> What we want is for the traditional unix way to save a file to (write to tempfile, rename over target) to *either* result in the old file or the new file.
This semantics (where data in the new file is magically committed) may or may not be a result of particular file system implementation. From the rename() man page:
> If newpath already exists it will be atomically replaced (subject to a few conditions; see ERRORS below), so that there is no point at which another process attempting to access newpath will find it missing.
Nowhere does it specify what _data_ will be in either file, just that the file will be there. ext4 dutifully obeys that.
In short, what you are referring to as "traditional unix way" doesn't really exist. Proof: emacs code.
PS. Sure, it would be nice to have such "one shot" API. But, the current API isn't it.