Refund NEAR tokens

Return attached NEAR tokens to the original user when a cross-contract call fails.

When your contract sends attached NEAR tokens in a cross-contract call and that call fails, the tokens return to your contract. They do not automatically return to the user who started the operation.

If the user should receive those tokens back, refund them in the callback.

---

The refund pattern

Save the original user and amount when creating the promise, then pass both to a private callback:

pub fn mint_nft(&mut self, amount: NearToken) -> Promise {
    let user = env::predecessor_account_id();

    Promise::new(self.nft.clone())
        .function_call(
            "mint".to_string(),
            {
                "token_id": "1",
                "receiver_id": user.clone()
            },
            Gas(5_000_000_000_000),
            amount.as_yoctonear()
        )
        .then(
            Self::ext(env::current_account_id())
                .mint_callback(user, amount),
        )
}

#[private]
pub fn mint_callback(
    &mut self,
    user: AccountId,
    amount: NearToken,
    #[callback_result] result: Result<(), PromiseError>,
) {
    if result.is_err() {
        // we got the tokens back, refund the user
        // do not use panic, that will stop the callback execution 
        Promise::new(user).transfer(amount);
    } else {
        // mint succeeded, do something else
    }
}