[Bug libstdc++/60966] std::call_once sometime hangs

["hideaki.kimura at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=60966

--- Comment #14 from Hideaki Kimura <hideaki.kimura at gmail dot com> ---
(In reply to Jonathan Wakely from comment #13)
> This means you are waiting on an object that has gone out of scope. WIthout
> more information it's not possible to tell if this is a bug in your program
> or the standard libary.
> 
> I'll try to reproduce it with Thomas's code...

Hey Jonathan,
I'm not sure if this helps, but could you try the following code snippet?

#include <future>
#include <iostream>
#include <thread>
#include <vector>

struct DummyTask {
    DummyTask(int id) : id_(id) {}
    int id_;
    std::promise<void> pr;
};

const int THREADS = 100;

void run_task(DummyTask* task) {
    std::this_thread::sleep_for(std::chrono::milliseconds(100));
    task->pr.set_value();
}

void wait_for_task(DummyTask* task) {
    task->pr.get_future().wait();
}

int main() {
    std::vector<DummyTask*> tasks;
    std::vector<std::thread*> threads;
    for (int i = 0; i < THREADS; ++i) {
        DummyTask* task = new DummyTask(i);
        tasks.push_back(task);
        threads.push_back(new std::thread(run_task, task));
    }

    for (int i = 0; i < THREADS; ++i) {
        wait_for_task(tasks[i]);
        // Because we returned from wait_for_task for this task, run_task is
surely done.
        // No one else is referring to the task. So, even before
threads[i]->join(),
        // it should be safe to delete it now.
        delete tasks[i];  // but here you get an invalid read!
    }
    for (int i = 0; i < THREADS; ++i) {
        threads[i]->join();
        delete threads[i];
    }
    return 0;
}


Run it a few times on valgrind. You will see what I got.
If you move threads[i]->join() to the line before delete tasks[i], you don't
get the issue.

[Assuming that I'm not terribly missing something..]
My bet is that std::promise puts something on thread-local, which causes some
issue when std::promise's destructor is called before the corresponding
thread's join() is called.