core/config: refactor change dispatcher (#4657)

* core/config: refactor change dispatcher

* update test

* close listener go routine when context is canceled

* use cancel cause

* use context

* add more time

* more time

config/config_source.go

@@ -10,6 +10,7 @@ import (
 	"github.com/google/uuid"
 	"github.com/rs/zerolog"
 
+	"github.com/pomerium/pomerium/internal/events"
 	"github.com/pomerium/pomerium/internal/fileutil"
 	"github.com/pomerium/pomerium/internal/log"
 	"github.com/pomerium/pomerium/internal/telemetry/metrics"
@@ -19,27 +20,27 @@ import (
 // A ChangeListener is called when configuration changes.
 type ChangeListener = func(context.Context, *Config)
 
+type changeDispatcherEvent struct {
+	cfg *Config
+}
+
 // A ChangeDispatcher manages listeners on config changes.
 type ChangeDispatcher struct {
-	sync.Mutex
-	onConfigChangeListeners []ChangeListener
+	target events.Target[changeDispatcherEvent]
 }
 
 // Trigger triggers a change.
 func (dispatcher *ChangeDispatcher) Trigger(ctx context.Context, cfg *Config) {
-	dispatcher.Lock()
-	defer dispatcher.Unlock()
-
-	for _, li := range dispatcher.onConfigChangeListeners {
-		li(ctx, cfg)
-	}
+	dispatcher.target.Dispatch(ctx, changeDispatcherEvent{
+		cfg: cfg,
+	})
 }
 
 // OnConfigChange adds a listener.
 func (dispatcher *ChangeDispatcher) OnConfigChange(_ context.Context, li ChangeListener) {
-	dispatcher.Lock()
-	defer dispatcher.Unlock()
-	dispatcher.onConfigChangeListeners = append(dispatcher.onConfigChangeListeners, li)
+	dispatcher.target.AddListener(func(ctx context.Context, evt changeDispatcherEvent) {
+		li(ctx, evt.cfg)
+	})
 }
 
 // A Source gets configuration.

config/layered_test.go

@@ -3,7 +3,9 @@ package config_test
 import (
 	"context"
 	"errors"
+	"sync/atomic"
 	"testing"
+	"time"
 
 	"github.com/stretchr/testify/assert"
 	"github.com/stretchr/testify/require"
@@ -13,6 +15,8 @@ import (
 )
 
 func TestLayeredConfig(t *testing.T) {
+	t.Parallel()
+
 	ctx := context.Background()
 
 	t.Run("error on initial build", func(t *testing.T) {
@@ -33,12 +37,15 @@ func TestLayeredConfig(t *testing.T) {
 		})
 		require.NoError(t, err)
 
-		var dst *config.Config
+		var dst atomic.Pointer[config.Config]
+		dst.Store(layered.GetConfig())
 		layered.OnConfigChange(ctx, func(ctx context.Context, c *config.Config) {
-			dst = c
+			dst.Store(c)
 		})
 
 		underlying.SetConfig(ctx, &config.Config{Options: &config.Options{DeriveInternalDomainCert: proto.String("b.com")}})
-		assert.Equal(t, "b.com", dst.Options.GetDeriveInternalDomain())
+		assert.Eventually(t, func() bool {
+			return dst.Load().Options.GetDeriveInternalDomain() == "b.com"
+		}, 10*time.Second, time.Millisecond)
 	})
 }