Merge pull request #305 from sigp/fork-choice-correction

Correct all fork choice rules for children with no votes.

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

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

eth2/fork_choice/src/protolambda_lmd_ghost.rs

@@ -1 +0,0 @@
-

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)

eth2/fork_choice/tests/bitwise_lmd_ghost_test_vectors.yaml

@@ -129,3 +129,16 @@ test_cases:
     - b9: 2
   heads:
     - id: 'b7'
+- blocks:
+    - id: 'b0'
+      parent: 'b0'
+    - id: 'b1'
+      parent: 'b0'
+    - id: 'b2'
+      parent: 'b0'
+  weights:
+    - b1: 0
+    - b2: 0
+  heads:
+    - id: 'b1'
+

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'