fixes data race in state.Slot (#5067)

* fixes data race in state/getters

beacon-chain/state/BUILD.bazel

@@ -34,6 +34,7 @@ go_library(
 go_test(
     name = "go_default_test",
     srcs = [
+        "getters_test.go",
         "references_test.go",
         "types_test.go",
     ],

beacon-chain/state/getters.go

@@ -134,6 +134,9 @@ func (b *BeaconState) Slot() uint64 {
 	if !b.HasInnerState() {
 		return 0
 	}
+	b.lock.RLock()
+	defer b.lock.RUnlock()
+
 	return b.state.Slot
 }
 

beacon-chain/state/getters_test.go

@@ -0,0 +1,25 @@
+package state
+
+import (
+	"sync"
+	"testing"
+
+	pb "github.com/prysmaticlabs/prysm/proto/beacon/p2p/v1"
+)
+
+func TestBeaconState_SlotDataRace(t *testing.T) {
+	headState, _ := InitializeFromProto(&pb.BeaconState{Slot: 1})
+
+	wg := sync.WaitGroup{}
+	wg.Add(2)
+	go func() {
+		headState.SetSlot(uint64(0))
+		wg.Done()
+	}()
+	go func() {
+		headState.Slot()
+		wg.Done()
+	}()
+
+	wg.Wait()
+}