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

lib/dialer/control_windows.go

@@ -0,0 +1,31 @@
+// 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 windows
+
+package dialer
+
+import (
+	"syscall"
+)
+
+var SupportsReusePort = true
+
+func ReusePortControl(_, _ string, c syscall.RawConn) error {
+	var opErr error
+	err := c.Control(func(fd uintptr) {
+		// On Windows, SO_REUSEADDR is equivalent to SO_REUSEPORT on Linux.
+		opErr = syscall.SetsockoptInt(syscall.Handle(fd), syscall.SOL_SOCKET, syscall.SO_REUSEADDR, 1)
+	})
+	if err != nil {
+		l.Debugln("ReusePortControl", err)
+		return err
+	}
+	if opErr != nil {
+		l.Debugln("ReusePortControl", opErr)
+	}
+	return opErr
+}