You are an expert in writing code in many different languages. Your goal is to perform code completion for the following code, keeping in mind the rest of the code and the file meta data.
    Metadata details:
    filename: single_source.rs
    path of file: ./repos/CYFS/src/component/cyfs-bdt/examples
    
    the code of the file until where you have to start completion: use async_std::{
    future, 
    task,
};
use cyfs_base::*;
use cyfs_bdt::{
    *, 
    ndn::{
        channel::{*, protocol::v0::*}
    }, 
};

mod utils;

#[async_std::main]
async fn main() {
    cyfs_util::process::check_cmd_and_exec("bdt-example");
    cyfs_debug::CyfsLoggerBuilder::new_app("bdt-example")
        .level("trace")
        .console("info")
        .build()
        .unwrap()
        .start();

    cyfs_debug::PanicBuilder::new("bdt-example", "bdt-example")
        .exit_on_panic(true)
        .build()
        .start();
    let (down_dev, down_secret) = utils::create_device(
        "5aSixgLuJjfrNKn9D4z66TEM6oxL3uNmWCWHk52cJDKR",
        &["W4udp127.0.0.1:10016"],
    )
    .unwrap();

    let (src_dev, src_secret) = utils::create_device(
        "5aSixgLuJjfrNKn9D4z66TEM6oxL3uNmWCWHk52cJDKR",
        &["W4udp127.0.0.1:10017"],
    )
    .unwrap();


    let (down_stack, down_store) = {
        let mut params = StackOpenParams::new("bdt-example");
       
        let store = MemChunkStore::new();
        params.chunk_store = Some(store.clone_as_reader());
      
        params.known_device = Som
    
async fn main() {
    cyfs_util::process::check_cmd_and_exec("bdt-example");
    cyfs_debug::CyfsLoggerBuilder::new_app("bdt-example")
        .level("trace")
        .console("info")
        .build()
        .unwrap()
        .start();

    cyfs_debug::PanicBuilder::new("bdt-example", "bdt-example")
        .exit_on_panic(true)
        .build()
        .start();
    let (down_dev, down_secret) = utils::create_device(
        "5aSixgLuJjfrNKn9D4z66TEM6oxL3uNmWCWHk52cJDKR",
        &["W4udp127.0.0.1:10016"],
    )
    .unwrap();

    let (src_dev, src_secret) = utils::create_device(
        "5aSixgLuJjfrNKn9D4z66TEM6oxL3uNmWCWHk52cJDKR",
        &["W4udp127.0.0.1:10017"],
    )
    .unwrap();


    let (down_stack, down_store) = {
        let mut params = StackOpenParams::new("bdt-example");
       
        let store = MemChunkStore::new();
        params.chunk_store = Some(store.clone_as_reader());
      
        params.known_device = Some(vec![src_dev.clone()]);
        (
            Stack::open(down_dev.clone(), down_secret, params)
                .await
                .unwrap(),
            store
        )
    };


    let (chunk_len, chunk_data) = utils::random_mem(1024, 1024);
    let chunk_hash = hash_data(&chunk_data[..]);
    let chunkid = ChunkId::new(&chunk_hash, chunk_len as u32);
    

    let src_stack = {
        let mut params = StackOpenParams::new("bdt-example");

        struct RespNotFound {

        }

        #[async_trait::async_trait]
        impl NdnEventHandler for RespNotFound {
            async fn on_newly_interest(
                &self, 
                _stack: &Stack, 
                interest: &Interest, 
                from: &Channel
            ) -> BuckyResult<()> {
                let resp = RespInterest {
                    session_id: interest.session_id.clone(), 
                    chunk: interest.chunk.clone(), 
                    err: BuckyErrorCode::NotFound, 
                    redirect: None,
                    redirect_referer: None,
                    to: None,
                };

                from.resp_interest(resp);
                Ok(())
            }
        
            fn on_unknown_piece_data(
                &self, 
                _stack: &Stack, 
                _piece: &PieceData, 
                _from: &Channel
            ) -> BuckyResult<DownloadSession> {
                unimplemented!()
            }
        }
        params.ndn_event = Some(Box::new(RespNotFound {}));

        Stack::open(src_dev, src_secret, params).await.unwrap()
    };


    let context = SingleSourceContext::from_desc("".to_owned(), src_stack.local_const().clone());
    let (path, reader) = download_chunk(
        &*down_stack,
        chunkid.clone(), 
        None, 
        context.clone()
    )
    .await.unwrap();

    log::info!("task path: {}", path);

    task::spawn(async move {
        let session = context.wait_session(future::pending::<BuckyError>()).await.unwrap();
        if let DownloadSessionState::Canceled(err) = session.wait_finish().await {
            assert_eq!(err.code(), BuckyErrorCode::NotFound);
        } else {
            unreachable!()
        }
    });

   
    down_store.write_chunk(&chunkid, reader).await.unwrap_err();
}