//go:build linux

package transport

import (
	"encoding/binary"
	"errors"
	"fmt"
	"io"
	"syscall"
	"time"

	"omnisocketgo/cmd/internal/latencylog"
	"omnisocketgo/cmd/internal/protocol"
)

const (
	linuxTimestampControlBufferSize = 256                    // 控制消息缓冲区。
	linuxTXTimestampWaitTimeout     = 250 * time.Millisecond // 等待 TX 时间戳的上限。
	linuxTXTimestampPollInterval    = time.Millisecond       // 轮询 errqueue 的间隔。
	linuxDataPollInterval           = time.Millisecond       // 轮询普通收发的间隔。

	linuxSOTimestampingNew      = 0x41
	linuxSCMTimestampingNew     = linuxSOTimestampingNew
	linuxSOEEOriginTimestamping = 4 // timestamping errqueue 事件。
	linuxSCMTstampSnd           = 0 // 对应 A_TX_SOFTWARE。
	linuxSCMTstampSched         = 1 // 对应 A_TX_SCHED。

	linuxSOFTimestampingTXSoftware = 1 << 1  // 打开 TX software timestamp。
	linuxSOFTimestampingRXSoftware = 1 << 3  // 打开 RX software timestamp。
	linuxSOFTimestampingSoftware   = 1 << 4  // software timestamp 总开关。
	linuxSOFTimestampingOptID      = 1 << 7  // 给时间戳关联 ID。
	linuxSOFTimestampingTXSched    = 1 << 8  // 打开 TX sched timestamp。
	linuxSOFTimestampingOptTSONLY  = 1 << 11 // 只回时间戳。
	linuxSOFTimestampingOptIDTCP   = 1 << 16 // 让 TCP 也带 timestamp ID。

	linuxTXTimestampPhasePreSendDrain    = "pre_send_drain"
	linuxTXTimestampPhasePostSendCollect = "post_send_collect"
	linuxTXTimestampPhasePostSelectDrain = "post_select_drain"
)

type txSendChunk struct {
	SendCallIndex    int
	FrameOffsetStart int
	FrameOffsetEnd   int // 包含当前 sendmsg 成功写出的最后一个 frame 字节偏移。
	BytesWritten     int
	ExpectedTXID     uint32
}

type txTimestampEvent struct {
	EventName  string
	TSUnixNano int64
	EEInfo     uint32
	EEData     uint32
}

type observedTXTimestampEvent struct {
	Phase     string
	ReadIndex int
	Event     txTimestampEvent
}

type txTimestampSelection struct {
	SelectedID uint32
	Timestamps map[string]int64
	HasEvent   bool
}

type socketExtendedErrInfo struct {
	Info uint32
	Data uint32
}

// 拿到底层 fd，并打开 Linux timestamping。
func (c *TCPConn) initLinuxTimestamping() error {
	sysConn, ok := c.conn.(interface {
		SyscallConn() (syscall.RawConn, error)
	})
	if !ok {
		return fmt.Errorf("transport: connection does not support SyscallConn")
	}

	rawConn, err := sysConn.SyscallConn()
	if err != nil || rawConn == nil {
		if err != nil {
			return fmt.Errorf("transport: get syscall conn: %w", err)
		}
		return fmt.Errorf("transport: missing syscall conn")
	}

	//socket是否可以成功打开 timestamping 取决于内核版本和配置，尝试多个 flag 组合直到成功或遇到非 EINVAL 错误。
	if err := enableLinuxTimestamping(rawConn); err != nil {
		return fmt.Errorf("transport: enable linux timestamping: %w", err)
	}
	//成功打开 timestamping 后，rawConn 就可以用来收 TX/RX 时间戳了。
	c.raw = rawConn
	return nil
}

// 给 socket开权限打开TX software timestamping。
func enableLinuxTimestamping(rawConn syscall.RawConn) error {
	flagCandidates := []int{ //不同linux版本可能支持不同的 flag 组合，尝试多个组合直到成功。
		linuxSOFTimestampingTXSched |
			linuxSOFTimestampingTXSoftware |
			linuxSOFTimestampingRXSoftware |
			linuxSOFTimestampingSoftware |
			linuxSOFTimestampingOptID | //TCP 协议栈给每个时间戳生成一个序列号
			linuxSOFTimestampingOptIDTCP |
			linuxSOFTimestampingOptTSONLY,
		linuxSOFTimestampingTXSched |
			linuxSOFTimestampingTXSoftware |
			linuxSOFTimestampingRXSoftware |
			linuxSOFTimestampingSoftware |
			linuxSOFTimestampingOptID |
			linuxSOFTimestampingOptTSONLY,
		linuxSOFTimestampingTXSched |
			linuxSOFTimestampingTXSoftware |
			linuxSOFTimestampingRXSoftware |
			linuxSOFTimestampingSoftware |
			linuxSOFTimestampingOptTSONLY,
	}

	var lastErr error
	for _, flags := range flagCandidates { //尝试不同的 flag 组合，直到成功或遇到非 EINVAL 错误。
		// 内核根据 fd 找到对应的内存结构体（Socket 缓冲区）
		err := rawConn.Control(func(fd uintptr) { //Control 方法保证在回调里 fd 是有效的，可以安全地调用 syscall.SetsockoptInt。
			lastErr = syscall.SetsockoptInt(int(fd), syscall.SOL_SOCKET, linuxSOTimestampingNew, flags)
		})
		if err != nil {
			return err
		}
		if lastErr == nil {
			return nil
		}
		if !errors.Is(lastErr, syscall.EINVAL) {
			return lastErr
		}
	}

	return lastErr
}

// sendMessageLinux 编码消息、写完整帧，再记录 TX 时间戳。
func (c *TCPConn) sendMessageLinux(msg protocol.Message) error {
	payload, err := protocol.EncodeMessage(msg)
	if err != nil {
		return fmt.Errorf("protocol: encode message: %w", err)
	}

	//编码后的消息 payload 前面加 4 字节长度，构成完整帧。
	frame := make([]byte, 4+len(payload))
	binary.BigEndian.PutUint32(frame[:4], uint32(len(payload)))
	copy(frame[4:], payload)

	readIndex := 0
	c.drainPendingTXTimestampEvents(msg, nil, linuxTXTimestampPhasePreSendDrain, &readIndex)

	chunks, err := c.writeFrameLinux(frame, msg)
	if err != nil {
		return fmt.Errorf("protocol: write frame: %w", err)
	}
	//记录发送延时日志
	c.logTXTimestampEvents(msg, chunks, &readIndex)
	return nil
}

// writeFrameLinux 用 sendmsg 写完整帧。
func (c *TCPConn) writeFrameLinux(frame []byte, msg protocol.Message) ([]txSendChunk, error) {
	written := 0
	sendCallIndex := 0
	chunks := make([]txSendChunk, 0, 1)

	for written < len(frame) {
		n, err := c.sendmsgDataOnce(frame[written:])
		switch {
		case err == nil:
			if n <= 0 {
				return nil, io.ErrShortWrite
			}
			chunk := txSendChunk{
				SendCallIndex:    sendCallIndex,
				FrameOffsetStart: written,
				FrameOffsetEnd:   written + n - 1,
				BytesWritten:     n,
				ExpectedTXID:     c.txWriteSeq + uint32(n) - 1,
			}
			c.txWriteSeq += uint32(n)
			chunks = append(chunks, chunk)
			c.logTXSendChunkDebugRecord(msg, chunk)
			sendCallIndex++
			written += n
		case isWouldBlock(err):
			time.Sleep(linuxDataPollInterval)
		default:
			return nil, err
		}
	}

	return chunks, nil
}

// 把 A_TX_SCHED / A_TX_SOFTWARE 写入日志。(发送过程中）
func (c *TCPConn) logTXTimestampEvents(msg protocol.Message, chunks []txSendChunk, readIndex *int) {
	timestamps := c.collectTXTimestampEvents(msg, chunks, readIndex)

	if ts, ok := timestamps[latencylog.EventATXSched]; ok {
		latencylog.LogMessageEventAt(c.logger, c.nodeRole, c.nodeID, latencylog.EventATXSched, ts, msg)
	}
	if ts, ok := timestamps[latencylog.EventATXSoftware]; ok {
		latencylog.LogMessageEventAt(c.logger, c.nodeRole, c.nodeID, latencylog.EventATXSoftware, ts, msg)
	}
}

// 在 errqueue 里等两类 TX 时间戳。
func (c *TCPConn) collectTXTimestampEvents(msg protocol.Message, chunks []txSendChunk, readIndex *int) map[string]int64 {
	deadline := time.Now().Add(linuxTXTimestampWaitTimeout)
	observed := make([]observedTXTimestampEvent, 0, 4)
	finalChunk, hasFinalChunk := finalTXSendChunk(chunks)

	// 轮询 errqueue，优先等待本次消息最后一个 sendmsg chunk 的 sched/software 两类时间戳。
	for time.Now().Before(deadline) {
		event, err := c.recvTXTimestampOnce()
		if err != nil {
			if isWouldBlock(err) {
				time.Sleep(linuxTXTimestampPollInterval)
				continue
			}
			break
		}
		if event.EventName == "" || event.TSUnixNano <= 0 {
			continue
		}
		observed = append(observed, observedTXTimestampEvent{
			Phase:     linuxTXTimestampPhasePostSendCollect,
			ReadIndex: *readIndex,
			Event:     event,
		})
		*readIndex = *readIndex + 1

		selection := selectTXTimestampEvents(observed, finalChunk.ExpectedTXID, hasFinalChunk)
		if hasFinalChunk && selection.HasEvent && selection.SelectedID == finalChunk.ExpectedTXID && hasCompleteTXTimestampPair(selection.Timestamps) {
			break
		}
	}

	selection := selectTXTimestampEvents(observed, finalChunk.ExpectedTXID, hasFinalChunk)
	c.logObservedTXTimestampEvents(msg, chunks, observed, selection)
	c.drainPendingTXTimestampEvents(msg, chunks, linuxTXTimestampPhasePostSelectDrain, readIndex)
	return selection.Timestamps
}

// recvTXTimestampOnce 从 errqueue 读一次时间戳事件。
func (c *TCPConn) recvTXTimestampOnce() (txTimestampEvent, error) {
	var (
		event txTimestampEvent
		opErr error
	)

	err := c.raw.Control(func(fd uintptr) {
		//设置足够大的 oob buffer 来接收控制消息，调用 recvmsg 从 errqueue 读一条消息。
		oob := make([]byte, linuxTimestampControlBufferSize)
		//recvmsg 的 flags 里必须带 MSG_ERRQUEUE，才能从 errqueue 里读消息，非阻塞模式下如果没有消息可读会返回 EAGAIN。
		_, oobn, _, _, recvErr := syscall.Recvmsg(int(fd), nil, oob, syscall.MSG_ERRQUEUE|syscall.MSG_DONTWAIT)
		if recvErr != nil {
			opErr = recvErr
			return
		}
		//解析控制消息，看看是不是我们关心的 TX 时间戳事件，如果是就拿到事件名和时间戳。
		event, _ = parseTXTimestampControlMessages(oob[:oobn])
	})
	if err != nil {
		return txTimestampEvent{}, err
	}
	if opErr != nil {
		return txTimestampEvent{}, opErr
	}

	return event, nil
}

// 把底层时间戳映射成日志事件名。
func parseTXTimestampControlMessages(oob []byte) (txTimestampEvent, bool) {
	if len(oob) == 0 {
		return txTimestampEvent{}, false
	}
	//解析控制消息，看看是不是我们关心的 TX 时间戳事件，如果是就拿到事件名和时间戳。
	controlMessages, err := syscall.ParseSocketControlMessage(oob)
	if err != nil {
		return txTimestampEvent{}, false
	}

	return parseTXTimestampSocketControlMessages(controlMessages)
}

func parseTXTimestampSocketControlMessages(controlMessages []syscall.SocketControlMessage) (txTimestampEvent, bool) {
	var (
		event   txTimestampEvent
		hasTS   bool
		hasKind bool
	)

	for _, controlMessage := range controlMessages {
		switch {
		case controlMessage.Header.Level == syscall.SOL_SOCKET && controlMessage.Header.Type == linuxSCMTimestampingNew:
			if ts := parseSCMTimestampingData(controlMessage.Data); ts > 0 {
				event.TSUnixNano = ts
				hasTS = true
			}
		case isSocketExtendedErr(controlMessage): //判断时间戳是否进入了errqueue，
			if info, ok := parseSocketExtendedErrInfo(controlMessage.Data); ok {
				event.EEInfo = info.Info
				event.EEData = info.Data
				event.EventName = mapLinuxTXTimestampEventName(info.Info)
				hasKind = true
			}
		}
	}

	if !hasTS || !hasKind || event.EventName == "" {
		return txTimestampEvent{}, false
	}

	return event, true
}

// 判断控制消息是否来自 socket extended err。
// 内核产生的时间戳并不会混合在普通的数据流里，而是被包装成一种特殊的“错误消息”丢进 Error Queue。
func isSocketExtendedErr(controlMessage syscall.SocketControlMessage) bool {
	switch {
	case controlMessage.Header.Level == syscall.SOL_IP && controlMessage.Header.Type == syscall.IP_RECVERR:
		return true
	case controlMessage.Header.Level == syscall.SOL_IPV6 && controlMessage.Header.Type == syscall.IPV6_RECVERR:
		return true
	default:
		return false
	}
}

// 从 socket extended err 的数据里取 origin timestamping 信息。
func parseSocketExtendedErrInfo(data []byte) (socketExtendedErrInfo, bool) {
	if len(data) < 16 {
		return socketExtendedErrInfo{}, false
	}
	if data[4] != linuxSOEEOriginTimestamping {
		return socketExtendedErrInfo{}, false
	}

	return socketExtendedErrInfo{
		Info: binary.NativeEndian.Uint32(data[8:12]),
		Data: binary.NativeEndian.Uint32(data[12:16]),
	}, true
}

func mapLinuxTXTimestampEventName(tsKind uint32) string {
	switch tsKind {
	case linuxSCMTstampSched:
		return latencylog.EventATXSched
	case linuxSCMTstampSnd:
		return latencylog.EventATXSoftware
	default:
		return fmt.Sprintf("TX_TIMESTAMP_KIND_%d", tsKind)
	}
}

func finalTXSendChunk(chunks []txSendChunk) (txSendChunk, bool) {
	if len(chunks) == 0 {
		return txSendChunk{}, false
	}

	return chunks[len(chunks)-1], true
}

func isBusinessTXTimestampEventName(eventName string) bool {
	return eventName == latencylog.EventATXSched || eventName == latencylog.EventATXSoftware
}

func hasCompleteTXTimestampPair(timestamps map[string]int64) bool {
	if len(timestamps) == 0 {
		return false
	}

	return timestamps[latencylog.EventATXSched] > 0 && timestamps[latencylog.EventATXSoftware] > 0
}

func selectTXTimestampEvents(events []observedTXTimestampEvent, expectedFinalTXID uint32, hasExpectedFinalTXID bool) txTimestampSelection {
	byID := make(map[uint32]map[string]int64)
	selection := txTimestampSelection{
		Timestamps: make(map[string]int64, 2),
	}

	var (
		highestID   uint32
		haveHighest bool
	)
	for _, observed := range events {
		event := observed.Event
		selection.HasEvent = true
		if !haveHighest || event.EEData > highestID {
			highestID = event.EEData
			haveHighest = true
		}

		if !isBusinessTXTimestampEventName(event.EventName) {
			continue
		}
		if _, ok := byID[event.EEData]; !ok {
			byID[event.EEData] = make(map[string]int64, 2)
		}
		if existing, exists := byID[event.EEData][event.EventName]; !exists || event.TSUnixNano < existing {
			byID[event.EEData][event.EventName] = event.TSUnixNano
		}
	}

	if !selection.HasEvent {
		return selection
	}

	switch {
	case hasExpectedFinalTXID && len(byID[expectedFinalTXID]) > 0:
		selection.SelectedID = expectedFinalTXID
		selection.Timestamps = copyTXTimestampMap(byID[expectedFinalTXID])
	case len(byID[highestID]) > 0:
		selection.SelectedID = highestID
		selection.Timestamps = copyTXTimestampMap(byID[highestID])
	default:
		selection.SelectedID = highestID
	}

	return selection
}

func copyTXTimestampMap(src map[string]int64) map[string]int64 {
	dst := make(map[string]int64, len(src))
	for key, value := range src {
		dst[key] = value
	}
	return dst
}

func (c *TCPConn) drainPendingTXTimestampEvents(msg protocol.Message, chunks []txSendChunk, phase string, readIndex *int) {
	for {
		event, err := c.recvTXTimestampOnce()
		if err != nil {
			if isWouldBlock(err) {
				return
			}
			return
		}
		if event.EventName == "" || event.TSUnixNano <= 0 {
			continue
		}
		c.logTXErrqueueDebugRecord(msg, chunks, observedTXTimestampEvent{
			Phase:     phase,
			ReadIndex: *readIndex,
			Event:     event,
		}, false)
		*readIndex = *readIndex + 1
	}
}

func (c *TCPConn) logObservedTXTimestampEvents(msg protocol.Message, chunks []txSendChunk, observed []observedTXTimestampEvent, selection txTimestampSelection) {
	if len(observed) == 0 {
		return
	}

	for _, entry := range observed {
		selected := selection.HasEvent &&
			entry.Event.EEData == selection.SelectedID &&
			isBusinessTXTimestampEventName(entry.Event.EventName) &&
			selection.Timestamps[entry.Event.EventName] == entry.Event.TSUnixNano
		c.logTXErrqueueDebugRecord(msg, chunks, entry, selected)
	}
}

func (c *TCPConn) logTXSendChunkDebugRecord(msg protocol.Message, chunk txSendChunk) {
	if c.txTimestampDebugLogger == nil {
		return
	}

	sendCallIndex := chunk.SendCallIndex
	frameOffsetStart := chunk.FrameOffsetStart
	frameOffsetEnd := chunk.FrameOffsetEnd
	bytesWritten := chunk.BytesWritten
	expectedTXID := chunk.ExpectedTXID

	record := c.newTXTimestampDebugRecord(msg)
	record.RecordType = txTimestampDebugRecordTypeSendChunk
	record.SendCallIndex = &sendCallIndex
	record.FrameOffsetStart = &frameOffsetStart
	record.FrameOffsetEnd = &frameOffsetEnd
	record.BytesWritten = &bytesWritten
	record.ExpectedTXID = &expectedTXID
	c.logTXTimestampDebugRecord(record)
}

func (c *TCPConn) logTXErrqueueDebugRecord(msg protocol.Message, chunks []txSendChunk, observed observedTXTimestampEvent, selected bool) {
	if c.txTimestampDebugLogger == nil {
		return
	}

	readIndex := observed.ReadIndex
	tsUnixNano := observed.Event.TSUnixNano
	eeInfo := observed.Event.EEInfo
	eeData := observed.Event.EEData
	selectedForLatency := selected

	record := c.newTXTimestampDebugRecord(msg)
	record.RecordType = txTimestampDebugRecordTypeErrqueueEvent
	record.Phase = observed.Phase
	record.ReadIndex = &readIndex
	record.EventName = observed.Event.EventName
	record.TSUnixNano = &tsUnixNano
	record.EEInfo = &eeInfo
	record.EEData = &eeData
	record.SelectedForLatency = &selectedForLatency
	if matchedSendCallIndex, ok := matchTXTimestampEventToSendChunk(observed.Event.EEData, chunks); ok {
		record.MatchedSendCallIndex = &matchedSendCallIndex
	}
	c.logTXTimestampDebugRecord(record)
}

func matchTXTimestampEventToSendChunk(txID uint32, chunks []txSendChunk) (int, bool) {
	for _, chunk := range chunks {
		if chunk.ExpectedTXID == txID {
			return chunk.SendCallIndex, true
		}
	}

	return 0, false
}

func (c *TCPConn) newTXTimestampDebugRecord(msg protocol.Message) TXTimestampDebugRecord {
	return TXTimestampDebugRecord{
		NodeRole:    c.nodeRole,
		NodeID:      c.nodeID,
		MessageType: msg.Type,
		MessageID:   msg.ID,
		From:        msg.From,
		To:          msg.To,
		FileName:    msg.FileName,
		BodySize:    len(msg.Body),
	}
}

func (c *TCPConn) logTXTimestampDebugRecord(record TXTimestampDebugRecord) {
	if c.txTimestampDebugLogger == nil {
		return
	}

	_ = c.txTimestampDebugLogger.LogTXTimestampDebugRecord(record)
}

// 读一条完整消息，并记录 B_RX_SOFTWARE。
func (c *TCPConn) receiveMessageLinux() (protocol.Message, error) {
	payload, rxTimestamp, err := c.readFrameLinux()
	if err != nil {
		return protocol.Message{}, fmt.Errorf("protocol: read frame: %w", err)
	}

	msg, err := protocol.DecodeMessage(payload)
	if err != nil {
		return protocol.Message{}, fmt.Errorf("protocol: decode message: %w", err)
	}

	if rxTimestamp > 0 {
		latencylog.LogMessageEventAt(c.logger, c.nodeRole, c.nodeID, latencylog.EventBRXSoftware, rxTimestamp, msg)
	}

	return msg, nil
}

// readFrameLinux 先读 4 字节长度，再读整条 payload。
func (c *TCPConn) readFrameLinux() ([]byte, int64, error) {
	var frameHeader [4]byte
	rxTimestamp, err := c.readFullLinux(frameHeader[:])
	if err != nil {
		return nil, rxTimestamp, err
	}

	size := binary.BigEndian.Uint32(frameHeader[:])
	switch {
	case size == 0:
		return nil, rxTimestamp, protocol.ErrInvalidFrameLength
	case size > protocol.MaxFrameSize:
		return nil, rxTimestamp, protocol.ErrFrameTooLarge
	}

	payload := make([]byte, int(size))
	bodyTimestamp, err := c.readFullLinux(payload)
	if rxTimestamp == 0 {
		rxTimestamp = bodyTimestamp
	}
	if err != nil {
		return nil, rxTimestamp, err
	}

	return payload, rxTimestamp, nil
}

// 读满 buf，并保留首个 RX_SOFTWARE（返回进入tcp协议栈的时间戳）。
func (c *TCPConn) readFullLinux(buf []byte) (int64, error) {
	if len(buf) == 0 {
		return 0, nil
	}

	var (
		offset      int
		firstRXTime int64
	)

	for offset < len(buf) {
		n, rxTimestamp, err := c.recvmsgLinux(buf[offset:])
		if firstRXTime == 0 && rxTimestamp > 0 {
			firstRXTime = rxTimestamp
		}
		if err != nil {
			if errors.Is(err, io.EOF) && offset > 0 {
				return firstRXTime, io.ErrUnexpectedEOF
			}
			return firstRXTime, err
		}

		offset += n
	}

	return firstRXTime, nil
}

// recvmsgLinux 用 recvmsg 同时读取数据和控制消息。
func (c *TCPConn) recvmsgLinux(buf []byte) (int, int64, error) {
	for {
		n, rxTimeNS, err := c.recvmsgDataOnce(buf)
		switch {
		case err == nil:
			if n == 0 {
				return 0, 0, io.EOF
			}
			return n, rxTimeNS, nil
		case isWouldBlock(err):
			time.Sleep(linuxDataPollInterval)
		default:
			return 0, 0, err
		}
	}
}

// 从控制消息里取 RX_SOFTWARE。
func parseRXTimestampControlMessages(oob []byte) int64 {
	if len(oob) == 0 {
		return 0
	}

	controlMessages, err := syscall.ParseSocketControlMessage(oob)
	if err != nil {
		return 0
	}

	for _, controlMessage := range controlMessages {
		if controlMessage.Header.Level != syscall.SOL_SOCKET || controlMessage.Header.Type != linuxSCMTimestampingNew {
			continue
		}

		if ts := parseSCMTimestampingData(controlMessage.Data); ts > 0 {
			return ts
		}
	}

	return 0
}

// 取第一个非零 timespec。
func parseSCMTimestampingData(data []byte) int64 {
	const timespec64Size = 16

	for offset := 0; offset+timespec64Size <= len(data); offset += timespec64Size {
		sec := int64(binary.NativeEndian.Uint64(data[offset : offset+8]))
		nsec := int64(binary.NativeEndian.Uint64(data[offset+8 : offset+16]))
		if sec == 0 && nsec == 0 {
			continue
		}
		return sec*int64(time.Second) + nsec
	}

	return 0
}

// 判断错误是否是 EAGAIN 或 EWOULDBLOCK。
func isWouldBlock(err error) bool {
	return errors.Is(err, syscall.EAGAIN) || errors.Is(err, syscall.EWOULDBLOCK)
}

func (c *TCPConn) sendmsgDataOnce(buf []byte) (int, error) {
	var (
		n     int
		opErr error
	)

	err := c.raw.Control(func(fd uintptr) {
		n, opErr = syscall.SendmsgN(int(fd), buf, nil, nil, 0)
	})
	if err != nil {
		return 0, err
	}

	return n, opErr
}

func (c *TCPConn) recvmsgDataOnce(buf []byte) (int, int64, error) {
	var (
		n        int
		rxTimeNS int64
		opErr    error
	)

	err := c.raw.Control(func(fd uintptr) {
		oob := make([]byte, linuxTimestampControlBufferSize)
		readN, oobN, _, _, recvErr := syscall.Recvmsg(int(fd), buf, oob, 0)
		if recvErr != nil {
			opErr = recvErr
			return
		}
		n = readN
		rxTimeNS = parseRXTimestampControlMessages(oob[:oobN])
	})
	if err != nil {
		return 0, 0, err
	}

	return n, rxTimeNS, opErr
}