app/updater: drain background update-check goroutine before returning (#17446)

DownloadNewRelease spawned a background checkForUpdate loop that read
package-level knobs (UpdateCheckInterval et al.) and returned without
waiting for it, so under -race the next test rewrote those globals while
the orphaned goroutine was still reading them. waitDownloadIdle (from

Cancel and WaitGroup-drain the loop before DownloadNewRelease returns,
and have TestCancelOngoingDownload join its download goroutine so the
drain is observable before the test exits.

app/updater/updater.go

@@ -153,10 +153,12 @@ func (u *Updater) DownloadNewRelease(ctx context.Context, updateResp UpdateRespo
 		return err
 	}
 
-	// In case of slow downloads, continue the update check in the background
+	// In case of slow downloads, continue the update check in the background.
+	// Drain the goroutine before returning: it reads package-level knobs
+	// (e.g. UpdateCheckInterval), which callers may mutate once we return.
 	bgctx, bgcancel := context.WithCancel(downloadCtx)
-	defer bgcancel()
-	go func() {
+	var bgwg sync.WaitGroup
+	bgwg.Go(func() {
 		for {
 			select {
 			case <-bgctx.Done():
@@ -165,6 +167,10 @@ func (u *Updater) DownloadNewRelease(ctx context.Context, updateResp UpdateRespo
 				u.checkForUpdate(bgctx)
 			}
 		}
+	})
+	defer func() {
+		bgcancel()
+		bgwg.Wait()
 	}()
 
 	resp, err := http.DefaultClient.Do(req)

app/updater/updater_test.go

@@ -577,7 +577,9 @@ func TestCancelOngoingDownload(t *testing.T) {
 	_, resp := updater.checkForUpdate(ctx)
 
 	// Start download in goroutine
+	downloadDone := make(chan struct{})
 	go func() {
+		defer close(downloadDone)
 		_ = updater.DownloadNewRelease(ctx, resp)
 	}()
 
@@ -598,6 +600,10 @@ func TestCancelOngoingDownload(t *testing.T) {
 	case <-time.After(2 * time.Second):
 		t.Fatal("download cancellation was not received by server")
 	}
+
+	// Wait for the download goroutine to unwind: it drags along a background
+	// update-check loop that reads package-level knobs the next test rewrites.
+	<-downloadDone
 }
 
 func TestTriggerImmediateCheck(t *testing.T) {