execution is finalised, rather than get terminated like the other 2 tests.

I was not able to figure out why.

It is interesting that:

1. When multiple Runnables are executed by CompletableFuture.runAsync in a row in the runAsyncWithMock test, only the first one waits, the others not.
2. When having multiple duplicated runAsyncWithMock tests, all of them run for 3s when the whole specification is executed.
3. When using Class instance rather than a Mock, the test is finalised immediately.

Any idea what I got wrong?

My configuration:

• Spock 1.3-groovy-2.4
• Groovy 2.4.15

The code:

class SpockCompletableFutureTest extends Specification {
    def runnable = Stub(Runnable) {
        run() >> {
            println "${Date.newInstance()} BEGIN1 in thread ${Thread.currentThread()}"
            sleep(3000)
            println "${Date.newInstance()} END1   in thread ${Thread.currentThread()}"
        }
    }

    def "runAsyncWithMock"() {
        when:
        CompletableFuture.runAsync(runnable)

        then:
        true
    }

    def "runAsyncWithMockAndClosure"() {
        when:
        CompletableFuture.runAsync({ runnable.run() })

        then:
        true
    }

    def "runAsyncWithClass"() {
        when:
        CompletableFuture.runAsync(new Runnable() {
            void run() {
                println "${Date.newInstance()} BEGIN2 in thread ${Thread.currentThread()}"
                sleep(3000)
                println "${Date.newInstance()} END2   in thread ${Thread.currentThread()}"
            }
        })

        then:
        true
    }
}