From 3f13c25c05a4c6084ec8e51bca7c31297b337bb8 Mon Sep 17 00:00:00 2001
From: Paul Hauner <paul@paulhauner.com>
Date: Thu, 31 Jan 2019 18:28:54 +1100
Subject: [PATCH] Fix issues with per_epoch block_root calls

They were being called with the wrong slot.
---
 beacon_node/beacon_chain/src/attestation_production.rs |  3 ++-
 eth2/types/src/beacon_state/committees.rs              |  8 ++++++++
 eth2/types/src/beacon_state/epoch_processing.rs        | 10 +++-------
 3 files changed, 13 insertions(+), 8 deletions(-)

diff --git a/beacon_node/beacon_chain/src/attestation_production.rs b/beacon_node/beacon_chain/src/attestation_production.rs
index 5adf70a4a..9755f48af 100644
--- a/beacon_node/beacon_chain/src/attestation_production.rs
+++ b/beacon_node/beacon_chain/src/attestation_production.rs
@@ -30,8 +30,9 @@ where
             .ok_or_else(|| Error::SlotTooOld)?;
 
         let head_slot = self.head().beacon_block.slot;
+        let previous_epoch_start_slot = head_slot - (head_slot % self.spec.epoch_length);
         let epoch_boundary_root = *state
-            .get_block_root(head_slot % self.spec.epoch_length, &self.spec)
+            .get_block_root(previous_epoch_start_slot, &self.spec)
             .ok_or_else(|| Error::SlotTooOld)?;
 
         Ok(AttestationData {
diff --git a/eth2/types/src/beacon_state/committees.rs b/eth2/types/src/beacon_state/committees.rs
index d4fd725da..1e1689e92 100644
--- a/eth2/types/src/beacon_state/committees.rs
+++ b/eth2/types/src/beacon_state/committees.rs
@@ -27,6 +27,14 @@ impl BeaconState {
         self.current_epoch(spec).saturating_sub(1)
     }
 
+    pub fn current_epoch_start_slot(&self, spec: &ChainSpec) -> u64 {
+        self.current_epoch(spec) * spec.epoch_length
+    }
+
+    pub fn previous_epoch_start_slot(&self, spec: &ChainSpec) -> u64 {
+        self.previous_epoch(spec) * spec.epoch_length
+    }
+
     /// Returns the number of committees per slot.
     ///
     /// Note: this is _not_ the committee size.
diff --git a/eth2/types/src/beacon_state/epoch_processing.rs b/eth2/types/src/beacon_state/epoch_processing.rs
index 2ff6d58c3..f38024181 100644
--- a/eth2/types/src/beacon_state/epoch_processing.rs
+++ b/eth2/types/src/beacon_state/epoch_processing.rs
@@ -71,8 +71,7 @@ impl BeaconState {
             current_epoch_attestations
                 .iter()
                 .filter(|a| {
-                    // TODO: ensure this saturating sub is correct.
-                    match self.get_block_root(self.slot.saturating_sub(spec.epoch_length), spec) {
+                    match self.get_block_root(self.current_epoch_start_slot(spec), spec) {
                         Some(block_root) => {
                             (a.data.epoch_boundary_root == *block_root)
                                 && (a.data.justified_slot == self.justified_slot)
@@ -142,9 +141,7 @@ impl BeaconState {
             previous_epoch_justified_attestations
                 .iter()
                 .filter(|a| {
-                    // TODO: ensure this saturating sub is correct.
-                    match self.get_block_root(self.slot.saturating_sub(2 * spec.epoch_length), spec)
-                    {
+                    match self.get_block_root(self.previous_epoch_start_slot(spec), spec) {
                         Some(block_root) => a.data.epoch_boundary_root == *block_root,
                         // Protected by a check that latest_block_roots isn't empty.
                         //
@@ -167,8 +164,7 @@ impl BeaconState {
             previous_epoch_attestations
                 .iter()
                 .filter(|a| {
-                    match self.get_block_root(self.slot.saturating_sub(2 * spec.epoch_length), spec)
-                    {
+                    match self.get_block_root(a.data.slot, spec) {
                         Some(block_root) => a.data.beacon_block_root == *block_root,
                         // Protected by a check that latest_block_roots isn't empty.
                         //