No runtime check to ensure transaction fee-payer is a debitable account

[CriesofCarrots]

Problem

Currently, all credit-debit checks happen inside message_processor::process_message. However, transaction fees get charged upstream in accounts::load_tx_accounts. This means that if a credit-only account is the first signer, it gets successfully charged the fee if it can afford it, potentially adding non-determinism to the chain. 🙀

Proposed Solution

Update runtime to reject transactions pre-fee if the fee payer is not locked as credit-debit

[garious]

Can we reject the transaction before any accounts are loaded? Otherwise you can DoS the leader by having it load a bunch of huge accounts without having to pay anything. Before SigVerify or any deserialize() would be ideal.

[CriesofCarrots]

Can we reject the transaction before any accounts are loaded? Otherwise you can DoS the leader by having it load a bunch of huge accounts without having to pay anything. Before SigVerify or any deserialize() would be ideal.

Yes, definitely. I am inclined to build off #6236, actually, and put this check in do_get_packet_offsets