lib/connections, vendor: Change KCP mux to SMUX

Closes #4032

vendor/github.com/xtaci/smux/mux.go

@@ -0,0 +1,80 @@
+package smux
+
+import (
+	"fmt"
+	"io"
+	"time"
+
+	"github.com/pkg/errors"
+)
+
+// Config is used to tune the Smux session
+type Config struct {
+	// KeepAliveInterval is how often to send a NOP command to the remote
+	KeepAliveInterval time.Duration
+
+	// KeepAliveTimeout is how long the session
+	// will be closed if no data has arrived
+	KeepAliveTimeout time.Duration
+
+	// MaxFrameSize is used to control the maximum
+	// frame size to sent to the remote
+	MaxFrameSize int
+
+	// MaxReceiveBuffer is used to control the maximum
+	// number of data in the buffer pool
+	MaxReceiveBuffer int
+}
+
+// DefaultConfig is used to return a default configuration
+func DefaultConfig() *Config {
+	return &Config{
+		KeepAliveInterval: 10 * time.Second,
+		KeepAliveTimeout:  30 * time.Second,
+		MaxFrameSize:      4096,
+		MaxReceiveBuffer:  4194304,
+	}
+}
+
+// VerifyConfig is used to verify the sanity of configuration
+func VerifyConfig(config *Config) error {
+	if config.KeepAliveInterval == 0 {
+		return errors.New("keep-alive interval must be positive")
+	}
+	if config.KeepAliveTimeout < config.KeepAliveInterval {
+		return fmt.Errorf("keep-alive timeout must be larger than keep-alive interval")
+	}
+	if config.MaxFrameSize <= 0 {
+		return errors.New("max frame size must be positive")
+	}
+	if config.MaxFrameSize > 65535 {
+		return errors.New("max frame size must not be larger than 65535")
+	}
+	if config.MaxReceiveBuffer <= 0 {
+		return errors.New("max receive buffer must be positive")
+	}
+	return nil
+}
+
+// Server is used to initialize a new server-side connection.
+func Server(conn io.ReadWriteCloser, config *Config) (*Session, error) {
+	if config == nil {
+		config = DefaultConfig()
+	}
+	if err := VerifyConfig(config); err != nil {
+		return nil, err
+	}
+	return newSession(config, conn, false), nil
+}
+
+// Client is used to initialize a new client-side connection.
+func Client(conn io.ReadWriteCloser, config *Config) (*Session, error) {
+	if config == nil {
+		config = DefaultConfig()
+	}
+
+	if err := VerifyConfig(config); err != nil {
+		return nil, err
+	}
+	return newSession(config, conn, true), nil
+}