[client] Add bind activity listener to bypass udp sockets (#4646)

client/internal/lazyconn/activity/lazy_conn.go

@@ -0,0 +1,82 @@
+package activity
+
+import (
+	"context"
+	"io"
+	"net"
+	"time"
+)
+
+// lazyConn detects activity when WireGuard attempts to send packets.
+// It does not deliver packets, only signals that activity occurred.
+type lazyConn struct {
+	activityCh chan struct{}
+	ctx        context.Context
+	cancel     context.CancelFunc
+}
+
+// newLazyConn creates a new lazyConn for activity detection.
+func newLazyConn() *lazyConn {
+	ctx, cancel := context.WithCancel(context.Background())
+	return &lazyConn{
+		activityCh: make(chan struct{}, 1),
+		ctx:        ctx,
+		cancel:     cancel,
+	}
+}
+
+// Read blocks until the connection is closed.
+func (c *lazyConn) Read(_ []byte) (n int, err error) {
+	<-c.ctx.Done()
+	return 0, io.EOF
+}
+
+// Write signals activity detection when ICEBind routes packets to this endpoint.
+func (c *lazyConn) Write(b []byte) (n int, err error) {
+	if c.ctx.Err() != nil {
+		return 0, io.EOF
+	}
+
+	select {
+	case c.activityCh <- struct{}{}:
+	default:
+	}
+
+	return len(b), nil
+}
+
+// ActivityChan returns the channel that signals when activity is detected.
+func (c *lazyConn) ActivityChan() <-chan struct{} {
+	return c.activityCh
+}
+
+// Close closes the connection.
+func (c *lazyConn) Close() error {
+	c.cancel()
+	return nil
+}
+
+// LocalAddr returns the local address.
+func (c *lazyConn) LocalAddr() net.Addr {
+	return &net.UDPAddr{IP: net.IP{127, 0, 0, 1}, Port: lazyBindPort}
+}
+
+// RemoteAddr returns the remote address.
+func (c *lazyConn) RemoteAddr() net.Addr {
+	return &net.UDPAddr{IP: net.IP{127, 0, 0, 1}, Port: lazyBindPort}
+}
+
+// SetDeadline sets the read and write deadlines.
+func (c *lazyConn) SetDeadline(_ time.Time) error {
+	return nil
+}
+
+// SetReadDeadline sets the deadline for future Read calls.
+func (c *lazyConn) SetReadDeadline(_ time.Time) error {
+	return nil
+}
+
+// SetWriteDeadline sets the deadline for future Write calls.
+func (c *lazyConn) SetWriteDeadline(_ time.Time) error {
+	return nil
+}