From da26341011eca01064ec1579e7cdc345be433204 Mon Sep 17 00:00:00 2001
From: Paul Hauner <paul@paulhauner.com>
Date: Sat, 14 Sep 2019 22:26:09 -0400
Subject: [PATCH] Add beacon chain event for finalization

---
 beacon_node/beacon_chain/src/beacon_chain.rs | 57 +++++++++++---------
 beacon_node/beacon_chain/src/events.rs       |  6 ++-
 2 files changed, 37 insertions(+), 26 deletions(-)

diff --git a/beacon_node/beacon_chain/src/beacon_chain.rs b/beacon_node/beacon_chain/src/beacon_chain.rs
index 33205e97c..d3f5e9d7d 100644
--- a/beacon_node/beacon_chain/src/beacon_chain.rs
+++ b/beacon_node/beacon_chain/src/beacon_chain.rs
@@ -1379,8 +1379,10 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
             let previous_slot = self.head().beacon_block.slot;
             let new_slot = beacon_block.slot;
 
+            let is_reorg = self.head().beacon_block_root != beacon_block.parent_root;
+
             // If we switched to a new chain (instead of building atop the present chain).
-            if self.head().beacon_block_root != beacon_block.parent_root {
+            if is_reorg {
                 metrics::inc_counter(&metrics::FORK_CHOICE_REORG_COUNT);
                 warn!(
                     self.log,
@@ -1415,12 +1417,34 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
                     new_epoch: new_finalized_epoch,
                 })
             } else {
-                self.update_canonical_head(CheckPoint {
+                let previous_head_beacon_block_root = self.canonical_head.read().beacon_block_root;
+                let current_head_beacon_block_root = beacon_block_root;
+
+                let mut new_head = CheckPoint {
                     beacon_block,
                     beacon_block_root,
                     beacon_state,
                     beacon_state_root,
-                })?;
+                };
+
+                new_head.beacon_state.build_all_caches(&self.spec)?;
+
+                let timer = metrics::start_timer(&metrics::UPDATE_HEAD_TIMES);
+
+                // Update the checkpoint that stores the head of the chain at the time it received the
+                // block.
+                *self.canonical_head.write() = new_head;
+
+                metrics::stop_timer(timer);
+
+                // Save `self` to `self.store`.
+                self.persist()?;
+
+                let _ = self.event_handler.register(EventKind::BeaconHeadChanged {
+                    reorg: is_reorg,
+                    previous_head_beacon_block_root,
+                    current_head_beacon_block_root,
+                });
 
                 if new_finalized_epoch != old_finalized_epoch {
                     self.after_finalization(old_finalized_epoch, finalized_root)?;
@@ -1442,28 +1466,6 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
         result
     }
 
-    /// Update the canonical head to `new_head`.
-    fn update_canonical_head(&self, mut new_head: CheckPoint<T::EthSpec>) -> Result<(), Error> {
-        let timer = metrics::start_timer(&metrics::UPDATE_HEAD_TIMES);
-
-        new_head.beacon_state.build_all_caches(&self.spec)?;
-
-        trace!(self.log, "Taking write lock on head");
-
-        // Update the checkpoint that stores the head of the chain at the time it received the
-        // block.
-        *self.canonical_head.write() = new_head;
-
-        trace!(self.log, "Dropping write lock on head");
-
-        // Save `self` to `self.store`.
-        self.persist()?;
-
-        metrics::stop_timer(timer);
-
-        Ok(())
-    }
-
     /// Called after `self` has had a new block finalized.
     ///
     /// Performs pruning and finality-based optimizations.
@@ -1495,6 +1497,11 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
 
             self.op_pool.prune_all(&finalized_state, &self.spec);
 
+            let _ = self.event_handler.register(EventKind::BeaconFinalization {
+                epoch: new_finalized_epoch,
+                root: finalized_block_root,
+            });
+
             Ok(())
         }
     }
diff --git a/beacon_node/beacon_chain/src/events.rs b/beacon_node/beacon_chain/src/events.rs
index d690eabf1..c93a13c8a 100644
--- a/beacon_node/beacon_chain/src/events.rs
+++ b/beacon_node/beacon_chain/src/events.rs
@@ -1,6 +1,6 @@
 use serde_derive::{Deserialize, Serialize};
 use std::marker::PhantomData;
-use types::{Attestation, BeaconBlock, EthSpec, Hash256};
+use types::{Attestation, BeaconBlock, Epoch, EthSpec, Hash256};
 
 pub trait EventHandler<T: EthSpec>: Sized + Send + Sync {
     fn register(&self, kind: EventKind<T>) -> Result<(), String>;
@@ -33,6 +33,10 @@ pub enum EventKind<T: EthSpec> {
         current_head_beacon_block_root: Hash256,
         previous_head_beacon_block_root: Hash256,
     },
+    BeaconFinalization {
+        epoch: Epoch,
+        root: Hash256,
+    },
     BeaconBlockImported {
         block_root: Hash256,
         block: Box<BeaconBlock<T>>,