MaximumClientWritesPending建议添加文档说明其意义及影响

[CendealChen]

MaximumClientWritesPending这个参数默认给的值太大了，也没有在文档中给出需要注意的问题。尤其是它对运行内存影响极大的问题。

// 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
		MaximumPacketSize:            0,              // no maximum packet size
		maximumPacketID:              math.MaxUint16,
		ReceiveMaximum:               1024,           // maximum number of concurrent qos messages per client
		MaximumInflight:              1024 * 8,       // maximum number of qos > 0 messages can be stored
		TopicAliasMaximum:            math.MaxUint16, // maximum topic alias value
		SharedSubAvailable:           1,              // shared subscriptions are available
		MinimumProtocolVersion:       3,              // minimum supported mqtt version (3.0.0)
		MaximumQos:                   2,              // maximum qos value available to clients
		RetainAvailable:              1,              // retain messages is available
		WildcardSubAvailable:         1,              // wildcard subscriptions are available
		SubIDAvailable:               1,              // subscription identifiers are available
	}
}

根据实际测试，发现默认给的1024*8过大了，在高活IoT设备场景相当的吃内存，在没有任何数据交互情况下，仅仅只是保持连接，就已经消耗了大量的内存，比如同时在线1w的时候，服务器的内存开销会很大， @werbenhu

Type: inuse_space
Time: Jun 15, 2024 at 7:31pm (CST)
Entering interactive mode (type "help" for commands, "o" for options)
(pprof) top
Showing nodes accounting for 707.87MB, 98.03% of 722.12MB total
Dropped 32 nodes (cum <= 3.61MB)
Showing top 10 nodes out of 51
      flat  flat%   sum%        cum   cum%
  643.35MB 89.09% 89.09%   665.38MB 92.14%  github.com/mochi-mqtt/server/v2.newClient
   20.04MB  2.77% 91.87%    20.04MB  2.77%  bufio.NewReaderSize (inline)
       9MB  1.25% 93.11%        9MB  1.25%  bytes.growSlice
    8.47MB  1.17% 94.29%     8.47MB  1.17%  github.com/mochi-mqtt/server/v2.(*Subscriptions).Add
       8MB  1.11% 95.39%        8MB  1.11%  runtime.malg
       5MB  0.69% 96.09%        5MB  0.69%  crypto/tls.Server
       5MB  0.69% 96.78%        5MB  0.69%  net.newFD
    4.50MB  0.62% 97.40%     4.50MB  0.62%  crypto/aes.(*aesCipherGCM).NewGCM
    4.50MB  0.62% 98.03%     4.50MB  0.62%  crypto/aes.newCipher
         0     0% 98.03%    21.01MB  2.91%  bufio.(*Reader).ReadByte

(pprof) list github.com/mochi-mqtt/server/v2.newClient 
Total: 722.12MB
ROUTINE ======================== github.com/mochi-mqtt/server/v2.newClient in D:\Cendeal\SDK\go1.22.1\pkg\mod\github.com\mochi-mqtt\server\v[email protected]\clients.go
  643.35MB   665.38MB (flat, cum) 92.14% of Total
         .          .    164:func newClient(c net.Conn, o *ops) *Client {
         .   512.04kB    165:   ctx, cancel := context.WithCancel(context.Background())
       3MB        3MB    166:   cl := &Client{
         .          .    167:           State: ClientState{
         .   512.02kB    168:                   Inflight:      NewInflights(),
         .          .    169:                   Subscriptions: NewSubscriptions(),
         .        1MB    170:                   TopicAliases:  NewTopicAliases(o.options.Capabilities.TopicAliasMaximum),
         .          .    171:                   open:          ctx,
         .          .    172:                   cancelOpen:    cancel,
         .          .    173:                   Keepalive:     defaultKeepalive,
  640.34MB   640.34MB    174:                   outbound:      make(chan *packets.Packet, o.options.Capabilities.MaximumClientWritesPending),
         .          .    175:           },
         .          .    176:           Properties: ClientProperties{
         .          .    177:                   ProtocolVersion: defaultClientProtocolVersion, // default protocol version
         .          .    178:           },
         .          .    179:           ops: o,
         .          .    180:   }
         .          .    181:
         .          .    182:   if c != nil {
         .          .    183:           cl.Net = ClientConnection{
         .          .    184:                   Conn:   c,
         .    20.04MB    185:                   bconn:  bufio.NewReaderSize(c, o.options.ClientNetReadBufferSize),
         .          .    186:                   Remote: c.RemoteAddr().String(),
         .          .    187:           }
         .          .    188:   }
         .          .    189:
         .          .    190:   return cl