SplitHTTP client: Add xmux (multiplex controller) for H3 & H2 (#3613)

https://github.com/XTLS/Xray-core/pull/3613#issuecomment-2351954957

Closes https://github.com/XTLS/Xray-core/issues/3560#issuecomment-2247495778

---------

Co-authored-by: mmmray <142015632+mmmray@users.noreply.github.com>

transport/internet/splithttp/mux.go

@@ -0,0 +1,102 @@
+package splithttp
+
+import (
+	"context"
+	"math/rand"
+	"sync/atomic"
+	"time"
+
+	"github.com/xtls/xray-core/common/errors"
+)
+
+type muxResource struct {
+	Resource       interface{}
+	OpenRequests   atomic.Int32
+	leftUsage      int32
+	expirationTime time.Time
+}
+
+type muxManager struct {
+	newResourceFn func() interface{}
+	config        Multiplexing
+	concurrency   int32
+	connections   int32
+	instances     []*muxResource
+}
+
+func NewMuxManager(config Multiplexing, newResource func() interface{}) *muxManager {
+	return &muxManager{
+		config:        config,
+		concurrency:   config.GetNormalizedMaxConcurrency().roll(),
+		connections:   config.GetNormalizedMaxConnections().roll(),
+		newResourceFn: newResource,
+		instances:     make([]*muxResource, 0),
+	}
+}
+
+func (m *muxManager) GetResource(ctx context.Context) *muxResource {
+	m.removeExpiredConnections(ctx)
+
+	if m.connections > 0 && len(m.instances) < int(m.connections) {
+		errors.LogDebug(ctx, "xmux: creating client, connections=", len(m.instances))
+		return m.newResource()
+	}
+
+	if len(m.instances) == 0 {
+		errors.LogDebug(ctx, "xmux: creating client because instances is empty, connections=", len(m.instances))
+		return m.newResource()
+	}
+
+	clients := make([]*muxResource, 0)
+	if m.concurrency > 0 {
+		for _, client := range m.instances {
+			openRequests := client.OpenRequests.Load()
+			if openRequests < m.concurrency {
+				clients = append(clients, client)
+			}
+		}
+	} else {
+		clients = m.instances
+	}
+
+	if len(clients) == 0 {
+		errors.LogDebug(ctx, "xmux: creating client because concurrency was hit, total clients=", len(m.instances))
+		return m.newResource()
+	}
+
+	client := clients[rand.Intn(len(clients))]
+	if client.leftUsage > 0 {
+		client.leftUsage -= 1
+	}
+	return client
+}
+
+func (m *muxManager) newResource() *muxResource {
+	leftUsage := int32(-1)
+	if x := m.config.GetNormalizedCMaxReuseTimes().roll(); x > 0 {
+		leftUsage = x - 1
+	}
+	expirationTime := time.UnixMilli(0)
+	if x := m.config.GetNormalizedCMaxLifetimeMs().roll(); x > 0 {
+		expirationTime = time.Now().Add(time.Duration(x) * time.Millisecond)
+	}
+
+	client := &muxResource{
+		Resource:       m.newResourceFn(),
+		leftUsage:      leftUsage,
+		expirationTime: expirationTime,
+	}
+	m.instances = append(m.instances, client)
+	return client
+}
+
+func (m *muxManager) removeExpiredConnections(ctx context.Context) {
+	for i := 0; i < len(m.instances); i++ {
+		client := m.instances[i]
+		if client.leftUsage == 0 || (client.expirationTime != time.UnixMilli(0) && time.Now().After(client.expirationTime)) {
+			errors.LogDebug(ctx, "xmux: removing client, leftUsage = ", client.leftUsage, ", expirationTime = ", client.expirationTime)
+			m.instances = append(m.instances[:i], m.instances[i+1:]...)
+			i--
+		}
+	}
+}