Function sync_file 

pub fn sync_file(
    path: &Path,
    policy: &Policy,
    component: FsyncComponents,
) -> Result<()>

Open path for writing (no truncate, no create) and apply policy’s barrier for component. Convenience for post-hoc hardening of already published files; write paths that hold the handle open should prefer Policy::apply directly. Requires write access because sync_all degrades to a permission error on read-only handles (Windows FlushFileBuffers semantics).