allow null types to have properties

schema.go

@@ -1429,9 +1429,22 @@ func (s *FixedSchema) CacheFingerprint() [32]byte {
 
 // NullSchema is an Avro null type schema.
 type NullSchema struct {
+	properties
 	fingerprinter
 }
 
+// NewNullSchema creates a new NullSchema.
+func NewNullSchema(opts ...SchemaOption) *NullSchema {
+	var cfg schemaConfig
+	for _, opt := range opts {
+		opt(&cfg)
+	}
+
+	return &NullSchema{
+		properties: newProperties(cfg.props, primitiveReserved),
+	}
+}
+
 // Type returns the type of the schema.
 func (s *NullSchema) Type() Type {
 	return Null
@@ -1444,7 +1457,16 @@ func (s *NullSchema) String() string {
 
 // MarshalJSON marshals the schema to json.
 func (s *NullSchema) MarshalJSON() ([]byte, error) {
-	return []byte(`"null"`), nil
+	if len(s.props) == 0 {
+		return []byte(`"null"`), nil
+	}
+	buf := new(bytes.Buffer)
+	buf.WriteString(`{"type":"null"`)
+	if err := s.marshalPropertiesToJSON(buf); err != nil {
+		return nil, err
+	}
+	buf.WriteString("}")
+	return buf.Bytes(), nil
 }
 
 // Fingerprint returns the SHA256 fingerprint of the schema.

schema_json_test.go

@@ -23,6 +23,10 @@ func TestSchema_JSON(t *testing.T) {
 			input: `{"type":"null"}`,
 			json:  `"null"`,
 		},
+		{
+			input: `{"type":"null","other":"foo"}`,
+			json:  `{"type":"null","other":"foo"}`,
+		},
 		{
 			input: `"boolean"`,
 			json:  `"boolean"`,

schema_parse.go

@@ -135,13 +135,7 @@ func parseComplexType(namespace string, m map[string]any, seen seenCache, cache
 	typ := Type(str)
 
 	switch typ {
-	case Null:
-		// TODO: Per the spec, "null" is a primitive type so should be permitted to
-		//       have other properties/metadata.
-		//       	https://avro.apache.org/docs/1.12.0/specification/#primitive-types
-		return &NullSchema{}, nil
-
-	case String, Bytes, Int, Long, Float, Double, Boolean:
+	case String, Bytes, Int, Long, Float, Double, Boolean, Null:
 		return parsePrimitive(typ, m)
 
 	case Record, Error:
@@ -172,6 +166,9 @@ type primitiveSchema struct {
 
 func parsePrimitive(typ Type, m map[string]any) (Schema, error) {
 	if len(m) == 0 {
+		if typ == Null {
+			return &NullSchema{}, nil
+		}
 		return NewPrimitiveSchema(typ, nil), nil
 	}
 
@@ -191,6 +188,9 @@ func parsePrimitive(typ Type, m map[string]any) (Schema, error) {
 		}
 	}
 
+	if typ == Null {
+		return NewNullSchema(WithProps(p.Props)), nil
+	}
 	return NewPrimitiveSchema(typ, logical, WithProps(p.Props)), nil
 }
 

schema_test.go

@@ -58,6 +58,7 @@ func TestNullSchema(t *testing.T) {
 	schemas := []string{
 		`null`,
 		`{"type":"null"}`,
+		`{"type":"null", "other-property": 123, "another-property": ["a","b","c"]}`,
 	}
 
 	for _, schm := range schemas {
@@ -1892,6 +1893,27 @@ func TestParse_PreservesAllProperties(t *testing.T) {
 				}, rec.Props())
 			},
 		},
+		{
+			name: "null",
+			schema: `{
+				"type": "null",
+				"name": "SomeMap",
+				"logicalType": "weights",
+				"precision": "abc",
+				"scale": "def",
+				"other": [1,2,3]
+			}`,
+			check: func(t *testing.T, schema avro.Schema) {
+				rec := schema.(*avro.NullSchema)
+				assert.Equal(t, map[string]any{
+					"name":        "SomeMap",
+					"logicalType": "weights",
+					"precision":   "abc",
+					"scale":       "def",
+					"other":       []any{1.0, 2.0, 3.0},
+				}, rec.Props())
+			},
+		},
 	}
 	for _, testCase := range testCases {
 		t.Run(testCase.name, func(t *testing.T) {
@@ -2141,4 +2163,30 @@ func TestNewSchema_IgnoresInvalidProperties(t *testing.T) {
 			"other":     true,
 		}, rec.Props())
 	})
+	t.Run("null", func(t *testing.T) {
+		rec := avro.NewNullSchema(
+			avro.WithProps(map[string]any{
+				// invalid (conflict with other type properties)
+				"type": false,
+				// valid
+				"name":        123,
+				"namespace":   "abc",
+				"doc":         "blah",
+				"aliases":     "foo",
+				"other":       true,
+				"logicalType": "baz",
+				"precision":   "abc",
+				"scale":       "def",
+			}))
+		assert.Equal(t, map[string]any{
+			"name":        123,
+			"namespace":   "abc",
+			"doc":         "blah",
+			"aliases":     "foo",
+			"other":       true,
+			"logicalType": "baz",
+			"precision":   "abc",
+			"scale":       "def",
+		}, rec.Props())
+	})
 }