Function write_json_atomic 

pub fn write_json_atomic<T, P>(path: P, value: &T) -> Result<()>
where
    T: Serialize,
    P: AsRef<Path>,

Serializes value as compact JSON and writes it to path atomically.

Writes to a temporary file in the same directory, fsyncs it, then renames it over path, so a concurrent reader never observes a partially written file. Used by the quota-cache writers (the background quota workers each persist their last-known-good snapshot).

Errors

Returns an error if the parent directory cannot be created, the temp file cannot be written, or the final rename fails.