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| "version": "3.8.7" | ||
| }, | ||
| "nbsphinx": { | ||
| "timeout": 60 | ||
| "timeout": 60 | ||
| } | ||
| }, | ||
| "nbformat": 4, | ||
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| // Copyright (c) 2023 The pybind Community. | ||
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| #pragma once | ||
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| #include "detail/common.h" | ||
| //#include "gil.h" | ||
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| #include <cassert> | ||
| #include <mutex> | ||
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| PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE) | ||
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| // 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; | ||
| } | ||
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| // This must only be called after `call_once_and_store_result()` was called. | ||
| T &get_stored() { | ||
| assert(is_initialized_); | ||
| return *reinterpret_cast<T *>(storage_); | ||
| } | ||
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| constexpr gil_safe_call_once_and_store() = default; | ||
| PYBIND11_DTOR_CONSTEXPR ~gil_safe_call_once_and_store() = default; | ||
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| private: | ||
| alignas(T) char storage_[sizeof(T)] = {}; | ||
| std::once_flag once_flag_ = {}; | ||
| bool 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. | ||
| }; | ||
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| PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE) |
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