lib/connections: Try TCP punchthrough (fixes #4259) (#5753)

lib/dialer/control_unix.go

@@ -0,0 +1,51 @@
+// Copyright (C) 2019 The Syncthing Authors.
+//
+// This Source Code Form is subject to the terms of the Mozilla Public
+// License, v. 2.0. If a copy of the MPL was not distributed with this file,
+// You can obtain one at https://mozilla.org/MPL/2.0/.
+
+// +build !solaris,!windows
+
+package dialer
+
+import (
+	"syscall"
+
+	"golang.org/x/sys/unix"
+)
+
+var SupportsReusePort = false
+
+func init() {
+	fd, err := unix.Socket(unix.AF_INET, unix.SOCK_STREAM, unix.IPPROTO_IP)
+	if err != nil {
+		l.Debugln("Failed to create a socket", err)
+		return
+	}
+	defer func() { _ = unix.Close(fd) }()
+
+	err = unix.SetsockoptInt(fd, unix.SOL_SOCKET, unix.SO_REUSEPORT, 1)
+	switch {
+	case err == unix.ENOPROTOOPT || err == unix.EINVAL:
+		l.Debugln("SO_REUSEPORT not supported")
+	case err != nil:
+		l.Debugln("Unknown error when determining SO_REUSEPORT support", err)
+	default:
+		l.Debugln("SO_REUSEPORT supported")
+		SupportsReusePort = true
+	}
+}
+
+func ReusePortControl(_, _ string, c syscall.RawConn) error {
+	if !SupportsReusePort {
+		return nil
+	}
+	var opErr error
+	err := c.Control(func(fd uintptr) {
+		opErr = unix.SetsockoptInt(int(fd), unix.SOL_SOCKET, unix.SO_REUSEPORT, 1)
+	})
+	if err != nil {
+		return err
+	}
+	return opErr
+}