> The problem is the test on the "python initialization":
> PyUnicode_EncodeFSDefault / PyUnicode_DecodeFSDefaultAndSize
> check the global Py_FileSystemDefaultEncoding variable (use
> C functions if it is NULL).

The problem is a little bit more complex. interp->codecs_initialized is set to 1 by _PyCodecRegistry_Init(), but the filesystem codec is not loaded yet. We need another variable: interp->fscodec_initialized.

fscodec_subinterpreter.patch fixes this issue.

Note: the patch replaces also PyErr_Print() by PyErr_PrintEx(0) to avoid a crash on error, if PyErr_PrintEx(0) is called before the sys module is initialized. For example, if PyModule_GetDict(_PyImport_FindBuiltin("builtins")) is NULL. Anyway, it's useless to set sys attributes on error, because sys is destroyed just after printing the error.