Add regression UT

sync3/handler/ensure_polling_test.go

@@ -129,6 +129,71 @@ func TestEnsurePollerCachesResponses(t *testing.T) {
 	n.MustHaveNoSentPayloads(t)
 }
 
+// Regression test for when we did cache failures, causing no poller to start for the device
+func TestEnsurePollerDoesntCacheFailures(t *testing.T) {
+	n := &mockNotifier{ch: make(chan pubsub.Payload, 100)}
+	ctx := context.Background()
+	pid := sync2.PollerID{UserID: "@alice:localhost", DeviceID: "DEVICE"}
+	ep := NewEnsurePoller(n, false)
+
+	finished := make(chan bool) // dummy
+	var expired atomic.Bool
+	go func() {
+		exp := ep.EnsurePolling(ctx, pid, "tokenHash")
+		expired.Store(exp)
+		close(finished)
+	}()
+
+	n.WaitForNextPayload(t, time.Second) // wait for V3EnsurePolling
+	// send back the response, which failed
+	ep.OnInitialSyncComplete(&pubsub.V2InitialSyncComplete{
+		UserID:   pid.UserID,
+		DeviceID: pid.DeviceID,
+		Success:  false,
+	})
+
+	select {
+	case <-finished:
+	case <-time.After(time.Second):
+		t.Fatalf("EnsurePolling didn't unblock after response was sent")
+	}
+	if !expired.Load() {
+		t.Fatalf("EnsurePolling returned not expired, wanted expired due to Success=false")
+	}
+
+	// hitting EnsurePolling again should do a new request (i.e not cached the failure)
+	var expiredAgain atomic.Bool
+	finished = make(chan bool) // dummy
+	go func() {
+		exp := ep.EnsurePolling(ctx, pid, "tokenHash")
+		expiredAgain.Store(exp)
+		close(finished)
+	}()
+
+	p := n.WaitForNextPayload(t, time.Second) // wait for V3EnsurePolling
+	// check it's a V3EnsurePolling payload
+	pp, ok := p.(*pubsub.V3EnsurePolling)
+	if !ok {
+		t.Fatalf("unexpected payload: %+v", p)
+	}
+	assertVal(t, pp.UserID, pid.UserID)
+	assertVal(t, pp.DeviceID, pid.DeviceID)
+	assertVal(t, pp.AccessTokenHash, "tokenHash")
+
+	// send back the response, which succeeded this time
+	ep.OnInitialSyncComplete(&pubsub.V2InitialSyncComplete{
+		UserID:   pid.UserID,
+		DeviceID: pid.DeviceID,
+		Success:  true,
+	})
+
+	select {
+	case <-finished:
+	case <-time.After(time.Second):
+		t.Fatalf("EnsurePolling didn't unblock after response was sent")
+	}
+}
+
 func assertVal(t *testing.T, got, want interface{}) {
 	t.Helper()
 	if !reflect.DeepEqual(got, want) {