BatchGet with tokio::spawn test error the trait `std::marker::Send` is not implemented for `dyn Iterator<Item = tikv_client::Key>`

test code

#[tokio::test]
    async fn spawn_test(){

        use crate::tikv::*;
        use tikv_client::{Config, Key,TransactionClient as Client, Value};

        tokio::spawn(async {
            let config = Config::default();
            let client = Client::new_with_config(vec![PD_ADDR], config).await.unwrap();

            let tikv_client = TikvClient::new(vec![PD_ADDR.to_string()]).await;

            let key1: Key = b"key1".to_vec().into();
            let value1: Value = b"value1".to_vec();
            let key2: Key = b"key2".to_vec().into();
            let value2: Value = b"value2".to_vec();

            let mut txn = client.begin_optimistic().await.unwrap();
            // batch get
            let result: HashMap<Key,Value> = txn
                .batch_get(vec![key1,key2])
                .await.unwrap()
                .map(|pair| (pair.0, pair.1))
                .collect();
            txn.commit().await.unwrap();
            print!("{:?}",result);
        });

    }

error:

error: future cannot be sent between threads safely
--> src/store/src/tikv/mod.rs:118:9
|
118 | tokio::spawn(async {
| ^^^^^^^^^^^^ future created by async block is not Send
|
= help: the trait std::marker::Send is not implemented for dyn Iterator<Item = tikv_client::Key>
note: future is not Send as this value is used across an await
--> /home/yuchao/.cargo/registry/src/mirrors.ustc.edu.cn-61ef6e0cd06fb9b8/tikv-client-0.1.0/src/transaction/buffer.rs:97:61
|
97 | let fetched_results = f(Box::new(undetermined_keys)).await?;
| --------------------------- ^^^^^^ await occurs here, with Box::new(undetermined_keys) maybe used later
| |
| has type Box<dyn Iterator<Item = tikv_client::Key>> which is not Send
note: Box::new(undetermined_keys) is later dropped here
--> /home/yuchao/.cargo/registry/src/mirrors.ustc.edu.cn-61ef6e0cd06fb9b8/tikv-client-0.1.0/src/transaction/buffer.rs:97:68
|
97 | let fetched_results = f(Box::new(undetermined_keys)).await?;
| ^
note: required by a bound in tokio::spawn
--> /home/yuchao/.cargo/registry/src/mirrors.ustc.edu.cn-61ef6e0cd06fb9b8/tokio-1.21.0/src/task/spawn.rs:127:21
|
127 | T: Future + Send + 'static,
| ^^^^ required by this bound in tokio::spawn

error: could not compile store due to previous error

Start at the BatchGet call in src/store/src/tikv/mod.rs around line 118 and inspect the tikv-client transaction buffering code referenced in transaction/buffer.rs. Reproduce the inline tokio::spawn test and trace the future's Send requirements; done means the test compiles and runs without the reported non-Send error.