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 // Copyright (c) 2023 The pybind Community.
 
 #pragma once
 
 #include "detail/common.h"
 #include "gil.h"
 
 #include <cassert>
 #include <mutex>
 
 #ifdef Py_GIL_DISABLED
 #    include <atomic>
 #endif
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
 // Use the `gil_safe_call_once_and_store` class below instead of the naive
 //
 //   static auto imported_obj = py::module_::import("module_name"); // BAD, DO NOT USE!
 //
 // which has two serious issues:
 //
 //     1. Py_DECREF() calls potentially after the Python interpreter was finalized already, and
 //     2. deadlocks in multi-threaded processes (because of missing lock ordering).
 //
 // The following alternative avoids both problems:
 //
 //   PYBIND11_CONSTINIT static py::gil_safe_call_once_and_store<py::object> storage;
 //   auto &imported_obj = storage // Do NOT make this `static`!
 //       .call_once_and_store_result([]() {
 //           return py::module_::import("module_name");
 //       })
 //       .get_stored();
 //
 // The parameter of `call_once_and_store_result()` must be callable. It can make
 // CPython API calls, and in particular, it can temporarily release the GIL.
 //
 // `T` can be any C++ type, it does not have to involve CPython API types.
 //
 // The behavior with regard to signals, e.g. `SIGINT` (`KeyboardInterrupt`),
 // is not ideal. If the main thread is the one to actually run the `Callable`,
 // then a `KeyboardInterrupt` will interrupt it if it is running normal Python
 // code. The situation is different if a non-main thread runs the
 // `Callable`, and then the main thread starts waiting for it to complete:
 // a `KeyboardInterrupt` will not interrupt the non-main thread, but it will
 // get processed only when it is the main thread's turn again and it is running
 // normal Python code. However, this will be unnoticeable for quick call-once
 // functions, which is usually the case.
 template <typename T>
 class gil_safe_call_once_and_store {
 public:
     // PRECONDITION: The GIL must be held when `call_once_and_store_result()` is called.
     template <typename Callable>
     gil_safe_call_once_and_store &call_once_and_store_result(Callable &&fn) {
         if (!is_initialized_) { // This read is guarded by the GIL.
             // Multiple threads may enter here, because the GIL is released in the next line and
             // CPython API calls in the `fn()` call below may release and reacquire the GIL.
             gil_scoped_release gil_rel; // Needed to establish lock ordering.
             std::call_once(once_flag_, [&] {
                 // Only one thread will ever enter here.
                 gil_scoped_acquire gil_acq;
                 ::new (storage_) T(fn()); // fn may release, but will reacquire, the GIL.
                 is_initialized_ = true;   // This write is guarded by the GIL.
             });
             // All threads will observe `is_initialized_` as true here.
         }
         // Intentionally not returning `T &` to ensure the calling code is self-documenting.
         return *this;
     }
 
     // This must only be called after `call_once_and_store_result()` was called.
     T &get_stored() {
         assert(is_initialized_);
         PYBIND11_WARNING_PUSH
 #if !defined(__clang__) && defined(__GNUC__) && __GNUC__ < 5
         // Needed for gcc 4.8.5
         PYBIND11_WARNING_DISABLE_GCC("-Wstrict-aliasing")
 #endif
         return *reinterpret_cast<T *>(storage_);
         PYBIND11_WARNING_POP
     }
 
     constexpr gil_safe_call_once_and_store() = default;
     PYBIND11_DTOR_CONSTEXPR ~gil_safe_call_once_and_store() = default;
 
 private:
     alignas(T) char storage_[sizeof(T)] = {};
     std::once_flag once_flag_ = {};
 #ifdef Py_GIL_DISABLED
     std::atomic_bool
 #else
     bool
 #endif
         is_initialized_{false};
     // The `is_initialized_`-`storage_` pair is very similar to `std::optional`,
     // but the latter does not have the triviality properties of former,
     // therefore `std::optional` is not a viable alternative here.
 };
 
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)

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