Feature or enhancement

Some CPython internals require initialization exactly once. Some of these one time initializations are not thread-safe without the GIL or have data races according to the C11 memory model.

We should add a lightweight, thread-safe one-time initialization API similar to C++11'sstd::call_once¹. The proposed internal-only API follows C++11'sstd::call_once, but adapted for C (i.e., error returns and function pointers):

typedefstruct {uint8_tv;}_PyOnceFlag;typedefint_Py_once_fn_t(void*arg);// Calls `fn` once using `flag`. The `arg` is passed to the call to `fn`.//// Returns 1 on success and 0 on failure.//// If `fn` returns 1 (success), then subsequent calls immediately return 1.// If `fn` returns 0 (failure), then subsequent calls will retry the call.int_PyOnceFlag_CallOnce(_PyOnceFlag*flag,_Py_once_fn_t*fn,void*arg);

As an example, thePython-ast.c relies on the GIL and aninitialized variable to ensure that it is only initialized once:

Linked PRs

• gh-111956: Add thread-safe one-time initialization. #111960