Add nil check for head in IsOptimistic (#13439)

beacon-chain/blockchain/chain_info.go

@@ -340,6 +340,10 @@ func (s *Service) IsOptimistic(_ context.Context) (bool, error) {
 		return false, nil
 	}
 	s.headLock.RLock()
+	if s.head == nil {
+		s.headLock.RUnlock()
+		return false, ErrNilHead
+	}
 	headRoot := s.head.root
 	headSlot := s.head.slot
 	headOptimistic := s.head.optimistic

beacon-chain/blockchain/chain_info_test.go

@@ -429,6 +429,11 @@ func TestService_IsOptimistic(t *testing.T) {
 	opt, err = c.IsOptimistic(ctx)
 	require.NoError(t, err)
 	require.Equal(t, true, opt)
+
+	// If head is nil, for some reason, an error should be returned rather than panic.
+	c = &Service{}
+	_, err = c.IsOptimistic(ctx)
+	require.ErrorIs(t, err, ErrNilHead)
 }
 
 func TestService_IsOptimisticBeforeBellatrix(t *testing.T) {

beacon-chain/blockchain/error.go

@@ -28,6 +28,8 @@ var (
 	// ErrNotCheckpoint is returned when a given checkpoint is not a
 	// checkpoint in any chain known to forkchoice
 	ErrNotCheckpoint = errors.New("not a checkpoint in forkchoice")
+	// ErrNilHead is returned when no head is present in the blockchain service.
+	ErrNilHead = errors.New("nil head")
 )
 
 var errMaxBlobsExceeded = errors.New("Expected commitments in block exceeds MAX_BLOBS_PER_BLOCK")