Function atomic_write_file 

pub fn atomic_write_file<F>(path: &Path, write_fn: F) -> Result<()>
where
    F: FnOnce(&mut File) -> Result<()>,

Write a file through a same-directory temporary file, then atomically rename it into place.

This avoids leaving critical index files truncated if a process is interrupted after opening with File::create but before the full payload is written.