[libcxx-commits] [libcxx] [libc++] std::condition_variable_any overloads accepting std::stop_token don't register for stop callbacks (PR #77099)

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sn-rbroker wrote:

Hi @huixie90,

Thank you for taking the time to reply!

> It is almost the same except that my patch does not take the internal lock when checks stop requested and only takes the lock before unlock the user lock and wait.

I think the change to whether or not the internal lock, or the user lock is held is also important to avoid hangs. I think if we only hold the user's lock, there is a window where `request_stop` can be called between us checking `stop_requested` but before entering the wait. e.g. with the changes from #77127 if I update the new test case in `wait_until_token_pred.pass.cpp` and force it to run many times in a loop like this, then the unit tests hang in my environment:
```
  for (int i = 0; i < 50000; ++i)
  {
    class MyThread {
    public:
      MyThread() {
        thread_ = support::make_test_jthread([this](std::stop_token st) {
          while (!st.stop_requested()) {
            std::unique_lock lock{m_};
            cv_.wait_for(lock, st, 1h, [] { return false; });
          }
        });
      }

    private:
      std::mutex m_;
      std::condition_variable_any cv_;
      std::jthread thread_;
    };

    [[maybe_unused]] MyThread my_thread;
  }
```
If I change the `condition_variable` header back to using my changes, the unit tests can run with this tight loop.

> I would keep the existing tests and add new tests ( I added new tests in my patch)

Ah sorry, I will keep that in mind for next time!



https://github.com/llvm/llvm-project/pull/77099