Fix flaky ActivityLog unit test (#10860)

* Wait for initial retention run to finish before adding segments.

vault/activity_log.go

@@ -137,6 +137,10 @@ type ActivityLog struct {
 	// channel closed by delete worker when done
 	deleteDone chan struct{}
 
+	// channel closed when deletion at startup is done
+	// (for unit test robustness)
+	retentionDone chan struct{}
+
 	// for testing: is config currently being invalidated. protected by l
 	configInvalidationInProgress bool
 }
@@ -975,7 +979,12 @@ func (c *Core) setupActivityLog(ctx context.Context, wg *sync.WaitGroup) error {
 		go manager.precomputedQueryWorker()
 
 		// Catch up on garbage collection
-		go manager.retentionWorker(time.Now(), manager.retentionMonths)
+		// Signal when this is done so that unit tests can proceed.
+		manager.retentionDone = make(chan struct{})
+		go func() {
+			manager.retentionWorker(time.Now(), manager.retentionMonths)
+			close(manager.retentionDone)
+		}()
 	}
 
 	// Link the token store to this core

vault/activity_log_test.go

@@ -2307,11 +2307,26 @@ func TestActivityLog_NextMonthStart(t *testing.T) {
 	}
 }
 
+// The retention worker is called on unseal; wait for it to finish before
+// proceeding with the test.
+func waitForRetentionWorkerToFinish(t *testing.T, a *ActivityLog) {
+	t.Helper()
+	timeout := time.After(30 * time.Second)
+	select {
+	case <-a.retentionDone:
+		return
+	case <-timeout:
+		t.Fatal("timeout waiting for retention worker to finish")
+	}
+
+}
+
 func TestActivityLog_Deletion(t *testing.T) {
 	timeutil.SkipAtEndOfMonth(t)
 
 	core, _, _ := TestCoreUnsealed(t)
 	a := core.activityLog
+	waitForRetentionWorkerToFinish(t, a)
 
 	times := []time.Time{
 		time.Date(2019, 1, 15, 1, 2, 3, 0, time.UTC), // 0