Function atomic_write_fsync 

pub fn atomic_write_fsync(dir: &Path, name: &str, bytes: &[u8]) -> Result<()>

Atomically write bytes to dir/name via a dir/name.tmp staging file with full durability.

Order of operations (must not change):

1. Create / truncate the tmp file and write bytes.
2. sync_data() on the tmp file — forces file data pages to stable storage.
3. rename(tmp, dst) — atomic on POSIX filesystems.
4. fsync_directory(dir) — forces the directory entry durable so the new name survives power loss.

name must be one plain path component; anything else returns InvalidInput before a byte is written. Both paths are built here from dir, so the rename stays inside one filesystem directory and the single parent fsync covers both entries.