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/beacon_chain/src/fork_choice.rs

@@ -125,14 +125,14 @@ impl<T: BeaconChainTypes> ForkChoice<T> {
         state: &BeaconState<T::EthSpec>,
         attestation: &Attestation,
     ) -> Result<()> {
-        // Note: `get_attesting_indices_unsorted` requires that the beacon state caches be built.
         let validator_indices = get_attesting_indices_unsorted(
             state,
             &attestation.data,
             &attestation.aggregation_bitfield,
         )?;
+        let block_slot = state.get_attestation_slot(&attestation.data)?;
 
-        let block_hash = attestation.data.target_root;
+        let block_hash = attestation.data.beacon_block_root;
 
         // Ignore any attestations to the zero hash.
         //
@@ -148,11 +148,6 @@ impl<T: BeaconChainTypes> ForkChoice<T> {
         // fine because votes to the genesis block are not useful; all validators implicitly attest
         // to genesis just by being present in the chain.
         if block_hash != Hash256::zero() {
-            let block_slot = attestation
-                .data
-                .target_epoch
-                .start_slot(T::EthSpec::slots_per_epoch());
-
             for validator_index in validator_indices {
                 self.backend
                     .process_attestation(validator_index, block_hash, block_slot)?;