Add conntracker api

common/conntrack/tracker.go

@@ -0,0 +1,41 @@
+package conntrack
+
+import (
+	"io"
+	"sync"
+
+	"github.com/sagernet/sing/common"
+	"github.com/sagernet/sing/common/x/list"
+)
+
+type Tracker struct {
+	access      sync.Mutex
+	connections list.List[io.Closer]
+}
+
+func (m *Tracker) Track(conn io.Closer) *Registration {
+	m.access.Lock()
+	element := m.connections.PushBack(conn)
+	m.access.Unlock()
+	return &Registration{m, element}
+}
+
+func (m *Tracker) Reset() {
+	m.access.Lock()
+	defer m.access.Unlock()
+	for element := m.connections.Front(); element != nil; element = element.Next() {
+		common.Close(element.Value)
+	}
+	m.connections = list.List[io.Closer]{}
+}
+
+type Registration struct {
+	manager *Tracker
+	element *list.Element[io.Closer]
+}
+
+func (t *Registration) Leave() {
+	t.manager.access.Lock()
+	defer t.manager.access.Unlock()
+	t.manager.connections.Remove(t.element)
+}