tf::Semaphore with exception may cause deadlocks in some situations #732

void semaphore_complex1(int semaphore_size) {
 
    tf::Executor executor(semaphore_size * 4);
    tf::Taskflow taskflow;
 
    tf::Semaphore semaphore(semaphore_size);
 
    tf::Task init = taskflow.emplace([]() {});
    for (size_t i = 0; i < semaphore_size * 4; ++i)
    {
        tf::Task A = taskflow.emplace([]() {});
        tf::Task B = taskflow.emplace([]() {
            std::this_thread::sleep_for(std::chrono::seconds(1));
            throw std::runtime_error("exception");
            });
        tf::Task C = taskflow.emplace([]() {});
        tf::Task D = taskflow.emplace([]() {});
 
        init.precede(A);
        A.precede(B);
        B.precede(C);
        C.precede(D);
 
        A.acquire(semaphore);
        D.release(semaphore);
    }
 
    REQUIRE(semaphore.value() == semaphore_size);
 
    auto future = executor.run(taskflow);
    auto fn = [&]() {
        auto res = future.wait_for(std::chrono::seconds(60));
        if (res == std::future_status::timeout)
        {
            FAIL("Timeout");
        }
        FAIL("No exception?!");
    };
 
    // when B throws the exception, D will not run and thus semaphore is not released
    REQUIRE_THROWS_WITH_AS(fn(), "exception", std::runtime_error);
 
    REQUIRE(semaphore.value() == 0);
 
    // reset the semaphore to a clean state before running the taskflow again
    semaphore.reset();
 
    REQUIRE(semaphore.value() == semaphore_size);
 
    // run it again
    REQUIRE_THROWS_WITH_AS(fn(), "exception", std::runtime_error);
}
 
TEST_CASE("Exception.Semaphore.Complex.4threads" * doctest::timeout(300)) {
    semaphore_complex1(4);
}