[Bug libstdc++/118757] Waiting for stored pointer inside std::atomic>

[redi at gcc dot gnu.org via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=118757

Jonathan Wakely <redi at gcc dot gnu.org> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
     Ever confirmed|0                           |1
   Last reconfirmed|                            |2025-02-05
             Status|UNCONFIRMED                 |NEW

--- Comment #1 from Jonathan Wakely <redi at gcc dot gnu.org> ---
There's no such requirement in the standard, it's perfectly valid for p.wait(q)
to take more than a second to wake up.

But the bug is real, we fail to wake up at all.

Here's a version that the standard requires to exit normally, without assuming
p.wait(q) executes in less than a second:

#include <memory>
#include <chrono>
#include <thread>
#include <barrier>

std::shared_ptr<int> q = std::make_shared<int>(42);
std::atomic<std::shared_ptr<int>> p = q;

std::barrier bar(2);

void signaler()
{
    std::this_thread::sleep_for(std::chrono::seconds(1));
    p.store({ q, nullptr });
    p.notify_one();
    bar.arrive_and_wait();
}

int main(int, char**)
{
    std::thread thr(signaler);
    p.wait(q);
    bar.arrive_and_wait();
    thr.join();
}