Relax Fee-Payer Constraint

Summary

This proposal aims to relax the handling of invalid fee-payers for blockhash transactions in Solana. Currently, if a transaction with an invalid fee-payer is included in a block, the entire block is rejected. This proposal suggests that instead of rejecting the entire block, the blockhash transaction with the invalid fee-payer should simply skip execution.

Motivation

The current constraint forces block-validation to be synchronous since in order to determine if a block is valid or not, some subset of transactions must be executed in order to determine if the fee-payer has sufficient funds to pay the transaction fees. By relaxing this constraint, we move one step closer towards asynchronous validation/execution.

New Terminology

A Blockhash Transaction is defined as a transaction that uses a real blockhash, as opposed to a durable nonce, for its recent blockhash / lifetime specifier.

A System Fee-Payer is defined as an account owned by the System Program with data length 0.

A Nonce Fee-Payer is defined as an account owned by the System Program with data length 80 and the first eight bytes being 01 00 00 00 01 00 00 00 (a Current Initialized nonce) or 00 00 00 00 01 00 00 00 (a Legacy Initialized nonce). The remaining 72 bytes are arbitrary.

A Nonce Fee-Payer is unrelated to the concept of a nonce transaction: a transaction that uses a Nonce Fee-Payer and uses a real blockhash is a Blockhash Transaction.

Detailed Design

Any transaction has a statically determined fee fee lamports. A transaction can successfully pay fees if:

• The fee-payer account is a System Fee-Payer or a Nonce Fee-Payer.
• If it is a System Fee-Payer, it has exactly fee lamports, or has at least fee + rent_exempt_reserve lamports.
• If it is a Nonce Fee-Payer, it has at least fee + rent_exempt_reserve lamports.

If the fee-payer account does not meet these conditions, the transaction may be included in a block, but it must not be executed. The transaction will have no effect on account state.

Invalid fee-payer transactions will count their requested, or default, cost units towards block limits. This is intended to make it strictly cheaper to process invalid fee-payer transactions compared to valid fee-payer transactions of the same construction.

This SIMD only applies to blockhash transactions. Handling any failure of nonce transactions is more involved, and nonce transaction fee-payer failure will be deferred to SIMD-0297, which also relaxes the nonce account validity constraint.

Alternatives Considered

• Requiring some sort of fee-payer lock up/bonding mechanism to ensure that fee- payers have sufficient funds to pay for the transaction fees. - This is more complex compared to this proposal.

Impact

Transactions that are unable to pay fees may be included in blocks.

Security Considerations

• Possible attack vector where a malicious leader can spam transactions with invalid fee-payers. Mitigation for this is charging full cost units for these.

Drawbacks

• If there is no interest in simplifying block-validation to allow for asynchronous, this proposal is not necesary.
• Concern about data propagation for without paying fees to the network (burn): - This concern has been raised in the past when this has been discussed. - However, the concern is largely invalid since even without this proposal, a malicious leader could still propagate data through the network for free by simply using an invalid fee-payer.

Backwards Compatibility

• All previously valid transactions and blocks are still valid.
• Blocks produced after the change may be rejected by previous versions of the validator client(s).