[BUG]: gil_scoped_acquire blocks

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What version (or hash if on master) of pybind11 are you using?

pybind11_bazel commit: fc56ce8a8b51e3dd941139d329b63ccfea1d304b

Problem description

I have a struct Middleware in C++ and I want to hold scoped_interpreter py::object and event_loop_thread as fields.
In the method initialize I have some std::thread, that will invoke some python code there.

Since it is multithreaded, I want to acquire in the thread, but it just blocks and hangs, no error is thrown, the release before the thread start didn't help as well.

Any suggestions please?

No other code running.

struct Middleware{
  void initialize(){
    py::gil_scoped_release release; // didn't help ???
    event_loop_runner = std::thread([&]() {
      {
        {
          py::gil_scoped_acquire acquire; // ??? blocks for some reason
          py::object module = py::module::import("simple.python");
          event_loop_thread = module.attr("EventLoopThread")();
          event_loop_thread.attr("start")(); // non-blocking
        }
        std::unique_lock<std::mutex> lock(python_thread_lock_);
        cv.wait(lock, [&]() { return !is_running; }); // block until stop called.
      }
    });
  }

  pybind11::scoped_interpreter guard;
  py::object event_loop_thread;
   std::thread event_loop_runner;
};

Reproducible example code

No response

Is this a regression? Put the last known working version here if it is.

Not a regression

The issue names only Middleware::initialize, py::gil_scoped_acquire, py::gil_scoped_release, and the scoped_interpreter member; no repository file or test is provided. Start by reviewing the complete threading and interpreter-lifetime example and reproducing the hang. Done means identifying the cause and documenting an actionable correction or a confirmed missing prerequisite.