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 // Copyright (c) 2023 The pybind Community.
 
 #pragma once
 
 #include "detail/common.h"
 #include "detail/internals.h"
 #include "gil.h"
 
 #include <cassert>
 #include <mutex>
 
 #if defined(Py_GIL_DISABLED) || defined(PYBIND11_HAS_SUBINTERPRETER_SUPPORT)
 #    include <atomic>
 #endif
 #ifdef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
 #    include <cstdint>
 #    include <memory>
 #    include <string>
 #endif
 
 PYBIND11_NAMESPACE_BEGIN(PYBIND11_NAMESPACE)
 
 PYBIND11_NAMESPACE_BEGIN(detail)
 #if defined(Py_GIL_DISABLED) || defined(PYBIND11_HAS_SUBINTERPRETER_SUPPORT)
 using atomic_bool = std::atomic_bool;
 #else
 using atomic_bool = bool;
 #endif
 PYBIND11_NAMESPACE_END(detail)
 
 // 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.
 //
 // For in-depth background, see docs/advanced/deadlock.md
 #ifndef PYBIND11_HAS_SUBINTERPRETER_SUPPORT
 // Subinterpreter support is disabled.
 // In this case, we can store the result globally, because there is only a single interpreter.
 //
 // The life span of the stored result is the entire process lifetime. It is leaked on process
 // termination to avoid destructor calls after the Python interpreter was finalized.
 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.
     //
     // NOTE: The second parameter (finalize callback) is intentionally unused when subinterpreter
     // support is disabled. In that case, storage is process-global and intentionally leaked to
     // avoid calling destructors after the Python interpreter has been finalized.
     template <typename Callable>
     gil_safe_call_once_and_store &call_once_and_store_result(Callable &&fn,
                                                              void (*)(T &) /*unused*/ = nullptr) {
         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;
     // The instance is a global static, so its destructor runs when the process
     // is terminating. Therefore, do nothing here because the Python interpreter
     // may have been finalized already.
     PYBIND11_DTOR_CONSTEXPR ~gil_safe_call_once_and_store() = default;
 
     // Disable copy and move operations.
     gil_safe_call_once_and_store(const gil_safe_call_once_and_store &) = delete;
     gil_safe_call_once_and_store(gil_safe_call_once_and_store &&) = delete;
     gil_safe_call_once_and_store &operator=(const gil_safe_call_once_and_store &) = delete;
     gil_safe_call_once_and_store &operator=(gil_safe_call_once_and_store &&) = delete;
 
 private:
     // The global static storage (per-process) when subinterpreter support is disabled.
     alignas(T) char storage_[sizeof(T)] = {};
     std::once_flag once_flag_;
 
     // 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.
     detail::atomic_bool is_initialized_{false};
 };
 #else
 // Subinterpreter support is enabled.
 // In this case, we should store the result per-interpreter instead of globally, because each
 // subinterpreter has its own separate state. The cached result may not shareable across
 // interpreters (e.g., imported modules and their members).
 
 PYBIND11_NAMESPACE_BEGIN(detail)
 
 template <typename T>
 struct call_once_storage {
     alignas(T) char storage[sizeof(T)] = {};
     std::once_flag once_flag;
     void (*finalize)(T &) = nullptr;
     std::atomic_bool is_initialized{false};
 
     call_once_storage() = default;
     ~call_once_storage() {
         if (is_initialized) {
             if (finalize != nullptr) {
                 finalize(*reinterpret_cast<T *>(storage));
             } else {
                 reinterpret_cast<T *>(storage)->~T();
             }
         }
     }
     call_once_storage(const call_once_storage &) = delete;
     call_once_storage(call_once_storage &&) = delete;
     call_once_storage &operator=(const call_once_storage &) = delete;
     call_once_storage &operator=(call_once_storage &&) = delete;
 };
 
 PYBIND11_NAMESPACE_END(detail)
 
 // Prefix for storage keys in the interpreter state dict.
 #    define PYBIND11_CALL_ONCE_STORAGE_KEY_PREFIX PYBIND11_INTERNALS_ID "_call_once_storage__"
 
 // The life span of the stored result is the entire interpreter lifetime. An additional
 // `finalize_fn` can be provided to clean up the stored result when the interpreter is destroyed.
 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,
                                                              void (*finalize_fn)(T &) = nullptr) {
         if (!is_last_storage_valid()) {
             // 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.
             // There can be multiple threads going through here.
             storage_type *value = nullptr;
             {
                 gil_scoped_acquire gil_acq; // Restore lock ordering.
                 // This function is thread-safe under free-threading.
                 value = get_or_create_storage_in_state_dict();
             }
             assert(value != nullptr);
             std::call_once(value->once_flag, [&] {
                 // Only one thread will ever enter here.
                 gil_scoped_acquire gil_acq;
                 // fn may release, but will reacquire, the GIL.
                 ::new (value->storage) T(fn());
                 value->finalize = finalize_fn;
                 value->is_initialized = true;
                 last_storage_ptr_ = reinterpret_cast<T *>(value->storage);
                 is_initialized_by_at_least_one_interpreter_ = true;
             });
             // All threads will observe `is_initialized_by_at_least_one_interpreter_` 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() {
         T *result = last_storage_ptr_;
         if (!is_last_storage_valid()) {
             gil_scoped_acquire gil_acq;
             auto *value = get_or_create_storage_in_state_dict();
             result = last_storage_ptr_ = reinterpret_cast<T *>(value->storage);
         }
         assert(result != nullptr);
         return *result;
     }
 
     constexpr gil_safe_call_once_and_store() = default;
     // The instance is a global static, so its destructor runs when the process
     // is terminating. Therefore, do nothing here because the Python interpreter
     // may have been finalized already.
     PYBIND11_DTOR_CONSTEXPR ~gil_safe_call_once_and_store() = default;
 
     // Disable copy and move operations because the memory address is used as key.
     gil_safe_call_once_and_store(const gil_safe_call_once_and_store &) = delete;
     gil_safe_call_once_and_store(gil_safe_call_once_and_store &&) = delete;
     gil_safe_call_once_and_store &operator=(const gil_safe_call_once_and_store &) = delete;
     gil_safe_call_once_and_store &operator=(gil_safe_call_once_and_store &&) = delete;
 
 private:
     using storage_type = detail::call_once_storage<T>;
 
     // Indicator of fast path for single-interpreter case.
     bool is_last_storage_valid() const {
         return is_initialized_by_at_least_one_interpreter_
                && !detail::has_seen_non_main_interpreter();
     }
 
     // Get the unique key for this storage instance in the interpreter's state dict.
     // The return type should not be `py::str` because PyObject is interpreter-dependent.
     std::string get_storage_key() const {
         // The instance is expected to be global static, so using its address as unique identifier.
         // The typical usage is like:
         //
         //   PYBIND11_CONSTINIT static gil_safe_call_once_and_store<T> storage;
         //
         return PYBIND11_CALL_ONCE_STORAGE_KEY_PREFIX
                + std::to_string(reinterpret_cast<std::uintptr_t>(this));
     }
 
     // Get or create per-storage capsule in the current interpreter's state dict.
     // The storage is interpreter-dependent and will not be shared across interpreters.
     storage_type *get_or_create_storage_in_state_dict() {
         return detail::atomic_get_or_create_in_state_dict<storage_type>(get_storage_key().c_str())
             .first;
     }
 
     // No storage needed when subinterpreter support is enabled.
     // The actual storage is stored in the per-interpreter state dict via
     // `get_or_create_storage_in_state_dict()`.
 
     // Fast local cache to avoid repeated lookups when there are no multiple interpreters.
     // This is only valid if there is a single interpreter. Otherwise, it is not used.
     // WARNING: We cannot use thread local cache similar to `internals_pp_manager::internals_p_tls`
     //          because the thread local storage cannot be explicitly invalidated when interpreters
     //          are destroyed (unlike `internals_pp_manager` which has explicit hooks for that).
     T *last_storage_ptr_ = nullptr;
     // This flag is true if the value has been initialized by any interpreter (may not be the
     // current one).
     detail::atomic_bool is_initialized_by_at_least_one_interpreter_{false};
 };
 #endif
 
 PYBIND11_NAMESPACE_END(PYBIND11_NAMESPACE)

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