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

//go:build linux

package dataplane

import (
	"net"
	"syscall"
)

// PMTUD socket-option values. Go's syscall package exports IP_MTU_DISCOVER and
// IPV6_MTU_DISCOVER but not the IP_PMTUDISC_* values, so they are spelled out
// here (include/linux/in.h, in6.h — stable ABI, same reasoning as the client's
// OsAbi.kt).
const (
	pmtudiscWant = 0 // per-route default: fragment locally when needed
	pmtudiscDo   = 2 // always set DF: oversized sends fail with EMSGSIZE, never fragment
)

// withDF runs fn with the Don't-Fragment bit forced on for conn, then puts the
// socket back the way it was found.
//
// The socket is shared by every session on that address family, so the caller
// must hold Server.dfMu: a concurrent big_send must not silently ride along
// with someone else's DF window (or, worse, clear it mid-flight).
func withDF(conn *net.UDPConn, 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_MTU_DISCOVER
	if v4 {
		level, opt = syscall.IPPROTO_IP, syscall.IP_MTU_DISCOVER
	}

	var setErr error
	prev := pmtudiscWant
	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, pmtudiscDo)
	}); 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()
}

// dfSupported reports whether withDF can actually set the DF bit here.
const dfSupported = true