Fix TSS key exhaustion in implicitly_convertible()#6020
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rwgk merged 1 commit intopybind:masterfrom Mar 29, 2026
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Replace `static thread_specific_storage<int>` with `thread_local bool` in the implicit conversion reentrancy guard. Since implicitly_convertible is a template function, each unique <InputType, OutputType> pair created its own TSS key via PyThread_tss_create(). Projects with hundreds of modules and many implicit conversions could exhaust PTHREAD_KEYS_MAX (1024 on Linux, 512 on macOS), especially on Python 3.12+ where CPython itself consumes more TSS keys for subinterpreter support. thread_local bool is safe here because: - bool is trivially destructible, so it works on all C++11 platforms including older macOS (the concern that motivated the TSS approach in PR pybind#5777 applied only to types with non-trivial destructors needing __cxa_thread_atexit runtime support) - Each thread gets its own copy, so it is thread-safe for free-threading - Subinterpreter sharing is benign: the guard prevents recursive implicit conversions on the same thread regardless of which interpreter is active - The v3.0.0 code already used thread_local bool under Py_GIL_DISABLED This effectively reverts the core change from PR pybind#5777 while keeping the non-copyable/non-movable set_flag guard. Made-with: Cursor
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…ybind#6020) Replace `static thread_specific_storage<int>` with `thread_local bool` in the implicit conversion reentrancy guard. Since implicitly_convertible is a template function, each unique <InputType, OutputType> pair created its own TSS key via PyThread_tss_create(). Projects with hundreds of modules and many implicit conversions could exhaust PTHREAD_KEYS_MAX (1024 on Linux, 512 on macOS), especially on Python 3.12+ where CPython itself consumes more TSS keys for subinterpreter support. thread_local bool is safe here because: - bool is trivially destructible, so it works on all C++11 platforms including older macOS (the concern that motivated the TSS approach in PR pybind#5777 applied only to types with non-trivial destructors needing __cxa_thread_atexit runtime support) - Each thread gets its own copy, so it is thread-safe for free-threading - Subinterpreter sharing is benign: the guard prevents recursive implicit conversions on the same thread regardless of which interpreter is active - The v3.0.0 code already used thread_local bool under Py_GIL_DISABLED This effectively reverts the core change from PR pybind#5777 while keeping the non-copyable/non-movable set_flag guard. Made-with: Cursor
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Description
Fixes #5975
PR #5777 (included in v3.0.1) replaced the
static bool/thread_local boolreentrancy guard inimplicitly_convertible()withstatic thread_specific_storage<int>to make it sub-interpreter and free-threading safe. Becauseimplicitly_convertibleis a template function parametrized on<InputType, OutputType>, every unique type pair instantiation creates its own staticthread_specific_storage, each of which allocates a TSS key viaPyThread_tss_create().Projects with hundreds of modules and many implicit conversions (the reporters have 590+ and 1000+
PYBIND11_MODULEs) exhaust the OS TSS key limit (PTHREAD_KEYS_MAX: 1024 on Linux, 512 on macOS). The problem manifests on Python 3.12+ because CPython itself consumes more TSS keys for subinterpreter support, reducing the budget available to user code.The fix replaces
static thread_specific_storage<int>withthread_local bool:thread_localuses compiler/OS TLS storage (e.g.,__threadsegment on Linux), which has effectively unlimited capacity.booldoes not require__cxa_thread_atexitruntime support, so it works on all C++11 platforms including older macOS targets. (This was the concern that motivated using TSS in fix: make implicitly_convertable sub-interpreter and free-threading safe #5777, but it only applies to types with non-trivial destructors.)thread_local boolunder#ifdef Py_GIL_DISABLED.The non-copyable/non-movable
set_flagRAII guard (added in #5777) is retained but simplified back to wrappingbool&.Suggested changelog entry:
thread_specific_storageallocation inimplicitly_convertible().