SIMD-0290: Relax Fee Payer Constraint (#290)

proposals/0290-relax-fee-payer-constraint.md

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+---
+simd: '0290'
+title: Relax Fee Payer Constraint
+authors:
+  - Andrew Fitzgerald (Anza)
+category: Standard
+type: Core
+status: Review
+created: 2025-05-29
+feature:
+---
+
+## Summary
+
+This proposal aims to relax the handling of invalid fee payers 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
+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
+
+None.
+
+## Detailed Design
+
+A transaction has a statically determined fee `fee` lamports.
+A transaction can successfully pay fees if:
+
+- The fee payer account has exactly `fee` lamports.
+- The fee payer account has at least X + `rent_exempt_reserve` lamports.
+
+If the fee payer account does not meet either of these conditions, the transaction
+may be included in a block, but it must not be executed.
+The transaction will have no effect on the state.
+
+Invalid fee payer transactions will count their requested,
+or default, compute units (CUs) 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.
+
+## 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 CUs 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).