Let’s revert this change and revisit what better approaches we can take.
If we can’t guarantee that the Python version is thread-safe, then we should document that StringIO is not thread-safe. it also means we don’t need to guarantee thread safety in the C implementation either.
However, that doesn’t mean we should allow segmentation faults. (thread-safe and segmentation fault are fully different issues, at least we should guarantee thread-unsafe with memory-safe implementation.)