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

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)
+}