Fix unmarshaler offset error in struct fields

When custom unmarshalers decoded container types in struct fields, the
reflection decoder would fail with "no next offset available" when trying
to advance to the next field.

Author: oschwald

CHANGELOG.md

@@ -1,5 +1,12 @@
 # Changes
 
+## 2.0.0-beta.8 - 2025-07-15
+
+- Fixed "no next offset available" error that occurred when using custom
+  unmarshalers that decode container types (maps, slices) in struct fields.
+  The reflection decoder now correctly calculates field positions when
+  advancing to the next field after custom unmarshaling.
+
 ## 2.0.0-beta.7 - 2025-07-07
 
 * Update capitalization of "uint" in `ReadUInt*` to match `KindUint*` as well

internal/decoder/decoder.go

@@ -1,7 +1,6 @@
 package decoder
 
 import (
-	"errors"
 	"fmt"
 	"iter"
 
@@ -370,13 +369,6 @@ func (d *Decoder) setNextOffset(offset uint) {
 	}
 }
 
-func (d *Decoder) getNextOffset() (uint, error) {
-	if !d.hasNextOffset {
-		return 0, errors.New("no next offset available")
-	}
-	return d.nextOffset, nil
-}
-
 func unexpectedKindErr(expectedKind, actualKind Kind) error {
 	return fmt.Errorf("unexpected kind %d, expected %d", actualKind, expectedKind)
 }

internal/decoder/nested_unmarshaler_test.go

@@ -3,6 +3,7 @@ package decoder
 import (
 	"testing"
 
+	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
 )
 
@@ -220,3 +221,82 @@ func TestNestedUnmarshalerInMap(t *testing.T) {
 		require.Equal(t, "map:value2", result["key2"].Data)
 	})
 }
+
+// testMapIterator uses ReadMap() iterator to simulate mmdbtype.Map behavior.
+type testMapIterator struct {
+	Values map[string]string
+	custom bool
+}
+
+func (m *testMapIterator) UnmarshalMaxMindDB(d *Decoder) error {
+	m.custom = true
+	iter, size, err := d.ReadMap()
+	if err != nil {
+		return err
+	}
+
+	m.Values = make(map[string]string, size)
+	for key, iterErr := range iter {
+		if iterErr != nil {
+			return iterErr
+		}
+
+		// Read the value as a string
+		value, err := d.ReadString()
+		if err != nil {
+			return err
+		}
+
+		m.Values[string(key)] = value
+	}
+	return nil
+}
+
+// TestCustomUnmarshalerWithIterator tests that custom unmarshalers using iterators
+// work correctly in struct fields. This reproduces the original "no next offset available"
+// issue that occurred when mmdbtype.Map was used in structs.
+func TestCustomUnmarshalerWithIterator(t *testing.T) {
+	type Record struct {
+		Name     string
+		Location testMapIterator // This field uses ReadMap() iterator
+		Country  string
+	}
+
+	data := []byte{
+		// Map with 3 items
+		0xe3,
+		// Key "Name"
+		0x44, 'N', 'a', 'm', 'e',
+		// Value "Test" (string)
+		0x44, 'T', 'e', 's', 't',
+		// Key "Location"
+		0x48, 'L', 'o', 'c', 'a', 't', 'i', 'o', 'n',
+		// Value: Map with 2 items (latitude and longitude)
+		0xe2,
+		// Key "lat"
+		0x43, 'l', 'a', 't',
+		// Value "40.7"
+		0x44, '4', '0', '.', '7',
+		// Key "lng"
+		0x43, 'l', 'n', 'g',
+		// Value "-74.0"
+		0x45, '-', '7', '4', '.', '0',
+		// Key "Country"
+		0x47, 'C', 'o', 'u', 'n', 't', 'r', 'y',
+		// Value "US"
+		0x42, 'U', 'S',
+	}
+
+	d := New(data)
+	var result Record
+
+	err := d.Decode(0, &result)
+	require.NoError(t, err)
+
+	require.Equal(t, "Test", result.Name)
+	assert.True(t, result.Location.custom, "Custom unmarshaler should be called")
+	assert.Len(t, result.Location.Values, 2)
+	assert.Equal(t, "40.7", result.Location.Values["lat"])
+	assert.Equal(t, "-74.0", result.Location.Values["lng"])
+	assert.Equal(t, "US", result.Country)
+}

internal/decoder/reflection.go

@@ -64,7 +64,7 @@ func (d *ReflectionDecoder) Decode(offset uint, v any) error {
 	return mmdberrors.WrapWithContext(err, offset, nil)
 }
 
-// DecodePath decodes the data value at offset and stores the value assocated
+// DecodePath decodes the data value at offset and stores the value associated
 // with the path in the value pointed at by v.
 func (d *ReflectionDecoder) DecodePath(
 	offset uint,
@@ -300,7 +300,7 @@ func (d *ReflectionDecoder) decodeValue(
 			if err := unmarshaler.UnmarshalMaxMindDB(decoder); err != nil {
 				return 0, err
 			}
-			return decoder.getNextOffset()
+			return d.nextValueOffset(offset, 1)
 		}
 	}