Function blocking 

async fn blocking(
    field: &'static str,
    f: impl FnOnce() -> Result<Results> + Send + 'static,
) -> Result<String, String>

Same rule as the HTTP API, through the same door: a cold mmap fault stalls the OS thread it lands on, and tokio has no way to see that happen. Sharing api::scan also shares its permit, so an agent’s reads and a browser’s are bounded together rather than each getting the whole pool.