[chore][confmap] Do not merge: demonstrate empty slice handling

confmap/confmap.go

@@ -133,6 +133,10 @@ func sanitizeExpanded(a any, useOriginal bool) any {
 		return c
 	case []any:
 		var newSlice []any
+		if m == nil {
+			return newSlice
+		}
+		newSlice = []any{}
 		for _, e := range m {
 			newSlice = append(newSlice, sanitizeExpanded(e, useOriginal))
 		}
@@ -219,7 +223,6 @@ func decodeConfig(m *Conf, result any, errorUnused bool, skipTopLevelUnmarshaler
 			// after the main unmarshaler hook is called,
 			// we unmarshal the embedded structs if present to merge with the result:
 			unmarshalerEmbeddedStructsHookFunc(),
-			zeroSliceHookFunc(),
 		),
 	}
 	decoder, err := mapstructure.NewDecoder(dc)
@@ -513,37 +516,6 @@ type Marshaler interface {
 	Marshal(component *Conf) error
 }
 
-// This hook is used to solve the issue: https://github.com/open-telemetry/opentelemetry-collector/issues/4001
-// We adopt the suggestion provided in this issue: https://github.com/mitchellh/mapstructure/issues/74#issuecomment-279886492
-// We should empty every slice before unmarshalling unless user provided slice is nil.
-// Assume that we had a struct with a field of type slice called `keys`, which has default values of ["a", "b"]
-//
-//	type Config struct {
-//	  Keys []string `mapstructure:"keys"`
-//	}
-//
-// The configuration provided by users may have following cases
-// 1. configuration have `keys` field and have a non-nil values for this key, the output should be overridden
-//   - for example, input is {"keys", ["c"]}, then output is Config{ Keys: ["c"]}
-//
-// 2. configuration have `keys` field and have an empty slice for this key, the output should be overridden by empty slices
-//   - for example, input is {"keys", []}, then output is Config{ Keys: []}
-//
-// 3. configuration have `keys` field and have nil value for this key, the output should be default config
-//   - for example, input is {"keys": nil}, then output is Config{ Keys: ["a", "b"]}
-//
-// 4. configuration have no `keys` field specified, the output should be default config
-//   - for example, input is {}, then output is Config{ Keys: ["a", "b"]}
-func zeroSliceHookFunc() mapstructure.DecodeHookFuncValue {
-	return func(from reflect.Value, to reflect.Value) (interface{}, error) {
-		if to.CanSet() && to.Kind() == reflect.Slice && from.Kind() == reflect.Slice {
-			to.Set(reflect.MakeSlice(to.Type(), from.Len(), from.Cap()))
-		}
-
-		return from.Interface(), nil
-	}
-}
-
 type moduleFactory[T any, S any] interface {
 	Create(s S) T
 }

confmap/confmap_test.go

@@ -691,6 +691,50 @@ func TestZeroSliceHookFunc(t *testing.T) {
 	}
 }
 
+func TestNilValuesUnchanged(t *testing.T) {
+	type structWithSlices struct {
+		Strings []string `mapstructure:"strings"`
+	}
+
+	slicesStruct := &structWithSlices{}
+
+	nilCfg := map[string]any{
+		"strings": []any(nil),
+	}
+	nilConf := NewFromStringMap(nilCfg)
+	err := nilConf.Unmarshal(slicesStruct)
+	require.NoError(t, err)
+
+	confFromStruct := New()
+	err = confFromStruct.Marshal(slicesStruct)
+	require.NoError(t, err)
+
+	require.Equal(t, nilCfg, nilConf.ToStringMap())
+	require.EqualValues(t, nilConf.ToStringMap(), confFromStruct.ToStringMap())
+}
+
+func TestEmptySliceUnchanged(t *testing.T) {
+	type structWithSlices struct {
+		Strings []string `mapstructure:"strings"`
+	}
+
+	slicesStruct := &structWithSlices{}
+
+	nilCfg := map[string]any{
+		"strings": []any{},
+	}
+	nilConf := NewFromStringMap(nilCfg)
+	err := nilConf.Unmarshal(slicesStruct)
+	require.NoError(t, err)
+
+	confFromStruct := New()
+	err = confFromStruct.Marshal(slicesStruct)
+	require.NoError(t, err)
+
+	require.Equal(t, nilCfg, nilConf.ToStringMap())
+	require.EqualValues(t, nilConf.ToStringMap(), confFromStruct.ToStringMap())
+}
+
 type C struct {
 	Modifiers []string `mapstructure:"modifiers"`
 }

confmap/confmaptest/configtest_test.go

@@ -30,11 +30,16 @@ func TestLoadConf(t *testing.T) {
 	assert.Equal(t, map[string]any{"floating": 3.14}, cfg.ToStringMap())
 }
 
-func TestToStringMapSanitizeEmptySlice(t *testing.T) {
+func TestToStringMapSanitizeNil(t *testing.T) {
+	cfg, err := LoadConf(filepath.Join("testdata", "nil.yaml"))
+	require.NoError(t, err)
+	assert.Equal(t, map[string]any{"slice": nil}, cfg.ToStringMap())
+}
+
+func TestToStringMapEmptySlice(t *testing.T) {
 	cfg, err := LoadConf(filepath.Join("testdata", "empty-slice.yaml"))
 	require.NoError(t, err)
-	var nilSlice []interface{}
-	assert.Equal(t, map[string]any{"slice": nilSlice}, cfg.ToStringMap())
+	assert.Equal(t, map[string]any{"slice": []any{}}, cfg.ToStringMap())
 }
 
 func TestValidateProviderScheme(t *testing.T) {

confmap/confmaptest/testdata/empty-slice.yaml

@@ -1 +1 @@
-slice: [] # empty slices are sanitized to nil in ToStringMap
+slice: []

confmap/confmaptest/testdata/nil.yaml

@@ -0,0 +1 @@
+slice: 

confmap/internal/mapstructure/encoder.go

@@ -143,6 +143,9 @@ func (e *Encoder) encodeSlice(value reflect.Value) (any, error) {
 			Kind:   value.Kind(),
 		}
 	}
+	if value.IsNil() {
+		return []any(nil), nil
+	}
 	result := make([]any, value.Len())
 	for i := 0; i < value.Len(); i++ {
 		var err error