Fork choice bug fixes (#449)

* Change reduced tree for adding weightless node

* Add more comments for reduced tree fork choice

* Small refactor on reduced tree for readability

* Move test_harness forking logic into itself

* Add new `AncestorIter` trait to store

* Add unfinished tests to fork choice

* Make `beacon_state.genesis_block_root` public

* Add failing lmd_ghost fork choice tests

* Extend fork_choice tests, create failing test

* Implement Debug for generic ReducedTree

* Add lazy_static to fork choice tests

* Add verify_integrity fn to reduced tree

* Fix bugs in reduced tree

* Ensure all reduced tree tests verify integrity

* Slightly alter reduce tree test params

* Add (failing) reduced tree test

* Fix bug in fork choice

Iter ancestors was not working well with skip slots

* Put maximum depth for common ancestor search

Ensures that we don't search back past the finalized root.

* Add basic finalization tests for reduced tree

* Change fork choice to use beacon_block_root

Previously it was using target_root, which was wrong

* Make ancestor iter return option

* Disable fork choice test when !debug_assertions

* Fix type, removed code fragment

* Tidy some borrow-checker evading

* Lower reduced tree random test iterations

beacon_node/store/src/iter.rs

@@ -3,6 +3,22 @@ use std::borrow::Cow;
 use std::sync::Arc;
 use types::{BeaconBlock, BeaconState, BeaconStateError, EthSpec, Hash256, Slot};
 
+/// Implemented for types that have ancestors (e.g., blocks, states) that may be iterated over.
+pub trait AncestorIter<U: Store, I: Iterator> {
+    /// Returns an iterator over the roots of the ancestors of `self`.
+    fn try_iter_ancestor_roots(&self, store: Arc<U>) -> Option<I>;
+}
+
+impl<'a, U: Store, E: EthSpec> AncestorIter<U, BestBlockRootsIterator<'a, E, U>> for BeaconBlock {
+    /// Iterates across all the prior block roots of `self`, starting at the most recent and ending
+    /// at genesis.
+    fn try_iter_ancestor_roots(&self, store: Arc<U>) -> Option<BestBlockRootsIterator<'a, E, U>> {
+        let state = store.get::<BeaconState<E>>(&self.state_root).ok()??;
+
+        Some(BestBlockRootsIterator::owned(store, state, self.slot))
+    }
+}
+
 #[derive(Clone)]
 pub struct StateRootsIterator<'a, T: EthSpec, U> {
     store: Arc<U>,