Amend SIMD-0123: clarify block revenue distribution mechanism

proposals/0123-block-revenue-distribution.md

@@ -137,46 +137,67 @@ vote state field `pending_delegator_rewards` and added to the balance of vote
 account. If either of these conditions is violated, the delegator rewards amount
 MUST be burned.
 
-### Runtime: Delegator Rewards Distribution
+### Runtime: Block Revenue Distribution
 
-When calculating stake delegation rewards for a particular completed reward
-epoch, construct a list of all vote accounts that were initialized at the
-beginning of the reward epoch and had a non-zero active stake delegation. For
-each vote account, retrieve its state at the end of the reward epoch and check
-the `pending_delegator_rewards` field in its vote state. Let this value be `P`.
-If `P` is non-zero, use it to calculate rewards for each of the stake accounts
-delegated to the vote account as follows: 
+At the beginning of each epoch, pending delegator rewards will be collected from
+actively staked vote accounts into the epoch rewards sysvar account and then
+distributed to stake accounts utilizing the existing partitioned rewards
+distribution mechanism described in [SIMD-0118].
 
-1. Sum all active stake delegated to the vote account during the reward epoch
-epoch. Let this total be `A`.
+#### Delegator Rewards Calculation
 
-2. For each individual stake account, multiply its active stake from the
-reward epoch by `P`, and divide the result by `A` using integer division.
-Discard any fractional lamports.
+Delegator rewards MUST be calculated during the beginning of the first block of
+an epoch `N` where the first block is any block with a parent in the previous
+epoch. Rewards MUST be recalculated if a node is restarted during the partitioned
+rewards distribution period as described in [SIMD-0118].
 
-After calculating all individual stake rewards, sum them to obtain `D`, the
-total distribution amount. Because of integer division, the full amount `P` may
-not be distributed so compute the amount to be burned, `B`, as the difference
-between `P` and `D`.
+For each vote account, get its total active stake delegation
+during the reward epoch `N - 1`. Let this value be `A`.
 
-If no blocks in the epoch following the completed reward epoch have been
-processed yet, subtract `B` from both the vote account’s lamport balance and its
-`pending_delegator_rewards` field and store the updated vote account. Finally,
-the burn amount `B` should also be deducted from the cluster capitalization.
+Then for each vote account, get its pending delegator rewards from the
+`pending_delegator_rewards` field in the vote state at the end of reward epoch
+`N - 1`. Let this value be `P`. If `P` is zero, skip this vote account as there
+is nothing to be distributed.
+
+Lastly, if this is the first block of epoch `N`, the vote state’s
+`pending_delegator_rewards` field MUST be reset to `0`. Then, if `A` (active
+stake) is zero, the delegator rewards will effectively returned to the voter
+after the `pending_delegator_rewards` field is reset to `0`. Otherwise, if `A`
+is non-zero, `P` lamports MUST be debited from the vote account’s lamport
+balance and credited to the epoch rewards sysvar account’s lamport balance
+before any transactions or votes are processed.
+
+Note that unlike inflation rewards distribution, block revenue distribution will
+not impact any internal epoch rewards sysvar state fields like `total_rewards`
+or `distributed_rewards` since block revenue will instead be tracked via the
+epoch rewards sysvar lamport balance.
 
 #### Individual Delegator Reward
 
-The stake reward distribution amounts for each stake account calculated above
-can then be used to construct a list of stake reward entries which MUST be
-partitioned and distributed according to [SIMD-0118].
+For each individual stake account with an active non-zero delegation, multiply
+its active stake in reward epoch `N - 1` by `P`, and divide the result by
+`A` using integer division to get the individual stake account’s block revenue
+reward distribution amount, truncating any sub-lamport portion.
+
+#### Delegator Rewards Distribution
 
-When reward entries are used to distribute rewards pool funds during partitioned
-rewards distribution, the delegated vote account for each rewarded stake account
-must have its `pending_delegator_rewards` field and its balance deducted with
-the amount of rewards distributed to keep capitalization consistent.
+The reward distribution amounts for each stake account can then be used to
+construct a list of stake reward entries which MUST be partitioned according to
+[SIMD-0118] which describes which stake rewards are distributed in each block
+during the partitioned rewards distribution period.
+
+During partitioned reward distribution for a given slot, when reward entries are
+used to distribute block revenue rewards, the epoch rewards sysvar account’s
+lamport balance MUST be debited by the block revenue reward distribution amount
+to keep capitalization consistent.
 
 [SIMD-0118]: https://github.com/solana-foundation/solana-improvement-documents/pull/118
 
+#### Delegator Rewards Completion
+
+After distributing all partitioned delegator rewards, the epoch rewards sysvar balance
+MUST be reset to its rent exemption balance and any surplus lamports are burned.
+
 ### Vote Program
 
 #### Withdraw
@@ -246,5 +267,5 @@ NA
 
 ## Backwards Compatibility
 
-A feature gate will be used to enable block reward distribution at an epoch
-boundary.
+A feature gate will be used to enable block revenue reward distribution at an
+epoch boundary.