Import Turbo Tunnel support code.

Copied and slightly modified from
https://gitweb.torproject.org/pluggable-transports/meek.git/log/?h=turbotunnel&id=7eb94209f857fc71c2155907b0462cc587fc76cc
https://github.com/net4people/bbs/issues/21

RedialPacketConn is adapted from clientPacketConn in
c64a61c6da/obfs4proxy/turbotunnel_client.go
https://github.com/net4people/bbs/issues/14#issuecomment-544747519

common/turbotunnel/redialpacketconn.go

@@ -0,0 +1,204 @@
+package turbotunnel
+
+import (
+	"context"
+	"errors"
+	"net"
+	"sync"
+	"sync/atomic"
+	"time"
+)
+
+// RedialPacketConn implements a long-lived net.PacketConn atop a sequence of
+// other, transient net.PacketConns. RedialPacketConn creates a new
+// net.PacketConn by calling a provided dialContext function. Whenever the
+// net.PacketConn experiences a ReadFrom or WriteTo error, RedialPacketConn
+// calls the dialContext function again and starts sending and receiving packets
+// on the new net.PacketConn. RedialPacketConn's own ReadFrom and WriteTo
+// methods return an error only when the dialContext function returns an error.
+//
+// RedialPacketConn uses static local and remote addresses that are independent
+// of those of any dialed net.PacketConn.
+type RedialPacketConn struct {
+	localAddr   net.Addr
+	remoteAddr  net.Addr
+	dialContext func(context.Context) (net.PacketConn, error)
+	recvQueue   chan []byte
+	sendQueue   chan []byte
+	closed      chan struct{}
+	closeOnce   sync.Once
+	// The first dial error, which causes the clientPacketConn to be
+	// closed and is returned from future read/write operations. Compare to
+	// the rerr and werr in io.Pipe.
+	err atomic.Value
+}
+
+// NewQueuePacketConn makes a new RedialPacketConn, with the given static local
+// and remote addresses, and dialContext function.
+func NewRedialPacketConn(
+	localAddr, remoteAddr net.Addr,
+	dialContext func(context.Context) (net.PacketConn, error),
+) *RedialPacketConn {
+	c := &RedialPacketConn{
+		localAddr:   localAddr,
+		remoteAddr:  remoteAddr,
+		dialContext: dialContext,
+		recvQueue:   make(chan []byte, queueSize),
+		sendQueue:   make(chan []byte, queueSize),
+		closed:      make(chan struct{}),
+		err:         atomic.Value{},
+	}
+	go c.dialLoop()
+	return c
+}
+
+// dialLoop repeatedly calls c.dialContext and passes the resulting
+// net.PacketConn to c.exchange. It returns only when c is closed or dialContext
+// returns an error.
+func (c *RedialPacketConn) dialLoop() {
+	ctx, cancel := context.WithCancel(context.Background())
+	for {
+		select {
+		case <-c.closed:
+			cancel()
+			return
+		default:
+		}
+		conn, err := c.dialContext(ctx)
+		if err != nil {
+			c.closeWithError(err)
+			cancel()
+			return
+		}
+		c.exchange(conn)
+		conn.Close()
+	}
+}
+
+// exchange calls ReadFrom on the given net.PacketConn and places the resulting
+// packets in the receive queue, and takes packets from the send queue and calls
+// WriteTo on them, making the current net.PacketConn active.
+func (c *RedialPacketConn) exchange(conn net.PacketConn) {
+	readErrCh := make(chan error)
+	writeErrCh := make(chan error)
+
+	go func() {
+		defer close(readErrCh)
+		for {
+			select {
+			case <-c.closed:
+				return
+			case <-writeErrCh:
+				return
+			default:
+			}
+
+			var buf [1500]byte
+			n, _, err := conn.ReadFrom(buf[:])
+			if err != nil {
+				readErrCh <- err
+				return
+			}
+			p := make([]byte, n)
+			copy(p, buf[:])
+			select {
+			case c.recvQueue <- p:
+			default: // OK to drop packets.
+			}
+		}
+	}()
+
+	go func() {
+		defer close(writeErrCh)
+		for {
+			select {
+			case <-c.closed:
+				return
+			case <-readErrCh:
+				return
+			case p := <-c.sendQueue:
+				_, err := conn.WriteTo(p, c.remoteAddr)
+				if err != nil {
+					writeErrCh <- err
+					return
+				}
+			}
+		}
+	}()
+
+	select {
+	case <-readErrCh:
+	case <-writeErrCh:
+	}
+}
+
+// ReadFrom reads a packet from the currently active net.PacketConn. The
+// packet's original remote address is replaced with the RedialPacketConn's own
+// remote address.
+func (c *RedialPacketConn) ReadFrom(p []byte) (int, net.Addr, error) {
+	select {
+	case <-c.closed:
+		return 0, nil, &net.OpError{Op: "read", Net: c.LocalAddr().Network(), Source: c.LocalAddr(), Addr: c.remoteAddr, Err: c.err.Load().(error)}
+	default:
+	}
+	select {
+	case <-c.closed:
+		return 0, nil, &net.OpError{Op: "read", Net: c.LocalAddr().Network(), Source: c.LocalAddr(), Addr: c.remoteAddr, Err: c.err.Load().(error)}
+	case buf := <-c.recvQueue:
+		return copy(p, buf), c.remoteAddr, nil
+	}
+}
+
+// WriteTo writes a packet to the currently active net.PacketConn. The addr
+// argument is ignored and instead replaced with the RedialPacketConn's own
+// remote address.
+func (c *RedialPacketConn) WriteTo(p []byte, addr net.Addr) (int, error) {
+	// addr is ignored.
+	select {
+	case <-c.closed:
+		return 0, &net.OpError{Op: "write", Net: c.LocalAddr().Network(), Source: c.LocalAddr(), Addr: c.remoteAddr, Err: c.err.Load().(error)}
+	default:
+	}
+	buf := make([]byte, len(p))
+	copy(buf, p)
+	select {
+	case c.sendQueue <- buf:
+		return len(buf), nil
+	default:
+		// Drop the outgoing packet if the send queue is full.
+		return len(buf), nil
+	}
+}
+
+// closeWithError unblocks pending operations and makes future operations fail
+// with the given error. If err is nil, it becomes errClosedPacketConn.
+func (c *RedialPacketConn) closeWithError(err error) error {
+	var once bool
+	c.closeOnce.Do(func() {
+		// Store the error to be returned by future read/write
+		// operations.
+		if err == nil {
+			err = errors.New("operation on closed connection")
+		}
+		c.err.Store(err)
+		close(c.closed)
+		once = true
+	})
+	if !once {
+		return &net.OpError{Op: "close", Net: c.LocalAddr().Network(), Addr: c.LocalAddr(), Err: c.err.Load().(error)}
+	}
+	return nil
+}
+
+// Close unblocks pending operations and makes future operations fail with a
+// "closed connection" error.
+func (c *RedialPacketConn) Close() error {
+	return c.closeWithError(nil)
+}
+
+// LocalAddr returns the localAddr value that was passed to NewRedialPacketConn.
+func (c *RedialPacketConn) LocalAddr() net.Addr { return c.localAddr }
+
+func (c *RedialPacketConn) SetDeadline(t time.Time) error      { return errNotImplemented }
+func (c *RedialPacketConn) SetReadDeadline(t time.Time) error  { return errNotImplemented }
+func (c *RedialPacketConn) SetWriteDeadline(t time.Time) error { return errNotImplemented }