From 770a2ca03053e425410b58cad1fed96a64e3a680 Mon Sep 17 00:00:00 2001
From: Michael Sproul <michael@sigmaprime.io>
Date: Wed, 10 Mar 2021 00:20:50 +0000
Subject: [PATCH] Fix proposer cache priming upon state advance (#2252)
MIME-Version: 1.0
Content-Type: text/plain; charset=UTF-8
Content-Transfer-Encoding: 8bit

## Proposed Changes

While investigating an incorrect head + target vote for the epoch boundary block 708544, I noticed that the state advance failed to prime the proposer cache, as per these logs:

```
Mar 09 21:42:47.448 DEBG Subscribing to subnet                   target_slot: 708544, subnet: Y, service: attestation_service
Mar 09 21:49:08.063 DEBG Advanced head state one slot            current_slot: 708543, state_slot: 708544, head_root: 0xaf5e69de09f384ee3b4fb501458b7000c53bb6758a48817894ec3d2b030e3e6f, service: state_advance
Mar 09 21:49:08.063 DEBG Completed state advance                 initial_slot: 708543, advanced_slot: 708544, head_root: 0xaf5e69de09f384ee3b4fb501458b7000c53bb6758a48817894ec3d2b030e3e6f, service: state_advance
Mar 09 21:49:14.787 DEBG Proposer shuffling cache miss           block_slot: 708544, block_root: 0x9b14bf68667ab1d9c35e6fd2c95ff5d609aa9e8cf08e0071988ae4aa00b9f9fe, parent_slot: 708543, parent_root: 0xaf5e69de09f384ee3b4fb501458b7000c53bb6758a48817894ec3d2b030e3e6f, service: beacon
Mar 09 21:49:14.800 DEBG Successfully processed gossip block     root: 0x9b14bf68667ab1d9c35e6fd2c95ff5d609aa9e8cf08e0071988ae4aa00b9f9fe, slot: 708544, graffiti: , service: beacon
Mar 09 21:49:14.800 INFO New block received                      hash: 0x9b14…f9fe, slot: 708544
Mar 09 21:49:14.984 DEBG Head beacon block                       slot: 708544, root: 0x9b14…f9fe, finalized_epoch: 22140, finalized_root: 0x28ec…29a7, justified_epoch: 22141, justified_root: 0x59db…e451, service: beacon
Mar 09 21:49:15.055 INFO Unaggregated attestation                validator: XXXXX, src: api, slot: 708544, epoch: 22142, delay_ms: 53, index: Y, head: 0xaf5e69de09f384ee3b4fb501458b7000c53bb6758a48817894ec3d2b030e3e6f, service: val_mon
Mar 09 21:49:17.001 DEBG Slot timer                              sync_state: Synced, current_slot: 708544, head_slot: 708544, head_block: 0x9b14…f9fe, finalized_epoch: 22140, finalized_root: 0x28ec…29a7, peers: 55, service: slot_notifier
```

The reason for this is that the condition was backwards, so that whole block of code was unreachable.

Looking at the attestations for the block included in the block after, we can see that lots of validators missed it. Some of them may be Lighthouse v1.1.1-v1.2.0-rc.0, but it's probable that they would have missed even with the proposer cache primed, given how late the block 708544 arrived (the cache miss occurred 3.787s after the slot start): https://beaconcha.in/block/708545#attestations
---
 beacon_node/beacon_chain/src/state_advance_timer.rs | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/beacon_node/beacon_chain/src/state_advance_timer.rs b/beacon_node/beacon_chain/src/state_advance_timer.rs
index 36bd9795c..64b540641 100644
--- a/beacon_node/beacon_chain/src/state_advance_timer.rs
+++ b/beacon_node/beacon_chain/src/state_advance_timer.rs
@@ -252,9 +252,9 @@ fn advance_head<T: BeaconChainTypes>(
         "current_slot" => current_slot,
     );
 
-    // If the `pre_state` is in a later epoch than `state`, pre-emptively add the proposer
-    // shuffling for the next epoch into the cache.
-    if initial_epoch > state.current_epoch() {
+    // If the advanced state is in a later epoch than where it started, pre-emptively add the
+    // proposer shuffling for the new epoch into the cache.
+    if state.current_epoch() > initial_epoch {
         debug!(
             log,
             "Priming proposer cache";