// SPDX-FileCopyrightText: 2026 Echolot contributors
// SPDX-License-Identifier: GPL-3.0-or-later

//go:build linux

package dataplane

import (
	"net"
	"syscall"
)

// withTOS runs fn with the socket's TOS/traffic class set to dscp<<2 (ECN bits left zero — the
// test is about DSCP survival, and claiming ECN capability we do not use would pollute it), then
// restores what was there before.
//
// Same borrow discipline as withDF: the socket is shared by every session on that family, so the
// caller must hold Server.dfMu for the whole window or a concurrent burst rides along with — or
// clears — someone else's marking.
func withTOS(conn *net.UDPConn, dscp int, fn func() error) error {
	raw, err := conn.SyscallConn()
	if err != nil {
		return err
	}
	v4 := conn.LocalAddr().(*net.UDPAddr).IP.To4() != nil
	level, opt := syscall.IPPROTO_IPV6, syscall.IPV6_TCLASS
	if v4 {
		level, opt = syscall.IPPROTO_IP, syscall.IP_TOS
	}

	var setErr error
	prev := 0
	if err := raw.Control(func(fd uintptr) {
		if p, e := syscall.GetsockoptInt(int(fd), level, opt); e == nil {
			prev = p
		}
		setErr = syscall.SetsockoptInt(int(fd), level, opt, dscp<<2)
	}); err != nil {
		return err
	}
	if setErr != nil {
		return setErr
	}
	defer func() {
		_ = raw.Control(func(fd uintptr) {
			_ = syscall.SetsockoptInt(int(fd), level, opt, prev)
		})
	}()
	return fn()
}

// TOSSupported reports whether withTOS can actually mark packets here. Exported so the control
// plane can tell the client up front that its dscp request will not be honored, instead of the
// client measuring an unmarked burst and concluding the network stripped the marking.
const TOSSupported = true