Encode nil map value for UDTs as NULL

Previously, the driver was encoding nil map values for UDT-typed columns as
an initialized map with zero values for each UDT field.
This patch changes its behaviour to encode nil maps as NULL values in Cassandra.

Patch by Bohdan Siryk; reviewed by TBD for CASSGO-118

Author: worryg0d

cassandra_test.go

@@ -256,6 +256,55 @@ func TestObserve(t *testing.T) {
 	}
 }
 
+func TestObserve_KeyspaceOverrideAPI(t *testing.T) {
+	session := createSession(t)
+	defer session.Close()
+
+	if err := createTable(session, `CREATE KEYSPACE IF NOT EXISTS gocql_test2 WITH replication = { 'class' : 'SimpleStrategy', 'replication_factor' : 1 }`); err != nil {
+		t.Fatal("create:", err)
+	}
+
+	if err := createTable(session, `CREATE TABLE IF NOT EXISTS gocql_test2.observe (id int, PRIMARY KEY (id))`); err != nil {
+		t.Fatal("create:", err)
+	}
+
+	var observedKeyspace string
+
+	resetObserved := func() {
+		observedKeyspace = ""
+	}
+
+	observer := funcQueryObserver(func(ctx context.Context, o ObservedQuery) {
+		observedKeyspace = o.Keyspace
+	})
+
+	const overrideKeyspace = "gocql_test2"
+
+	// Session default keyspace
+	err := session.Query(`INSERT INTO observe (id) VALUES (?)`, 42).
+		Observer(observer).
+		SetKeyspace(overrideKeyspace).
+		Exec()
+	if err != nil {
+		t.Fatal("insert:", err)
+	}
+	if observedKeyspace != overrideKeyspace {
+		t.Fatalf("expected keyspace '%s', got %s", overrideKeyspace, observedKeyspace)
+	}
+
+	resetObserved()
+	var id int
+	if err := session.Query(`SELECT id FROM observe WHERE id = ?`, 42).
+		Observer(observer).
+		SetKeyspace(overrideKeyspace).
+		Scan(&id); err != nil {
+		t.Fatal("select:", err)
+	}
+	if observedKeyspace != overrideKeyspace {
+		t.Fatalf("expected keyspace '%s', got %s", overrideKeyspace, observedKeyspace)
+	}
+}
+
 func TestObserve_Pagination(t *testing.T) {
 	session := createSession(t)
 	defer session.Close()
@@ -4676,3 +4725,43 @@ func TestIterMapScanUDT(t *testing.T) {
 	require.NoError(t, err)
 	require.Nil(t, rawResult["value"])
 }
+
+func TestUDT_MarshalNilMap(t *testing.T) {
+	session := createSession(t)
+	defer session.Close()
+
+	err := createTable(session, `CREATE TYPE IF NOT EXISTS gocql_test.nil_map_udt (
+		field_a text,
+		field_b int
+	);`)
+	require.NoError(t, err)
+
+	err = createTable(session, `CREATE TABLE IF NOT EXISTS gocql_test.nil_map_udt_table (
+		id int PRIMARY KEY,
+		value frozen<nil_map_udt>
+	);`)
+	require.NoError(t, err)
+
+	nilMap := map[string]interface{}(nil)
+	err = session.Query("INSERT INTO nil_map_udt_table (id, value) VALUES (?, ?)", 1, nilMap).Exec()
+	require.NoError(t, err)
+
+	ctx := context.Background()
+
+	const selectQuery = "SELECT value FROM nil_map_udt_table WHERE id = ?"
+	t.Run("scan", func(t *testing.T) {
+		var scanned map[string]interface{}
+		err = session.Query(selectQuery, 1).ScanContext(ctx, &scanned)
+		require.NoError(t, err)
+		require.Nil(t, scanned)
+	})
+
+	t.Run("iter map scan", func(t *testing.T) {
+		rawResult := map[string]interface{}{}
+		rawResultIter := session.Query(selectQuery, 1).IterContext(ctx)
+		rawResultIter.MapScan(rawResult)
+		err = rawResultIter.Close()
+		require.NoError(t, err)
+		require.Nil(t, rawResult["value"])
+	})
+}

marshal.go

@@ -2855,6 +2855,12 @@ func (udt UDTTypeInfo) Marshal(value interface{}) ([]byte, error) {
 
 		return buf, nil
 	case map[string]interface{}:
+		if v == nil {
+			// If the map is nil, we should marshal it as NULL value.
+			// framer will encode this as NULL value
+			return nil, nil
+		}
+
 		var buf []byte
 		for i := range udt.Elements {
 			val, ok := v[udt.Elements[i].Name]

marshal_test.go

@@ -2131,6 +2131,20 @@ func TestMarshalUDTMap(t *testing.T) {
 		},
 	}
 
+	t.Run("nil map", func(t *testing.T) {
+		value := map[string]interface{}(nil)
+		expected := []byte(nil)
+
+		data, err := Marshal(typeInfo, value)
+		if err != nil {
+			t.Errorf("got error %#v", err)
+		}
+
+		if !reflect.DeepEqual(data, expected) {
+			t.Errorf("got value %#v", data)
+		}
+	})
+
 	t.Run("partially bound", func(t *testing.T) {
 		value := map[string]interface{}{
 			"y": 2,