Altair consensus changes and refactors (#2279)

## Proposed Changes

Implement the consensus changes necessary for the upcoming Altair hard fork.

## Additional Info

This is quite a heavy refactor, with pivotal types like the `BeaconState` and `BeaconBlock` changing from structs to enums. This ripples through the whole codebase with field accesses changing to methods, e.g. `state.slot` => `state.slot()`.


Co-authored-by: realbigsean <seananderson33@gmail.com>

beacon_node/store/src/impls/beacon_state.rs

@@ -1,6 +1,6 @@
 use crate::*;
-use ssz::{Decode, DecodeError, Encode};
-use ssz_derive::{Decode, Encode};
+use ssz::{DecodeError, Encode};
+use ssz_derive::Encode;
 use std::convert::TryInto;
 use types::beacon_state::{CloneConfig, CommitteeCache, CACHED_EPOCHS};
 
@@ -23,13 +23,14 @@ pub fn store_full_state<E: EthSpec>(
 pub fn get_full_state<KV: KeyValueStore<E>, E: EthSpec>(
     db: &KV,
     state_root: &Hash256,
+    spec: &ChainSpec,
 ) -> Result<Option<BeaconState<E>>, Error> {
     let total_timer = metrics::start_timer(&metrics::BEACON_STATE_READ_TIMES);
 
     match db.get_bytes(DBColumn::BeaconState.into(), state_root.as_bytes())? {
         Some(bytes) => {
             let overhead_timer = metrics::start_timer(&metrics::BEACON_STATE_READ_OVERHEAD_TIMES);
-            let container = StorageContainer::from_ssz_bytes(&bytes)?;
+            let container = StorageContainer::from_ssz_bytes(&bytes, spec)?;
 
             metrics::stop_timer(overhead_timer);
             metrics::stop_timer(total_timer);
@@ -44,7 +45,7 @@ pub fn get_full_state<KV: KeyValueStore<E>, E: EthSpec>(
 
 /// A container for storing `BeaconState` components.
 // TODO: would be more space efficient with the caches stored separately and referenced by hash
-#[derive(Encode, Decode)]
+#[derive(Encode)]
 pub struct StorageContainer<T: EthSpec> {
     state: BeaconState<T>,
     committee_caches: Vec<CommitteeCache>,
@@ -55,9 +56,28 @@ impl<T: EthSpec> StorageContainer<T> {
     pub fn new(state: &BeaconState<T>) -> Self {
         Self {
             state: state.clone_with(CloneConfig::none()),
-            committee_caches: state.committee_caches.to_vec(),
+            committee_caches: state.committee_caches().to_vec(),
         }
     }
+
+    pub fn from_ssz_bytes(bytes: &[u8], spec: &ChainSpec) -> Result<Self, ssz::DecodeError> {
+        // We need to use the slot-switching `from_ssz_bytes` of `BeaconState`, which doesn't
+        // compose with the other SSZ utils, so we duplicate some parts of `ssz_derive` here.
+        let mut builder = ssz::SszDecoderBuilder::new(bytes);
+
+        builder.register_anonymous_variable_length_item()?;
+        builder.register_type::<Vec<CommitteeCache>>()?;
+
+        let mut decoder = builder.build()?;
+
+        let state = decoder.decode_next_with(|bytes| BeaconState::from_ssz_bytes(bytes, spec))?;
+        let committee_caches = decoder.decode_next()?;
+
+        Ok(Self {
+            state,
+            committee_caches,
+        })
+    }
 }
 
 impl<T: EthSpec> TryInto<BeaconState<T>> for StorageContainer<T> {
@@ -73,7 +93,7 @@ impl<T: EthSpec> TryInto<BeaconState<T>> for StorageContainer<T> {
                 )));
             };
 
-            state.committee_caches[i] = self.committee_caches.remove(i);
+            state.committee_caches_mut()[i] = self.committee_caches.remove(i);
         }
 
         Ok(state)

beacon_node/store/src/impls/partial_beacon_state.rs

@@ -1,16 +0,0 @@
-use crate::*;
-use ssz::{Decode, Encode};
-
-impl<T: EthSpec> StoreItem for PartialBeaconState<T> {
-    fn db_column() -> DBColumn {
-        DBColumn::BeaconState
-    }
-
-    fn as_store_bytes(&self) -> Vec<u8> {
-        self.as_ssz_bytes()
-    }
-
-    fn from_store_bytes(bytes: &[u8]) -> Result<Self, Error> {
-        Ok(Self::from_ssz_bytes(bytes)?)
-    }
-}