Function ensure 

pub async fn ensure<F, Fut>(
    host: &HostSsh,
    connection: &str,
    tag: &str,
    announce: F,
) -> Result<(), String>
where
    F: FnOnce(usize) -> Fut,
    Fut: Future<Output = ()>,

Put this version’s helper on host if it is not there (spec §6.2): probe the platform (recorded under connection for Connectors → Remotes), get the matching build (checked against the release’s hashes), upload it over ssh, check its hash there, move it in. tag names this install’s own temp file (letters, digits, -, _), so two installs on one host never touch each other’s upload. announce hears the size just before the upload, for whoever is waiting to say what is happening.