BeaconState: Use copy on write for validator index map (#4713)

* Use copy on write for validator index map
* Merge refs/heads/master into copy-on-write-2

beacon-chain/state/getters.go

@@ -324,6 +324,15 @@ func (b *BeaconState) ValidatorIndexByPubkey(key [48]byte) (uint64, bool) {
 	return idx, ok
 }
 
+func (b *BeaconState) validatorIndexMap() map[[48]byte]uint64 {
+	m := make(map[[48]byte]uint64, len(b.valIdxMap))
+
+	for k, v := range b.valIdxMap {
+		m[k] = v
+	}
+	return m
+}
+
 // PubkeyAtIndex returns the pubkey at the given
 // validator index.
 func (b *BeaconState) PubkeyAtIndex(idx uint64) [48]byte {

beacon-chain/state/setters.go

@@ -228,10 +228,10 @@ func (b *BeaconState) UpdateValidatorAtIndex(idx uint64, val *ethpb.Validator) e
 // SetValidatorIndexByPubkey updates the validator index mapping maintained internally to
 // a given input 48-byte, public key.
 func (b *BeaconState) SetValidatorIndexByPubkey(pubKey [48]byte, validatorIdx uint64) {
-	b.lock.Lock()
-	b.valIdxMap[pubKey] = validatorIdx
-	b.markFieldAsDirty(validators)
-	b.lock.Unlock()
+	// Copy on write since this is a shared map.
+	m := b.validatorIndexMap()
+	m[pubKey] = validatorIdx
+	b.valIdxMap = m
 }
 
 // SetBalances for the beacon state. This PR updates the entire

beacon-chain/state/types.go

@@ -84,17 +84,15 @@ func (b *BeaconState) Copy() *BeaconState {
 			FinalizedCheckpoint:         b.FinalizedCheckpoint(),
 		},
 		dirtyFields: make(map[fieldIndex]interface{}, 20),
-		valIdxMap:   make(map[[48]byte]uint64, len(b.valIdxMap)),
+
+		// Copy on write validator index map.
+		valIdxMap:   b.valIdxMap,
 	}
 
 	for i := range b.dirtyFields {
 		dst.dirtyFields[i] = true
 	}
 
-	for i := range b.valIdxMap {
-		dst.valIdxMap[i] = b.valIdxMap[i]
-	}
-
 	dst.merkleLayers = make([][][]byte, len(b.merkleLayers))
 	for i, layer := range b.merkleLayers {
 		dst.merkleLayers[i] = make([][]byte, len(layer))