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Use re-entrant mutex to protect global state.
Add pymutex.hpp which implements a re-entrant mutex on top of Python's PyMutex. Add BOOST_PYTHON_LOCK_STATE() macro that uses RAII to lock mutable global state as required.
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‎include/boost/python/detail/pymutex.hpp‎

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// Copyright 2025 Boost.Python Contributors
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// Distributed under the Boost Software License, Version 1.0. (See
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// accompanying file LICENSE_1_0.txt or copy at
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// http://www.boost.org/LICENSE_1_0.txt)
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#ifndef BOOST_PYTHON_DETAIL_PYMUTEX_HPP
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#define BOOST_PYTHON_DETAIL_PYMUTEX_HPP
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#include <boost/python/detail/prefix.hpp>
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#ifdef Py_GIL_DISABLED
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// needed for pymutex wrapper
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#include <atomic>
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#include <cstddef>
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#endif
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namespace boost { namespace python { namespace detail {
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#ifdef Py_GIL_DISABLED
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// Re-entrant wrapper around PyMutex for free-threaded Python
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// Similar to _PyRecursiveMutex or threading.RLock
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class pymutex {
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PyMutex m_mutex;
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std::atomic<unsigned long> m_owner;
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std::size_t m_level;
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public:
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pymutex() : m_mutex({}), m_owner(0), m_level(0) {}
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// Non-copyable, non-movable
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pymutex(const pymutex&) = delete;
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pymutex& operator=(const pymutex&) = delete;
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void lock() {
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unsigned long thread = PyThread_get_thread_ident();
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if (m_owner.load(std::memory_order_relaxed) == thread) {
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m_level++;
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return;
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}
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PyMutex_Lock(&m_mutex);
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m_owner.store(thread, std::memory_order_relaxed);
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// m_level should be 0 when we acquire the lock
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}
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void unlock() {
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unsigned long thread = PyThread_get_thread_ident();
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// Verify current thread owns the lock
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if (m_owner.load(std::memory_order_relaxed) != thread) {
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// This should never happen - programming error
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return;
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}
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if (m_level > 0) {
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m_level--;
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return;
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}
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m_owner.store(0, std::memory_order_relaxed);
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PyMutex_Unlock(&m_mutex);
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}
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bool is_locked_by_current_thread() const {
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unsigned long thread = PyThread_get_thread_ident();
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return m_owner.load(std::memory_order_relaxed) == thread;
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}
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};
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// RAII lock guard for pymutex
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class pymutex_guard {
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pymutex& m_mutex;
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public:
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explicit pymutex_guard(pymutex& mutex) : m_mutex(mutex) {
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m_mutex.lock();
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}
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~pymutex_guard() {
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m_mutex.unlock();
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}
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// Non-copyable, non-movable
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pymutex_guard(const pymutex_guard&) = delete;
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pymutex_guard& operator=(const pymutex_guard&) = delete;
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};
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// Global mutex for protecting all Boost.Python internal state
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// Similar to pybind11's internals.mutex
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BOOST_PYTHON_DECL pymutex& get_global_mutex();
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// Macro for acquiring the global lock
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// Similar to pybind11's PYBIND11_LOCK_INTERNALS
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#define BOOST_PYTHON_LOCK_STATE() \
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::boost::python::detail::pymutex_guard lock(::boost::python::detail::get_global_mutex())
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#else
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// No-op macro when not in free-threaded mode
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#define BOOST_PYTHON_LOCK_STATE()
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#endif // Py_GIL_DISABLED
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}}} // namespace boost::python::detail
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#endif // BOOST_PYTHON_DETAIL_PYMUTEX_HPP

‎src/converter/from_python.cpp‎

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#include <boost/python/handle.hpp>
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#include <boost/python/detail/raw_pyobject.hpp>
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#include <boost/python/detail/pymutex.hpp>
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#include <boost/python/cast.hpp>
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#include <vector>
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inline bool visit(rvalue_from_python_chain const* chain)
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{
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BOOST_PYTHON_LOCK_STATE();
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visited_t::iterator const p = std::lower_bound(visited.begin(), visited.end(), chain);
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if (p != visited.end() && *p == chain)
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return false;
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{
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unvisit(rvalue_from_python_chain const* chain)
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: chain(chain) {}
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~unvisit()
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{
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BOOST_PYTHON_LOCK_STATE();
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visited_t::iterator const p = std::lower_bound(visited.begin(), visited.end(), chain);
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assert(p != visited.end());
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visited.erase(p);

‎src/converter/registry.cpp‎

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#include <boost/python/converter/registry.hpp>
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#include <boost/python/converter/registrations.hpp>
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#include <boost/python/converter/builtin_converters.hpp>
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#include <boost/python/detail/pymutex.hpp>
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#include <set>
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#include <stdexcept>
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namespace // <unnamed>
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{
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typedef registration entry;
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typedef std::set<entry> registry_t;
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#ifndef BOOST_PYTHON_CONVERTER_REGISTRY_APPLE_MACH_WORKAROUND
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registry_t& entries()
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{
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entry* get(type_info type, bool is_shared_ptr = false)
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{
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BOOST_PYTHON_LOCK_STATE();
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# ifdef BOOST_PYTHON_TRACE_REGISTRY
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registry_t::iterator p = entries().find(entry(type));
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registration const* query(type_info type)
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{
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BOOST_PYTHON_LOCK_STATE();
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registry_t::iterator p = entries().find(entry(type));
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# ifdef BOOST_PYTHON_TRACE_REGISTRY
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std::cout << "querying " << type

‎src/converter/type_id.cpp‎

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#include <boost/python/type_id.hpp>
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#include <boost/python/detail/decorated_type_id.hpp>
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#include <boost/python/detail/pymutex.hpp>
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#include <utility>
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#include <vector>
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#include <algorithm>
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{
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free_mem(char*p)
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: p(p) {}
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~free_mem()
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{
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std::free(p);
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bool cxxabi_cxa_demangle_is_broken()
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{
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BOOST_PYTHON_LOCK_STATE();
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static bool was_tested = false;
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static bool is_broken = false;
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if (!was_tested) {
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{
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BOOST_PYTHON_DECL char const* gcc_demangle(char const* mangled)
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{
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BOOST_PYTHON_LOCK_STATE();
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typedef std::vector<
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std::pair<char const*, char const*>
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> mangling_map;

‎src/errors.cpp‎

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#include <boost/python/errors.hpp>
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#include <boost/cast.hpp>
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#include <boost/python/detail/exception_handler.hpp>
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#include <boost/python/detail/pymutex.hpp>
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namespace boost { namespace python {
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#ifdef Py_GIL_DISABLED
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namespace detail {
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// Global mutex for protecting all Boost.Python internal state
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pymutex& get_global_mutex()
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{
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static pymutex mutex;
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return mutex;
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}
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}
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#endif
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error_already_set::~error_already_set() {}
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// IMPORTANT: this function may only be called from within a catch block!
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BOOST_PYTHON_DECL bool handle_exception_impl(function0<void> f)
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{
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try
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{
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if (detail::exception_handler::chain)
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return detail::exception_handler::chain->handle(f);
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detail::exception_handler* handler_chain = nullptr;
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{
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BOOST_PYTHON_LOCK_STATE();
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handler_chain = detail::exception_handler::chain;
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}
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if (handler_chain)
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return handler_chain->handle(f);
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f();
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return false;
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}
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: m_impl(impl)
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, m_next(0)
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{
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BOOST_PYTHON_LOCK_STATE();
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if (chain != 0)
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tail->m_next = this;
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else

‎src/object/inheritance.cpp‎

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// http://www.boost.org/LICENSE_1_0.txt)
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#include <boost/python/object/inheritance.hpp>
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#include <boost/python/type_id.hpp>
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#include <boost/python/detail/pymutex.hpp>
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#include <boost/graph/breadth_first_search.hpp>
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#if _MSC_FULL_VER >= 13102171 && _MSC_FULL_VER <= 13102179
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# include <boost/graph/reverse_graph.hpp>
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391392
inline void* convert_type(void* const p, class_id src_t, class_id dst_t, bool polymorphic)
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{
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BOOST_PYTHON_LOCK_STATE();
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// Quickly rule out unregistered types
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index_entry* src_p = seek_type(src_t);
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if (src_p == 0)
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BOOST_PYTHON_DECL void add_cast(
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class_id src_t, class_id dst_t, cast_function cast, bool is_downcast)
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{
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BOOST_PYTHON_LOCK_STATE();
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// adding an edge will invalidate any record of unreachability in
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// the cache.
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static std::size_t expected_cache_len = 0;
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BOOST_PYTHON_DECL void register_dynamic_id_aux(
491496
class_id static_id, dynamic_id_function get_dynamic_id)
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{
498+
BOOST_PYTHON_LOCK_STATE();
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tuples::get<kdynamic_id>(*demand_type(static_id)) = get_dynamic_id;
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}
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