fix: decode in the case of field missing in both writer binary and re…

…ader struct (#452)

codec_record.go

@@ -76,9 +76,17 @@ func decoderOfStruct(d *decoderContext, schema Schema, typ reflect2.Type) ValDec
 				}
 			}
 		}
-
-		// Skip field if it doesnt exist
+		// Skip field if it doesn't exist
 		if sf == nil {
+			// If the field value doesn't exist in the binary, ignore it instead of
+			// appending a 'SkipDecoder'.
+			//
+			// Note: 'SkipDecoder' performs a read and moves the cursor, which,
+			// in this case, will lead to a dirty read.
+			if field.action == FieldSetDefault {
+				continue
+			}
+
 			fields = append(fields, &structFieldDecoder{
 				decoder: createSkipDecoder(field.Type()),
 			})

schema_compatibility_test.go

@@ -976,3 +976,44 @@ func TestSchemaCompatibility_ResolveWithComplexUnion(t *testing.T) {
 	want := map[string]any{"b": []byte("foo")}
 	assert.Equal(t, want, result)
 }
+
+func TestSchemaCompatibility_ResolveWithFieldMissingInWriterAndReaderStruct(t *testing.T) {
+	w := avro.MustParse(`{
+				"type":"record", "name":"test", "namespace": "org.hamba.avro", 
+				"fields":[
+					{"name": "a", "type": "string"}
+				]
+			}`)
+
+	r := avro.MustParse(`{
+				"type":"record", "name":"test", "namespace": "org.hamba.avro", 
+				"fields":[
+					{"name": "a", "type": "string"},
+					{"name": "b", "type": "int", "default": 10},
+					{"name": "c", "type": "string", "default": "foo"}
+				]
+			}`)
+
+	type TestW struct {
+		A string `avro:"a"`
+	}
+	value := TestW{A: "abc"}
+
+	b, err := avro.Marshal(w, value)
+	assert.NoError(t, err)
+
+	schema, err := avro.NewSchemaCompatibility().Resolve(r, w)
+	assert.NoError(t, err)
+
+	type TestR struct {
+		A string `avro:"a"`
+		C string `avro:"c"`
+	}
+	want := TestR{A: "abc", C: "foo"}
+
+	var result TestR
+	err = avro.Unmarshal(schema, b, &result)
+	assert.NoError(t, err)
+
+	assert.Equal(t, want, result)
+}