Define invariants checks#

After each successful instruction execution, the check() method is called to check the account data invariants.

For each instruction, you can compare the account data before and after the instruction execution such as:

fn check(
    &self,
    pre_ix: Self::IxSnapshot,
    post_ix: Self::IxSnapshot,
    _ix_data: Self::IxData,
) -> Result<(), &'static str> {
    if let Some(escrow_pre) = pre_ix.escrow {
        // we can unwrap the receiver account because it
        // has to be initialized before the instruction
        // execution and it is not supposed to be closed
        // after the instruction execution either
        let receiver = pre_ix.receiver.unwrap();
        let receiver_lamports_before = receiver.lamports();
        let receiver_lamports_after = post_ix.receiver.unwrap().lamports();

        if receiver.key() != escrow_pre.receiver.key()
            && receiver_lamports_before < receiver_lamports_after
        {
            return Err("Un-authorized withdrawal");
        }
    }

    Ok(())
}