perf: expand gateway capacity and prevent receipt replay

gateway/internal/inbound/server_test.go

@@ -930,6 +930,42 @@ func TestFlushOnlineAccountsFetchesPendingDeliveries(t *testing.T) {
 	}
 }
 
+func TestFlushPendingCoalescesConcurrentRequestsPerAccount(t *testing.T) {
+	var calls atomic.Int32
+	release := make(chan struct{})
+	api := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		if r.URL.Path != "/api/gateway/events/downstream/pending" {
+			t.Fatalf("unexpected api path: %s", r.URL.Path)
+		}
+		calls.Add(1)
+		<-release
+		_ = json.NewEncoder(w).Encode([]pendingDelivery{})
+	}))
+	defer api.Close()
+
+	server := Server{APIBaseURL: api.URL + "/api", GatewayInstanceID: "gateway-a"}
+	results := make(chan error, 8)
+	for range 8 {
+		go func() {
+			_, err := server.flushPending("100001", log.Default())
+			results <- err
+		}()
+	}
+	deadline := time.Now().Add(time.Second)
+	for calls.Load() == 0 && time.Now().Before(deadline) {
+		time.Sleep(time.Millisecond)
+	}
+	close(release)
+	for range 8 {
+		if err := <-results; err != nil {
+			t.Fatalf("flush failed: %v", err)
+		}
+	}
+	if got := calls.Load(); got != 1 {
+		t.Fatalf("pending fetch calls = %d, want 1", got)
+	}
+}
+
 func TestRecoverPendingCandidatesFetchesPresenceAccounts(t *testing.T) {
 	resetDownstreamRegistry()
 	defer resetDownstreamRegistry()
@@ -1085,7 +1121,7 @@ func TestDownstreamDeliveryRequiresAcknowledgement(t *testing.T) {
 		messageID: "MSG-LONG-1", phoneNumber: "18821203795",
 		protocolLog: func(event protocolLogEvent) { protocolEvents <- event },
 	}
-	registerDownstreamAck(session, "delivery-1", 37, 9016479179509871733, time.Now().Add(time.Second))
+	registerDownstreamAck(session, "delivery-1", "", 37, 9016479179509871733, time.Now().Add(time.Second))
 	handleDownstreamAcknowledgement(conn, 37, 9016479179509871733, 0, log.Default())
 
 	select {
@@ -1165,6 +1201,7 @@ func TestSendDownstreamRejectsZeroMessageID(t *testing.T) {
 		&downstreamSession{mu: &sync.Mutex{}},
 		&cmpp.Cmpp2DeliverReqPkt{MsgId: 0},
 		"delivery-zero",
+		"",
 	)
 	if err == nil || !strings.Contains(err.Error(), "Msg_Id=0") {
 		t.Fatalf("expected zero Msg_Id rejection, got %v", err)
@@ -1180,7 +1217,7 @@ func TestDownstreamDeliveryReportsAckTimeout(t *testing.T) {
 		conn: &cmpp.Conn{}, connectionID: "conn-1",
 		deliveryReport: func(event downstreamDeliveryLifecycleEvent) { events <- event },
 	}
-	registerDownstreamAck(session, "delivery-timeout", 38, 9017467844344255865, time.Now().Add(20*time.Millisecond))
+	registerDownstreamAck(session, "delivery-timeout", "", 38, 9017467844344255865, time.Now().Add(20*time.Millisecond))
 
 	select {
 	case event := <-events: