Cannot use prepare phase length to determine reward set calculation

Signed-off-by: Jacinta Ferrant <[email protected]>

Author: jacinta-stacks

stacks-common/src/types/mod.rs

@@ -663,6 +663,31 @@ impl StacksEpochId {
         self >= &StacksEpochId::Epoch30
     }
 
+    /// Does this epoch use the nakamoto reward set, or the epoch2 reward set?
+    /// We use the epoch2 reward set in all pre-3.0 epochs.
+    /// We also use the epoch2 reward set in the first 3.0 reward cycle.
+    /// After that, we use the nakamoto reward set.
+    pub fn uses_nakamoto_reward_set(
+        &self,
+        cur_reward_cycle: u64,
+        first_epoch30_reward_cycle: u64,
+    ) -> bool {
+        match self {
+            StacksEpochId::Epoch10
+            | StacksEpochId::Epoch20
+            | StacksEpochId::Epoch2_05
+            | StacksEpochId::Epoch21
+            | StacksEpochId::Epoch22
+            | StacksEpochId::Epoch23
+            | StacksEpochId::Epoch24
+            | StacksEpochId::Epoch25 => false,
+            StacksEpochId::Epoch30
+            | StacksEpochId::Epoch31
+            | StacksEpochId::Epoch32
+            | StacksEpochId::Epoch33 => cur_reward_cycle > first_epoch30_reward_cycle,
+        }
+    }
+
     /// What is the coinbase (in uSTX) to award for the given burnchain height?
     /// Applies prior to SIP-029
     fn coinbase_reward_pre_sip029(

stackslib/src/chainstate/nakamoto/coordinator/mod.rs

@@ -362,15 +362,28 @@ pub fn load_nakamoto_reward_set<U: RewardSetProvider>(
     sort_db: &SortitionDB,
     provider: &U,
 ) -> Result<Option<(RewardCycleInfo, StacksHeaderInfo)>, Error> {
-    // If the prepare phase started in pre-Nakamoto, it should be using Epoch 2.5 reward
-    // set calculation rules.
     let cycle_start_height = burnchain.nakamoto_first_block_of_cycle(reward_cycle);
-    // This is safe to cast because a u32 always fits in a u64
-    let prepare_phase_start =
-        cycle_start_height.saturating_sub(u64::from(burnchain.pox_constants.prepare_length));
-    let epoch_at_height = SortitionDB::get_stacks_epoch(sort_db.conn(), prepare_phase_start)?
-        .unwrap_or_else(|| panic!("FATAL: no epoch defined for burn height {prepare_phase_start}"));
-    if epoch_at_height.epoch_id < StacksEpochId::Epoch30 {
+    let epoch_at_height = SortitionDB::get_stacks_epoch(sort_db.conn(), cycle_start_height)?
+        .unwrap_or_else(|| panic!("FATAL: no epoch defined for burn height {cycle_start_height}"));
+    let is_pre_naka_epoch = if epoch_at_height.epoch_id < StacksEpochId::Epoch30 {
+        true
+    } else {
+        let epoch_30 =
+            SortitionDB::get_stacks_epoch_by_epoch_id(sort_db.conn(), &StacksEpochId::Epoch30)?
+                .unwrap_or_else(|| panic!("FATAL: no Nakamoto epoch defined"));
+        // Find the first Stacks block in this reward cycle's preceding prepare phase.
+        // This block will have invoked `.signers.stackerdb-set-signer-slots()` with the reward set.
+        // Note that we may not have processed it yet. But, if we do find it, then it's
+        // unique (and since Nakamoto Stacks blocks are processed in order, the anchor block
+        // cannot change later).
+        let first_epoch30_reward_cycle = burnchain
+            .block_height_to_reward_cycle(epoch_30.start_height)
+            .expect("FATAL: no reward cycle for epoch 3.0 start height");
+        !epoch_at_height
+            .epoch_id
+            .uses_nakamoto_reward_set(reward_cycle, first_epoch30_reward_cycle)
+    };
+    if is_pre_naka_epoch {
         // in epoch 2.5, and in the first reward cycle of epoch 3.0, the reward set can *only* be found in the sortition DB.
         // The nakamoto chain-processing rules aren't active yet, so we can't look for the reward
         // cycle info in the nakamoto chain state.