Safeguard for rent-exempt minimum increase

Summary

Protective feature to increase rent-exempt minimum balance back to the legacy value of 6960 per-byte. This should only be activated in response to excessive state growth observed on mainnet beta.

Motivation

Reducing rent without a built-in safeguard to respond to excessive state growth is inherently risky. This proposal is designed to provide a rent increase feature gate proactively so core developers can respond to issues more promptly.

New Terminology

N/A

Detailed Design

Reset lamports_per_byte_year to the legacy value of 6960.

ACCOUNT_STORAGE_OVERHEAD = 128
lamports_per_byte_year = 6960 // post-activation
exemption_threshold = 1.0
effective_size = ACCOUNT_STORAGE_OVERHEAD + data_size_bytes
min_balance = effective_size 
                            * lamports_per_byte_year
                            * exemption_threshold
                       = effective_size * 6960

Alternatives Considered

SIMD-0389 introduces a supervisory controller that will detect and respond to excessive state growth automatically. Because that alternative is yet to be accepted, the manual intervention described in this proposal can allow for safely shipping moderate rent reduction sooner while further reduction can be withheld until the controller is shipped.

Impact

• New account creations and allocations will be subject to the increased rent value. SIMD-0392 maintains usability of existing accounts that are made sub-exempt after the rent increase.

Security Considerations

N/A

Backwards Compatibility

SIMD-0392 grandfathers in existing accounts so they will continue to be valid after a rent increase.