SIMD-0185: Vote Account v4

proposals/0185-vote-account-v4.md

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+---
+simd: '0185'
+title: Vote Account v4
+authors: Justin Starry (Anza)
+category: Standard
+type: Core
+status: Review
+created: 2024-10-17
+feature: (fill in with feature tracking issues once accepted)
+---
+
+## Summary
+
+Add a new version of vote account state which clears unused state to make
+room for new state information.
+
+## Motivation
+
+A new update for the vote program is proposed to improve authorized voter
+bookkeeping as well as initialize state fields that will allow validators to set
+commission rates and collector accounts for different revenue sources in the
+future.
+
+### Authorized voter bookkeeping
+
+- Over 40% of vote state size is reserved for tracking a history of 32 prior
+authorized voters for the vote account in the `prior_voters` field. Having such
+a long history of prior voters is arguably not very useful, tracking the most
+recent previous epoch's voter is probably sufficient and can be stored in the
+`authorized_voters` field instead.
+
+- The `authorized_voters` field doesn't store the voter for the previous epoch
+so it's impossible to have a transition epoch where both the previous and newly
+assigned voter can both sign votes.
+
+### Revenue Collection Customization
+
+- There is only one commission rate stored in vote account state but validators
+want to be able to use different commission rates for different income streams
+like block revenue.
+
+- It's not possible to customize which accounts income is collected into.
+Currently all block fee revenue is collected into the validator identity account
+which cannot be a cold wallet since the identity needs to sign a lot of messages
+for various network protocols used in Solana like turbine, gossip, and QUIC.
+
+## Alternatives Considered
+
+### Reuse vote commission
+
+Vote accounts already allow validators to set a commission rate for inflation
+rewards and so it's not unreasonable to expect that this commission rate could
+also be used to distribute block revenue. However, some validators have
+expressed a desire to be able to tune revenue streams independently.
+
+## New Terminology
+
+- Block Revenue Commission: The commission rate that determines how much of
+block base fee and priority fee revenue is collected by the validator before
+distributing remaining funds to stake delegators. Previously 100% of block
+revenue was distributed to the validator identity account.
+
+- Inflation Rewards Commission: The commission rate that determines how much of 
+stake inflation rewards are collected by the validator before distributing the
+remaining rewards to stake delegators. Previously referred to as simply the
+"commission" since there was no need to differentiate from other types of
+commission.
+
+- Block Revenue Commission Collector: The account used to collect commissioned
+block revenue for validators. Previously collected by default into the validator
+identity account.
+
+- Inflation Rewards Commission Collector: The account used to collect
+commissioned inflation rewards for validators. Previously collected by default
+into the vote account.
+
+## Detailed Design
+
+Currently, all block revenue, including both transaction base fees and priority
+fees, is collected into a validator's node id account. This proposal details
+changes to the vote account state that will allow validators to specify how
+different sources of income are collected in future SIMD's. This proposal also
+updates the bookkeeping for authorized voters in the vote account state.
+
+### Vote Account 
+
+A new version of vote state will be introduced with the enum discriminant value
+of `3u32` which will be little endian encoded in the first 4 bytes of account
+data. 
+
+```rust
+pub enum VoteStateVersions {
+    V1(..),
+    V2(..),
+    V3(..),
+    V4(..), // <- new variant
+}
+```
+
+This new version of vote state will include new fields for setting the
+commission and collector account for the following sources of validator income:
+inflation rewards and block revenue. It will also remove the `prior_voters`
+field.
+
+```rust
+pub struct VoteStateV4 {
+    pub node_pubkey: Pubkey,
+    pub authorized_withdrawer: Pubkey,
+
+    /// REMOVED
+    /// commission: u8,
+
+    /// NEW: the collector accounts for validator income
+    pub inflation_rewards_collector: Pubkey,
+    pub block_revenue_collector: Pubkey,
+
+    /// NEW: basis points (0-10,000) that represent how much of each income
+    /// source should be given to this VoteAccount
+    pub inflation_rewards_commission_bps: u16,
+    pub block_revenue_commission_bps: u16,
+
+    /// NEW: reward amount pending distribution to stake delegators
+    pub pending_delegator_rewards: u64,
+
+    pub votes: VecDeque<LandedVote>,
+    pub root_slot: Option<Slot>,
+    pub authorized_voters: AuthorizedVoters,
+
+    /// REMOVED
+    /// prior_voters: CircBuf<(Pubkey, Epoch, Epoch)>,
+
+    pub epoch_credits: Vec<(Epoch, u64, u64)>,
+    pub last_timestamp: BlockTimestamp,
+}
+```
+
+### Vote Program
+
+All vote instructions MUST be updated to support deserializing v4 vote accounts.
+
+Whenever a vote account is initialized OR modified by the vote program in a
+transaction AND hasn't been updated to v4 yet, the account state MUST be saved
+in the new format with the following default values for the new fields described
+above:
+
+```rust
+VoteStateV4 {
+    // ..
+
+    inflation_rewards_collector: Pubkey::default(),
+    block_rewards_collector: Pubkey::default(),
+    inflation_rewards_commission_bps: 100u16 * (old_vote_state.commission as u16),
+    block_rewards_commission_bps: 10_000u16,
+    pending_delegator_rewards: 0u64,
+
+    // ..
+}
+```
+
+If a modified vote account's size is smaller than `3762` bytes, first resize the
+account to `3762` bytes before updating the account data. The vote program does
+not need to check if the resulting account is rent exempt, the runtime will
+enforce that check. This differs from the prior vote program implementation which
+falls back to store vote state as v2 if the account size cannot be resized while
+keeping the account rent exempt.
+
+
+### `InitializeAccount`
+
+The required size for vote accounts previously set to `3762` bytes MUST remain
+unchanged despite freeing up space with the removal of the prior voters field.
+Keeping the same size requirement simplifies this proposal and leaves extra
+space for future fields.
+
+Note that a v4 vote account is ALWAYS considered initialized, because unlike
+other vote state versions, it's never stored with uninitialized state.
+
+#### `UpdateCommission`
+
+The existing `UpdateCommission` instruction will will continue to only update
+the inflation rewards commission in integer percentage values.
+
+When updating vote state v4 accounts, the new `inflation_rewards_commission_bps`
+field should be used instead of the old generic `commission` field.
+Additionally, the new commission value MUST be multiplied by `100` before being
+checked for commission or increases and before being stored in account data.
+
+#### `Authorize`, `AuthorizeChecked`, `AuthorizeWithSeed`, `AuthorizeCheckedWithSeed`
+
+Existing authorize instructions will be processed differently when setting new
+authorized voters. Rather than purging authorized voter entries from the
+`authorized_voters` field that correspond to epochs less than the current epoch,
+only purge entries less than the previous epoch (current epoch - 1). This will
+mean that the `authorized_voters` field can now hold up to 4 entries for the
+epochs in the range `[current_epoch - 1, current_epoch + 2]`. Keeping the
+authorized voter around from the previous epoch will allow the protocol to
+accept votes from both the current and previous authorized voters to make voter
+transitions smoother.
+
+Additionally, since the `prior_voters` field is removed from vote state v4,
+there's no need to read or modify this field when processing authorize
+instructions.
+
+#### `Withdraw`
+
+The existing withdraw instruction MUST be modified to completely zero vote
+account data for fully withdrawn vote accounts. The old behavior partially
+zeroed the account data following the vote state version discriminant and is
+less intuitive.
+
+### Stake Program
+
+The builtin stake program reads vote account state when creating, delegating,
+and deactivating stake accounts. The program MUST be updated to support v4 vote
+accounts.
+
+### Other
+
+The runtime stakes cache and epoch stakes stored in snapshots MUST also be updated
+to support initialized v4 vote accounts.
+
+The tower serialization format MUST remain unchanged and continue serializing tower
+vote state as vote state v2.
+
+## Impact
+
+This is a prerequisite for implementing other SIMD's like block reward
+distribution in [SIMD-0123] which give validators more flexibility in how
+inflation rewards and block revenue is collected and distributed.
+
+[SIMD-0123]: https://github.com/solana-foundation/solana-improvement-documents/pull/123
+
+## Security Considerations
+
+NA
+
+## Drawbacks *(Optional)*
+
+NA
+
+## Backwards Compatibility *(Optional)*
+
+Existing programs that read vote state will need to be updated to support the
+latest account state version.