feat: Create interface for eval events. (#1288)

## This PR

Before this change the eval services used a unexported
struct which prevented creating eval services outside of
this package.

This change creates a new IEvents interface that allows
providing custom impls of flag eval services.

### Related Issues


### Notes
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### Follow-up Tasks
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noted under this section -->
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### How to test
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Signed-off-by: Connor Hindley <connor.hindley@tanium.com>

flagd/pkg/service/flag-evaluation/connect_service.go

@@ -58,7 +58,7 @@ type ConnectService struct {
 	logger                *logger.Logger
 	eval                  evaluator.IEvaluator
 	metrics               *telemetry.MetricsRecorder
-	eventingConfiguration *eventingConfiguration
+	eventingConfiguration IEvents
 
 	server        *http.Server
 	metricsServer *http.Server
@@ -125,7 +125,7 @@ func (s *ConnectService) Serve(ctx context.Context, svcConf service.Configuratio
 
 // Notify emits change event notifications for subscriptions
 func (s *ConnectService) Notify(n service.Notification) {
-	s.eventingConfiguration.emitToAll(n)
+	s.eventingConfiguration.EmitToAll(n)
 }
 
 // nolint: funlen
@@ -209,7 +209,7 @@ func (s *ConnectService) AddMiddleware(mw middleware.IMiddleware) {
 
 func (s *ConnectService) Shutdown() {
 	s.readinessEnabled = false
-	s.eventingConfiguration.emitToAll(service.Notification{
+	s.eventingConfiguration.EmitToAll(service.Notification{
 		Type: service.Shutdown,
 		Data: map[string]interface{}{},
 	})

flagd/pkg/service/flag-evaluation/connect_service_test.go

@@ -182,7 +182,7 @@ func TestConnectServiceNotify(t *testing.T) {
 
 	sChan := make(chan iservice.Notification, 1)
 	eventing := service.eventingConfiguration
-	eventing.subs["key"] = sChan
+	eventing.Subscribe("key", sChan)
 
 	// notification type
 	ofType := iservice.ConfigurationChange
@@ -220,7 +220,7 @@ func TestConnectServiceShutdown(t *testing.T) {
 
 	sChan := make(chan iservice.Notification, 1)
 	eventing := service.eventingConfiguration
-	eventing.subs["key"] = sChan
+	eventing.Subscribe("key", sChan)
 
 	// notification type
 	ofType := iservice.Shutdown

flagd/pkg/service/flag-evaluation/eventing.go

@@ -6,20 +6,27 @@ import (
 	iservice "github.com/open-feature/flagd/core/pkg/service"
 )
 
+// IEvents is an interface for event subscriptions
+type IEvents interface {
+	Subscribe(id any, notifyChan chan iservice.Notification)
+	Unsubscribe(id any)
+	EmitToAll(n iservice.Notification)
+}
+
 // eventingConfiguration is a wrapper for notification subscriptions
 type eventingConfiguration struct {
 	mu   *sync.RWMutex
-	subs map[interface{}]chan iservice.Notification
+	subs map[any]chan iservice.Notification
 }
 
-func (eventing *eventingConfiguration) subscribe(id interface{}, notifyChan chan iservice.Notification) {
+func (eventing *eventingConfiguration) Subscribe(id any, notifyChan chan iservice.Notification) {
 	eventing.mu.Lock()
 	defer eventing.mu.Unlock()
 
 	eventing.subs[id] = notifyChan
 }
 
-func (eventing *eventingConfiguration) emitToAll(n iservice.Notification) {
+func (eventing *eventingConfiguration) EmitToAll(n iservice.Notification) {
 	eventing.mu.RLock()
 	defer eventing.mu.RUnlock()
 
@@ -28,7 +35,7 @@ func (eventing *eventingConfiguration) emitToAll(n iservice.Notification) {
 	}
 }
 
-func (eventing *eventingConfiguration) unSubscribe(id interface{}) {
+func (eventing *eventingConfiguration) Unsubscribe(id any) {
 	eventing.mu.Lock()
 	defer eventing.mu.Unlock()
 

flagd/pkg/service/flag-evaluation/eventing_test.go

@@ -22,8 +22,8 @@ func TestSubscribe(t *testing.T) {
 	chanB := make(chan iservice.Notification, 1)
 
 	// when
-	eventing.subscribe(idA, chanA)
+	eventing.Subscribe(idA, chanA)
-	eventing.subscribe(idB, chanB)
+	eventing.Subscribe(idB, chanB)
 
 	// then
 	require.Equal(t, chanA, eventing.subs[idA], "incorrect subscription association")
@@ -43,10 +43,10 @@ func TestUnsubscribe(t *testing.T) {
 	chanB := make(chan iservice.Notification, 1)
 
 	// when
-	eventing.subscribe(idA, chanA)
+	eventing.Subscribe(idA, chanA)
-	eventing.subscribe(idB, chanB)
+	eventing.Subscribe(idB, chanB)
 
-	eventing.unSubscribe(idA)
+	eventing.Unsubscribe(idA)
 
 	// then
 	require.Empty(t, eventing.subs[idA],

flagd/pkg/service/flag-evaluation/flag_evaluator.go

@@ -30,13 +30,13 @@ type OldFlagEvaluationService struct {
 	logger                *logger.Logger
 	eval                  evaluator.IEvaluator
 	metrics               *telemetry.MetricsRecorder
-	eventingConfiguration *eventingConfiguration
+	eventingConfiguration IEvents
 	flagEvalTracer        trace.Tracer
 }
 
 // NewOldFlagEvaluationService creates a OldFlagEvaluationService with provided parameters
 func NewOldFlagEvaluationService(log *logger.Logger,
-	eval evaluator.IEvaluator, eventingCfg *eventingConfiguration, metricsRecorder *telemetry.MetricsRecorder,
+	eval evaluator.IEvaluator, eventingCfg IEvents, metricsRecorder *telemetry.MetricsRecorder,
 ) *OldFlagEvaluationService {
 	return &OldFlagEvaluationService{
 		logger:                log,
@@ -117,8 +117,8 @@ func (s *OldFlagEvaluationService) EventStream(
 	stream *connect.ServerStream[schemaV1.EventStreamResponse],
 ) error {
 	requestNotificationChan := make(chan service.Notification, 1)
-	s.eventingConfiguration.subscribe(req, requestNotificationChan)
+	s.eventingConfiguration.Subscribe(req, requestNotificationChan)
-	defer s.eventingConfiguration.unSubscribe(req)
+	defer s.eventingConfiguration.Unsubscribe(req)
 
 	requestNotificationChan <- service.Notification{
 		Type: service.ProviderReady,

flagd/pkg/service/flag-evaluation/flag_evaluator_v2.go

@@ -23,14 +23,14 @@ type FlagEvaluationService struct {
 	logger                *logger.Logger
 	eval                  evaluator.IEvaluator
 	metrics               *telemetry.MetricsRecorder
-	eventingConfiguration *eventingConfiguration
+	eventingConfiguration IEvents
 	flagEvalTracer        trace.Tracer
 }
 
 // NewFlagEvaluationService creates a FlagEvaluationService with provided parameters
 func NewFlagEvaluationService(log *logger.Logger,
 	eval evaluator.IEvaluator,
-	eventingCfg *eventingConfiguration,
+	eventingCfg IEvents,
 	metricsRecorder *telemetry.MetricsRecorder,
 ) *FlagEvaluationService {
 	return &FlagEvaluationService{
@@ -116,8 +116,8 @@ func (s *FlagEvaluationService) EventStream(
 	stream *connect.ServerStream[evalV1.EventStreamResponse],
 ) error {
 	requestNotificationChan := make(chan service.Notification, 1)
-	s.eventingConfiguration.subscribe(req, requestNotificationChan)
+	s.eventingConfiguration.Subscribe(req, requestNotificationChan)
-	defer s.eventingConfiguration.unSubscribe(req)
+	defer s.eventingConfiguration.Unsubscribe(req)
 
 	requestNotificationChan <- service.Notification{
 		Type: service.ProviderReady,