Macro transaction 

macro_rules! transaction {
    ($db:expr, |$tx:ident| $body:block) => { ... };
}

Runs a block in a transaction: committed when it evaluates to Ok, cancelled when it evaluates to Err (? leaves the block, not the function). When the transaction conflicts with a concurrent one, the block runs again in a new transaction, up to TRANSACTION_ATTEMPTS times.

let user = slave_framework::transaction!(&db, |tx| {
    let user: Option<User> = tx.query("CREATE ...").await?.take(0)?;
    tx.query("CREATE ...").await?.check()?;
    Ok(user)
})?;

The error type must implement MaybeConflict. To make two transactions that read the same rows and write different ones conflict (e.g. “keep at least one owner”), have both write one shared record, such as the parent row.

A macro rather than a function taking an async closure: the futures of async closures are not provably Send, which MCP tools require.