[confmap] Fix handling for slices with default values (#12662)

#### Description

#11882
introduced issues with handling slices which have default values. It's
likely that the removal of `zeroSliceHookFunc` is causing problems with
overriding default lists.

Author: evan-bradley

.chloggen/revert-confmap-nil.yaml

@@ -0,0 +1,25 @@
+# Use this changelog template to create an entry for release notes.
+
+# One of 'breaking', 'deprecation', 'new_component', 'enhancement', 'bug_fix'
+change_type: bug_fix
+
+# The name of the component, or a single word describing the area of concern, (e.g. otlpreceiver)
+component: confmap
+
+# A brief description of the change.  Surround your text with quotes ("") if it needs to start with a backtick (`).
+note: Ensure slices with defaults containing struct values are correctly set.
+
+# One or more tracking issues or pull requests related to the change
+issues: [12661]
+
+# (Optional) One or more lines of additional information to render under the primary note.
+# These lines will be padded with 2 spaces and then inserted directly into the document.
+# Use pipe (|) for multiline entries.
+subtext: This reverts the changes made in https://github.com/open-telemetry/opentelemetry-collector/pull/11882.
+
+# Optional: The change log or logs in which this entry should be included.
+# e.g. '[user]' or '[user, api]'
+# Include 'user' if the change is relevant to end users.
+# Include 'api' if there is a change to a library API.
+# Default: '[user]'
+change_logs: []

confmap/confmap.go

@@ -141,10 +141,6 @@ 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))
 		}
@@ -240,6 +236,7 @@ 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)
@@ -534,6 +531,37 @@ 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) (any, 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

@@ -689,50 +689,6 @@ 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

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

confmap/confmaptest/testdata/empty-slice.yaml

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

confmap/confmaptest/testdata/nil.yaml

@@ -141,9 +141,6 @@ 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

confmap/internal/mapstructure/encoder.go

@@ -124,7 +124,7 @@ func TestConfmapMarshalConfig(t *testing.T) {
 									"host": "localhost",
 									"port": 8888,
 									"with_resource_constant_labels": map[string]any{
-										"included": []any{},
+										"included": []any(nil),
 									},
 									"without_scope_info":  true,
 									"without_type_suffix": true,