[MM-32390] Config logic refactor (#17578)

* 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>

* Move FF synchronizer to its own package

* Remove unidiomatic use of sync.Once

* Add some comments

* Rename function

* More comment

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

app/featureflag/feature_flags_sync.go

@@ -0,0 +1,111 @@
+// Copyright (c) 2015-present Mattermost, Inc. All Rights Reserved.
+// See LICENSE.txt for license information.
+
+package featureflag
+
+import (
+	"math"
+	"reflect"
+	"strconv"
+	"strings"
+
+	"github.com/pkg/errors"
+	"github.com/splitio/go-client/v6/splitio/client"
+	"github.com/splitio/go-client/v6/splitio/conf"
+
+	"github.com/mattermost/mattermost-server/v5/model"
+	"github.com/mattermost/mattermost-server/v5/shared/mlog"
+)
+
+type SyncParams struct {
+	ServerID            string
+	SplitKey            string
+	SyncIntervalSeconds int
+	Log                 *mlog.Logger
+	Attributes          map[string]interface{}
+}
+
+type Synchronizer struct {
+	SyncParams
+
+	client  *client.SplitClient
+	stop    chan struct{}
+	stopped chan struct{}
+}
+
+var featureNames = getStructFields(model.FeatureFlags{})
+
+func NewSynchronizer(params SyncParams) (*Synchronizer, error) {
+	cfg := conf.Default()
+	if params.Log != nil {
+		cfg.Logger = &splitLogger{wrappedLog: params.Log.With(mlog.String("service", "split"))}
+	} else {
+		cfg.LoggerConfig.LogLevel = math.MinInt32
+	}
+	factory, err := client.NewSplitFactory(params.SplitKey, cfg)
+	if err != nil {
+		return nil, errors.Wrap(err, "unable to create split factory")
+	}
+
+	return &Synchronizer{
+		SyncParams: params,
+		client:     factory.Client(),
+		stop:       make(chan struct{}),
+		stopped:    make(chan struct{}),
+	}, nil
+}
+
+// EnsureReady blocks until the syncronizer is ready to update feature flag values
+func (f *Synchronizer) EnsureReady() error {
+	if err := f.client.BlockUntilReady(10); err != nil {
+		return errors.Wrap(err, "split.io client could not initialize")
+	}
+
+	return nil
+}
+
+func (f *Synchronizer) UpdateFeatureFlagValues(base model.FeatureFlags) model.FeatureFlags {
+	featuresMap := f.client.Treatments(f.ServerID, featureNames, f.Attributes)
+	ffm := featureFlagsFromMap(featuresMap, base)
+	return ffm
+}
+
+func (f *Synchronizer) Close() {
+	f.client.Destroy()
+}
+
+// featureFlagsFromMap sets the feature flags from a map[string]string.
+// It starts with baseFeatureFlags and only sets values that are
+// given by the upstream management system.
+// Makes the assumption that all feature flags are strings or booleans.
+// Strings are converted to booleans by considering case insensitive "on" or any value considered by strconv.ParseBool as true and any other value as false.
+func featureFlagsFromMap(featuresMap map[string]string, baseFeatureFlags model.FeatureFlags) model.FeatureFlags {
+	refStruct := reflect.ValueOf(&baseFeatureFlags).Elem()
+	for fieldName, fieldValue := range featuresMap {
+		refField := refStruct.FieldByName(fieldName)
+		// "control" is returned by split.io if the treatment is not found, in this case we should use the default value.
+		if !refField.IsValid() || !refField.CanSet() || fieldValue == "control" {
+			continue
+		}
+
+		switch refField.Type().Kind() {
+		case reflect.Bool:
+			parsedBoolValue, _ := strconv.ParseBool(fieldValue)
+			refField.Set(reflect.ValueOf(strings.ToLower(fieldValue) == "on" || parsedBoolValue))
+		default:
+			refField.Set(reflect.ValueOf(fieldValue))
+		}
+
+	}
+	return baseFeatureFlags
+}
+
+func getStructFields(s interface{}) []string {
+	structType := reflect.TypeOf(s)
+	fieldNames := make([]string, 0, structType.NumField())
+	for i := 0; i < structType.NumField(); i++ {
+		fieldNames = append(fieldNames, structType.Field(i).Name)
+	}
+
+	return fieldNames
+}