[MM-28692] Include config diffs in audit record for config changing API calls (#17623)

* Replace config generator

* Cleanup

* Some renaming and docs additions to add clarity

* Cleanup logging related methods

* Cleanup emitter

* Fix TestDefaultsGenerator

* Move feature flags synchronization logic out of config package

* Remove unnecessary util functions

* Simplify load/set logic

* Refine semantics and add some test to cover them

* Remove unnecessary deep copies

* Improve logic further

* Fix license header

* Review file store tests

* Fix test

* Fix test

* Avoid additional write during initialization

* More consistent naming

* Update app/feature_flags.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Christopher Speller <crspeller@gmail.com>

* Update config/store.go

Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

* Make ConfigStore.Set() return both old and new configs

* Implement config diff function

* Make app.SaveConfig return previous and current configs

* Add config diff to audit record

* Fix returned configs

* Include high level test

* Move FF synchronizer to its own package

* Remove unidiomatic use of sync.Once

* Add some comments

* Rename function

* More comment

* Save config diff in audit record for local endpoints

* Enable audit for config set/reset commands

* Improve tests output

Co-authored-by: Christopher Speller <crspeller@gmail.com>
Co-authored-by: Ibrahim Serdar Acikgoz <serdaracikgoz86@gmail.com>

config/diff.go

@@ -0,0 +1,90 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package config
+
+import (
+	"fmt"
+	"reflect"
+
+	"github.com/mattermost/mattermost-server/v5/model"
+)
+
+type ConfigDiffs []ConfigDiff
+
+type ConfigDiff struct {
+	Path      string      `json:"path"`
+	BaseVal   interface{} `json:"base_val"`
+	ActualVal interface{} `json:"actual_val"`
+}
+
+func diff(base, actual reflect.Value, label string) ([]ConfigDiff, error) {
+	var diffs []ConfigDiff
+
+	if base.IsZero() && actual.IsZero() {
+		return diffs, nil
+	}
+
+	if base.IsZero() || actual.IsZero() {
+		return append(diffs, ConfigDiff{
+			Path:      label,
+			BaseVal:   base.Interface(),
+			ActualVal: actual.Interface(),
+		}), nil
+	}
+
+	baseType := base.Type()
+	actualType := actual.Type()
+
+	if baseType.Kind() == reflect.Ptr {
+		base = reflect.Indirect(base)
+		actual = reflect.Indirect(actual)
+		baseType = base.Type()
+		actualType = actual.Type()
+	}
+
+	if baseType != actualType {
+		return nil, fmt.Errorf("not same type %s %s", baseType, actualType)
+	}
+
+	switch baseType.Kind() {
+	case reflect.Struct:
+		if base.NumField() != actual.NumField() {
+			return nil, fmt.Errorf("not same number of fields in struct")
+		}
+		for i := 0; i < base.NumField(); i++ {
+			fieldLabel := baseType.Field(i).Name
+			if label != "" {
+				fieldLabel = label + "." + fieldLabel
+			}
+			d, err := diff(base.Field(i), actual.Field(i), fieldLabel)
+			if err != nil {
+				return nil, err
+			}
+			diffs = append(diffs, d...)
+		}
+	default:
+		if !reflect.DeepEqual(base.Interface(), actual.Interface()) {
+			diffs = append(diffs, ConfigDiff{
+				Path:      label,
+				BaseVal:   base.Interface(),
+				ActualVal: actual.Interface(),
+			})
+		}
+	}
+
+	return diffs, nil
+}
+
+func Diff(base, actual *model.Config) (ConfigDiffs, error) {
+	if base == nil || actual == nil {
+		return nil, fmt.Errorf("input configs should not be nil")
+	}
+	baseVal := reflect.Indirect(reflect.ValueOf(base))
+	actualVal := reflect.Indirect(reflect.ValueOf(actual))
+	return diff(baseVal, actualVal, "")
+}
+
+func (cd ConfigDiffs) String() string {
+	return fmt.Sprintf("%+v", []ConfigDiff(cd))
+}