From 086e9574d2bce180ae8a60ef0f2845d029f80d06 Mon Sep 17 00:00:00 2001
From: Age Manning <Age@AgeManning.com>
Date: Thu, 14 Mar 2019 15:22:45 +1100
Subject: [PATCH] Correct all fork choice rules for children with no votes.

---
 eth2/fork_choice/src/bitwise_lmd_ghost.rs     | 22 ++++++++++++++-----
 eth2/fork_choice/src/optimized_lmd_ghost.rs   | 22 ++++++++++++++-----
 eth2/fork_choice/src/protolambda_lmd_ghost.rs |  1 -
 eth2/fork_choice/src/slow_lmd_ghost.rs        |  7 ++++++
 .../tests/bitwise_lmd_ghost_test_vectors.yaml | 15 ++++++++++++-
 .../tests/lmd_ghost_test_vectors.yaml         | 12 ++++++++++
 6 files changed, 67 insertions(+), 12 deletions(-)
 delete mode 100644 eth2/fork_choice/src/protolambda_lmd_ghost.rs

diff --git a/eth2/fork_choice/src/bitwise_lmd_ghost.rs b/eth2/fork_choice/src/bitwise_lmd_ghost.rs
index fd1c3dea4..d7b10015b 100644
--- a/eth2/fork_choice/src/bitwise_lmd_ghost.rs
+++ b/eth2/fork_choice/src/bitwise_lmd_ghost.rs
@@ -409,11 +409,23 @@ impl<T: ClientDB + Sized> ForkChoice for BitwiseLMDGhost<T> {
                         *child_votes.entry(child).or_insert_with(|| 0) += vote;
                     }
                 }
-                // given the votes on the children, find the best child
-                current_head = self
-                    .choose_best_child(&child_votes)
-                    .ok_or(ForkChoiceError::CannotFindBestChild)?;
-                trace!("Best child found: {}", current_head);
+                // check if we have votes of children, if not select the smallest hash child
+                if child_votes.is_empty() {
+                    current_head = *children
+                        .iter()
+                        .min_by(|child1, child2| child1.cmp(child2))
+                        .expect("Must be children here");
+                    trace!(
+                        "Children have no votes - smallest hash chosen: {}",
+                        current_head
+                    );
+                } else {
+                    // given the votes on the children, find the best child
+                    current_head = self
+                        .choose_best_child(&child_votes)
+                        .ok_or(ForkChoiceError::CannotFindBestChild)?;
+                    trace!("Best child found: {}", current_head);
+                }
             }
 
             // didn't find head yet, proceed to next iteration
diff --git a/eth2/fork_choice/src/optimized_lmd_ghost.rs b/eth2/fork_choice/src/optimized_lmd_ghost.rs
index 636ccdabc..30c84e9e1 100644
--- a/eth2/fork_choice/src/optimized_lmd_ghost.rs
+++ b/eth2/fork_choice/src/optimized_lmd_ghost.rs
@@ -380,11 +380,23 @@ impl<T: ClientDB + Sized> ForkChoice for OptimizedLMDGhost<T> {
                         *child_votes.entry(child).or_insert_with(|| 0) += vote;
                     }
                 }
-                // given the votes on the children, find the best child
-                current_head = self
-                    .choose_best_child(&child_votes)
-                    .ok_or(ForkChoiceError::CannotFindBestChild)?;
-                trace!("Best child found: {}", current_head);
+                // check if we have votes of children, if not select the smallest hash child
+                if child_votes.is_empty() {
+                    current_head = *children
+                        .iter()
+                        .min_by(|child1, child2| child1.cmp(child2))
+                        .expect("Must be children here");
+                    trace!(
+                        "Children have no votes - smallest hash chosen: {}",
+                        current_head
+                    );
+                } else {
+                    // given the votes on the children, find the best child
+                    current_head = self
+                        .choose_best_child(&child_votes)
+                        .ok_or(ForkChoiceError::CannotFindBestChild)?;
+                    trace!("Best child found: {}", current_head);
+                }
             }
 
             // didn't find head yet, proceed to next iteration
diff --git a/eth2/fork_choice/src/protolambda_lmd_ghost.rs b/eth2/fork_choice/src/protolambda_lmd_ghost.rs
deleted file mode 100644
index 8b1378917..000000000
--- a/eth2/fork_choice/src/protolambda_lmd_ghost.rs
+++ /dev/null
@@ -1 +0,0 @@
-
diff --git a/eth2/fork_choice/src/slow_lmd_ghost.rs b/eth2/fork_choice/src/slow_lmd_ghost.rs
index ab4cd2ada..abf13f21b 100644
--- a/eth2/fork_choice/src/slow_lmd_ghost.rs
+++ b/eth2/fork_choice/src/slow_lmd_ghost.rs
@@ -210,6 +210,7 @@ impl<T: ClientDB + Sized> ForkChoice for SlowLMDGhost<T> {
             trace!("Children found: {:?}", children);
 
             let mut head_vote_count = 0;
+            head_hash = children[0];
             for child_hash in children {
                 let vote_count = self.get_vote_count(&latest_votes, &child_hash)?;
                 trace!("Vote count for child: {} is: {}", child_hash, vote_count);
@@ -218,6 +219,12 @@ impl<T: ClientDB + Sized> ForkChoice for SlowLMDGhost<T> {
                     head_hash = *child_hash;
                     head_vote_count = vote_count;
                 }
+                // resolve ties - choose smaller hash
+                else if vote_count == head_vote_count {
+                    if *child_hash < head_hash {
+                        head_hash = *child_hash;
+                    }
+                }
             }
         }
         Ok(head_hash)
diff --git a/eth2/fork_choice/tests/bitwise_lmd_ghost_test_vectors.yaml b/eth2/fork_choice/tests/bitwise_lmd_ghost_test_vectors.yaml
index 931d8decf..61b0b05c4 100644
--- a/eth2/fork_choice/tests/bitwise_lmd_ghost_test_vectors.yaml
+++ b/eth2/fork_choice/tests/bitwise_lmd_ghost_test_vectors.yaml
@@ -128,4 +128,17 @@ test_cases:
     - b8: 4
     - b9: 2
   heads:
-    - id: 'b7'
\ No newline at end of file
+    - id: 'b7'
+- blocks:
+    - id: 'b0'
+      parent: 'b0'
+    - id: 'b1'
+      parent: 'b0'
+    - id: 'b2'
+      parent: 'b0'
+  weights:
+    - b1: 0
+    - b2: 0
+  heads:
+    - id: 'b1'
+
diff --git a/eth2/fork_choice/tests/lmd_ghost_test_vectors.yaml b/eth2/fork_choice/tests/lmd_ghost_test_vectors.yaml
index dab998beb..e7847de11 100644
--- a/eth2/fork_choice/tests/lmd_ghost_test_vectors.yaml
+++ b/eth2/fork_choice/tests/lmd_ghost_test_vectors.yaml
@@ -51,3 +51,15 @@ test_cases:
     - b3: 6
   heads:
     - id: 'b2'
+- blocks:
+    - id: 'b0'
+      parent: 'b0'
+    - id: 'b1'
+      parent: 'b0'
+    - id: 'b2'
+      parent: 'b0'
+  weights:
+    - b1: 0
+    - b2: 0
+  heads:
+    - id: 'b1'