Add test showing that reusing TCP ports isn't a good idea

reuse_test.go

@@ -0,0 +1,70 @@
+package torrent
+
+import (
+	"context"
+	"errors"
+	"net"
+	"sync/atomic"
+	"syscall"
+	"testing"
+
+	"github.com/anacrolix/log"
+
+	qt "github.com/frankban/quicktest"
+)
+
+// Show that multiple connections from the same local TCP port to the same remote port will fail.
+func TestTcpPortReuseIsABadIdea(t *testing.T) {
+	remote, err := net.Listen("tcp", "localhost:0")
+	c := qt.New(t)
+	c.Assert(err, qt.IsNil)
+	defer remote.Close()
+	dialer := net.Dialer{}
+	dialer.Control = func(network, address string, c syscall.RawConn) (err error) {
+		return c.Control(func(fd uintptr) {
+			err = setReusePortSockOpts(fd)
+		})
+	}
+	first, err := dialer.Dial("tcp", remote.Addr().String())
+	c.Assert(err, qt.IsNil)
+	defer first.Close()
+	dialer.LocalAddr = first.LocalAddr()
+	_, err = dialer.Dial("tcp", remote.Addr().String())
+	c.Assert(errors.Is(err, syscall.EADDRINUSE), qt.IsTrue)
+}
+
+// Show that multiple connections from the same local utp socket to the same remote port will
+// succeed. This is necessary for ut_holepunch to work.
+func TestUtpLocalPortIsReusable(t *testing.T) {
+	const network = "udp"
+	c := qt.New(t)
+	remote, err := NewUtpSocket(network, "localhost:0", nil, log.Default)
+	c.Assert(err, qt.IsNil)
+	defer remote.Close()
+	var remoteAccepts int32
+	go func() {
+		for {
+			c, err := remote.Accept()
+			if err != nil {
+				if atomic.LoadInt32(&remoteAccepts) != 2 {
+					t.Logf("error accepting on remote: %v", err)
+				}
+				break
+			}
+			// This is not a leak, bugger off.
+			defer c.Close()
+			atomic.AddInt32(&remoteAccepts, 1)
+		}
+	}()
+	local, err := NewUtpSocket(network, "localhost:0", nil, log.Default)
+	c.Assert(err, qt.IsNil)
+	defer local.Close()
+	first, err := local.DialContext(context.Background(), network, remote.Addr().String())
+	c.Assert(err, qt.IsNil)
+	defer first.Close()
+	second, err := local.DialContext(context.Background(), network, remote.Addr().String())
+	c.Assert(err, qt.IsNil)
+	defer second.Close()
+	remote.Close()
+	c.Assert(atomic.LoadInt32(&remoteAccepts), qt.Equals, int32(2))
+}