add check and test (#7853)

Co-authored-by: terence tsao <terence@prysmaticlabs.com>
Co-authored-by: Radosław Kapka <rkapka@wp.pl>

beacon-chain/state/stategen/getter.go

@@ -193,6 +193,11 @@ func (s *State) loadStateByRoot(ctx context.Context, blockRoot [32]byte) (*state
 		return nil, errUnknownBoundaryState
 	}
 
+	// Return state early if we are retrieving it from our finalized state cache.
+	if startState.Slot() == targetSlot {
+		return startState, nil
+	}
+
 	blks, err := s.LoadBlocks(ctx, startState.Slot()+1, targetSlot, bytesutil.ToBytes32(summary.Root))
 	if err != nil {
 		return nil, errors.Wrap(err, "could not load blocks for hot state using root")

beacon-chain/state/stategen/getter_test.go

@@ -272,6 +272,33 @@ func TestLoadeStateByRoot_Cached(t *testing.T) {
 	}
 }
 
+func TestLoadeStateByRoot_FinalizedState(t *testing.T) {
+	ctx := context.Background()
+	db, _ := testDB.SetupDB(t)
+	service := New(db, cache.NewStateSummaryCache())
+
+	beaconState, _ := testutil.DeterministicGenesisState(t, 32)
+	genesisStateRoot, err := beaconState.HashTreeRoot(ctx)
+	require.NoError(t, err)
+	genesis := blocks.NewGenesisBlock(genesisStateRoot[:])
+	assert.NoError(t, db.SaveBlock(ctx, genesis))
+	gRoot, err := genesis.Block.HashTreeRoot()
+	require.NoError(t, err)
+	require.NoError(t, service.beaconDB.SaveStateSummary(ctx, &pb.StateSummary{Slot: 0, Root: gRoot[:]}))
+
+	service.finalizedInfo.state = beaconState
+	service.finalizedInfo.slot = beaconState.Slot()
+	service.finalizedInfo.root = gRoot
+
+	// This tests where hot state was already cached.
+	loadedState, err := service.loadStateByRoot(ctx, gRoot)
+	require.NoError(t, err)
+
+	if !proto.Equal(loadedState.InnerStateUnsafe(), beaconState.InnerStateUnsafe()) {
+		t.Error("Did not correctly retrieve finalized state")
+	}
+}
+
 func TestLoadeStateByRoot_EpochBoundaryStateCanProcess(t *testing.T) {
 	ctx := context.Background()
 	db, ssc := testDB.SetupDB(t)