When you use rename to overwrite a file, it guarentees that at any point in time, newpath will refer to either the original file, or the new file. This is atomic enough to give you effectivly atomic file creation with contents.

It is not technically atomic, because there is a period where oldpath and newpath both refer to the same file, but I don't think that is an issue for this usecase.

Linux has the O_TMPFILE flag to open, for which this is one of the use cases described in the man page. I can’t say about “guaranteed” though. I assume Microsoft Transactional NTFS (TxF) also had (or has) this capability—but deprecated.

TxF is a corner stone of Windows Update, I can't really imagine them removing it. I always assumed they marked it deprecated to avoid supporting it towards customers.