Sync in handler, future cannot be sent between threads safely

[allan2]

I'm trying to dynamically construct Postgres query parameters. The Postgres params type is &[&(dyn ToSql + Sync)].

I tried this:

async fn insert_multirow_once(DbConn(conn): DbConn) -> Result<impl IntoResponse, StatusCode> {
    let statement = "INSERT INTO foo (a, b) VALUES ($1, $2), ($3, $4)";

    let mut params = Vec::<Box<dyn ToSql + Sync>>::with_capacity(4);

    for i in 1..=2 {
        params.push(Box::new(i));
        params.push(Box::new("s".to_owned()));
    }

    // from https://github.com/sfackler/rust-postgres/issues/712
    let params = params.iter().map(|x| x.as_ref()).collect::<Vec<_>>();

    conn.execute(statement, &params)
        .await
        .map_err(internal_error)?;
    Ok(StatusCode::OK)
}

The function by itself is ok but it doesn't work as a handler. The error is future cannot be sent between threads safely.

I searched the issues for Send and Sync but I'm still not sure what to do. Any help would be appreciated.

Full example here, based on the tokio-postgres example

[davidpdrsn]

Hm I'm not sure. Try asking the tokio-postgres maintainers.

[allan2]

I got it working. I had read the solution but forgot to actually cast.

Changed this:

let mut params = Vec::<Box<dyn ToSql + Sync + Send>>::with_capacity(4);  // add Send

and this

let params = params
    .iter()
    .map(|x| x.as_ref() as &(dyn ToSql + Sync))  // need to cast
    .collect::<Vec<_>>();

and now it works.

Thanks for Axum!