Ability to limit maximum number of clients

server.go

@@ -45,6 +45,7 @@ var (
 
 // Capabilities indicates the capabilities and features provided by the server.
 type Capabilities struct {
+	MaximumClients               int64           `yaml:"maximum_clients" json:"maximum_clients"`                                 // maximum number of connected clients
 	MaximumMessageExpiryInterval int64           `yaml:"maximum_message_expiry_interval" json:"maximum_message_expiry_interval"` // maximum message expiry if message expiry is 0 or over
 	MaximumClientWritesPending   int32           `yaml:"maximum_client_writes_pending" json:"maximum_client_writes_pending"`     // maximum number of pending message writes for a client
 	MaximumSessionExpiryInterval uint32          `yaml:"maximum_session_expiry_interval" json:"maximum_session_expiry_interval"` // maximum number of seconds to keep disconnected sessions
@@ -65,6 +66,7 @@ type Capabilities struct {
 // NewDefaultServerCapabilities defines the default features and capabilities provided by the server.
 func NewDefaultServerCapabilities() *Capabilities {
 	return &Capabilities{
+		MaximumClients:               math.MaxInt64,  // maximum number of connected clients
 		MaximumMessageExpiryInterval: 60 * 60 * 24,   // maximum message expiry if message expiry is 0 or over
 		MaximumClientWritesPending:   1024 * 8,       // maximum number of pending message writes for a client
 		MaximumSessionExpiryInterval: math.MaxUint32, // maximum number of seconds to keep disconnected sessions
@@ -414,6 +416,16 @@ func (s *Server) attachClient(cl *Client, listener string) error {
 	}
 
 	cl.ParseConnect(listener, pk)
+	if atomic.LoadInt64(&s.Info.ClientsConnected) >= s.Options.Capabilities.MaximumClients {
+		if cl.Properties.ProtocolVersion < 5 {
+			s.SendConnack(cl, packets.ErrServerUnavailable, false, nil)
+		} else {
+			s.SendConnack(cl, packets.ErrServerBusy, false, nil)
+		}
+
+		return packets.ErrServerBusy
+	}
+
 	code := s.validateConnect(cl, pk) // [MQTT-3.1.4-1] [MQTT-3.1.4-2]
 	if code != packets.CodeSuccess {
 		if err := s.SendConnack(cl, code, false, nil); err != nil {

server_test.go

@@ -944,6 +944,41 @@ func TestServerEstablishConnectionInvalidConnect(t *testing.T) {
 	_ = r.Close()
 }
 
+func TestEstablishConnectionMaximumClientsReached(t *testing.T) {
+	cc := NewDefaultServerCapabilities()
+	cc.MaximumClients = 0
+	s := New(&Options{
+		Logger:       logger,
+		Capabilities: cc,
+	})
+	_ = s.AddHook(new(AllowHook), nil)
+	defer s.Close()
+
+	r, w := net.Pipe()
+	o := make(chan error)
+	go func() {
+		o <- s.EstablishConnection("tcp", r)
+	}()
+
+	go func() {
+		_, _ = w.Write(packets.TPacketData[packets.Connect].Get(packets.TConnectClean).RawBytes)
+	}()
+
+	// receive the connack
+	recv := make(chan []byte)
+	go func() {
+		buf, err := io.ReadAll(w)
+		require.NoError(t, err)
+		recv <- buf
+	}()
+
+	err := <-o
+	require.Error(t, err)
+	require.ErrorIs(t, err, packets.ErrServerBusy)
+
+	_ = r.Close()
+}
+
 // See https://github.com/mochi-mqtt/server/issues/178
 func TestServerEstablishConnectionZeroByteUsernameIsValid(t *testing.T) {
 	s := newServer()