core/config: refactor change dispatcher

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)
 	})
 }

internal/events/target.go

@@ -0,0 +1,166 @@
+package events
+
+import (
+	"context"
+	"errors"
+	"sync"
+
+	"github.com/google/uuid"
+)
+
+type (
+	// A Listener is a function that listens for events of type T.
+	Listener[T any] func(ctx context.Context, event T)
+	// A Handle represents a listener.
+	Handle string
+
+	addListenerEvent[T any] struct {
+		listener Listener[T]
+		handle   Handle
+	}
+	removeListenerEvent[T any] struct {
+		handle Handle
+	}
+	dispatchEvent[T any] struct {
+		ctx   context.Context
+		event T
+	}
+)
+
+// A Target is a target for events.
+//
+// Listeners are added with AddListener with a function to be called when the event occurs.
+// AddListener returns a Handle which can be used to remove a listener with RemoveListener.
+//
+// Dispatch dispatches events to all the registered listeners.
+//
+// Target is safe to use in its zero state.
+//
+// The first time any method of Target is called a background goroutine is started that handles
+// any requests and maintains the state of the listeners. Each listener also starts a
+// separate goroutine so that all listeners can be invoked concurrently.
+//
+// The channels to the main goroutine and to the listener goroutines have a size of 1 so typically
+// methods and dispatches will return immediately. However a slow listener will cause the next event
+// dispatch to block. This is the opposite behavior from Manager.
+//
+// Close will cancel all the goroutines. Subsequent calls to AddListener, RemoveListener, Close and
+// Dispatch are no-ops.
+type Target[T any] struct {
+	initOnce         sync.Once
+	ctx              context.Context
+	cancel           context.CancelCauseFunc
+	addListenerCh    chan addListenerEvent[T]
+	removeListenerCh chan removeListenerEvent[T]
+	dispatchCh       chan dispatchEvent[T]
+	listeners        map[Handle]chan dispatchEvent[T]
+}
+
+// AddListener adds a listener to the target.
+func (t *Target[T]) AddListener(listener Listener[T]) Handle {
+	t.init()
+
+	// using a handle is necessary because you can't use a function as a map key.
+	handle := Handle(uuid.NewString())
+
+	select {
+	case <-t.ctx.Done():
+	case t.addListenerCh <- addListenerEvent[T]{listener, handle}:
+	}
+
+	return handle
+}
+
+// Close closes the event target. This can be called multiple times safely.
+// Once closed the target cannot be used.
+func (t *Target[T]) Close() {
+	t.init()
+
+	t.cancel(errors.New("target closed"))
+}
+
+// Dispatch dispatches an event to all listeners.
+func (t *Target[T]) Dispatch(ctx context.Context, evt T) {
+	t.init()
+
+	select {
+	case <-t.ctx.Done():
+	case t.dispatchCh <- dispatchEvent[T]{ctx: ctx, event: evt}:
+	}
+}
+
+// RemoveListener removes a listener from the target.
+func (t *Target[T]) RemoveListener(handle Handle) {
+	t.init()
+
+	select {
+	case <-t.ctx.Done():
+	case t.removeListenerCh <- removeListenerEvent[T]{handle}:
+	}
+}
+
+func (t *Target[T]) init() {
+	t.initOnce.Do(func() {
+		t.ctx, t.cancel = context.WithCancelCause(context.Background())
+		t.addListenerCh = make(chan addListenerEvent[T], 1)
+		t.removeListenerCh = make(chan removeListenerEvent[T], 1)
+		t.dispatchCh = make(chan dispatchEvent[T], 1)
+		t.listeners = map[Handle]chan dispatchEvent[T]{}
+		go t.run()
+	})
+}
+
+func (t *Target[T]) run() {
+	// listen for add/remove/dispatch events and call functions
+	for {
+		select {
+		case <-t.ctx.Done():
+			return
+		case evt := <-t.addListenerCh:
+			t.addListener(evt.listener, evt.handle)
+		case evt := <-t.removeListenerCh:
+			t.removeListener(evt.handle)
+		case evt := <-t.dispatchCh:
+			t.dispatch(evt.ctx, evt.event)
+		}
+	}
+}
+
+// these functions are not thread-safe. They are intended to be called only by "run".
+
+func (t *Target[T]) addListener(listener Listener[T], handle Handle) {
+	ch := make(chan dispatchEvent[T], 1)
+	t.listeners[handle] = ch
+	// start a goroutine to send events to the listener
+	go func() {
+		for {
+			select {
+			case <-t.ctx.Done():
+			case evt := <-ch:
+				listener(evt.ctx, evt.event)
+			}
+		}
+	}()
+}
+
+func (t *Target[T]) removeListener(handle Handle) {
+	ch, ok := t.listeners[handle]
+	if !ok {
+		// nothing to do since the listener doesn't exist
+		return
+	}
+	// close the channel to kill the goroutine
+	close(ch)
+	delete(t.listeners, handle)
+}
+
+func (t *Target[T]) dispatch(ctx context.Context, evt T) {
+	// loop over all the listeners and send the event to them
+	for _, ch := range t.listeners {
+		select {
+		case <-t.ctx.Done():
+			return
+		case ch <- dispatchEvent[T]{ctx: ctx, event: evt}:
+		}
+	}
+}

internal/events/target_test.go

@@ -0,0 +1,53 @@
+package events_test
+
+import (
+	"context"
+	"sync/atomic"
+	"testing"
+	"time"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/pomerium/pomerium/internal/events"
+)
+
+func TestTarget(t *testing.T) {
+	t.Parallel()
+
+	var target events.Target[int64]
+	t.Cleanup(target.Close)
+
+	var calls1, calls2, calls3 atomic.Int64
+	h1 := target.AddListener(func(_ context.Context, i int64) {
+		calls1.Add(i)
+	})
+	h2 := target.AddListener(func(_ context.Context, i int64) {
+		calls2.Add(i)
+	})
+	h3 := target.AddListener(func(_ context.Context, i int64) {
+		calls3.Add(i)
+	})
+
+	shouldBe := func(i1, i2, i3 int64) {
+		t.Helper()
+
+		assert.Eventually(t, func() bool { return calls1.Load() == i1 }, time.Second, time.Millisecond)
+		assert.Eventually(t, func() bool { return calls2.Load() == i2 }, time.Second, time.Millisecond)
+		assert.Eventually(t, func() bool { return calls3.Load() == i3 }, time.Second, time.Millisecond)
+	}
+
+	target.Dispatch(context.Background(), 1)
+	shouldBe(1, 1, 1)
+
+	target.RemoveListener(h2)
+	target.Dispatch(context.Background(), 2)
+	shouldBe(3, 1, 3)
+
+	target.RemoveListener(h1)
+	target.Dispatch(context.Background(), 3)
+	shouldBe(3, 1, 6)
+
+	target.RemoveListener(h3)
+	target.Dispatch(context.Background(), 4)
+	shouldBe(3, 1, 6)
+}