nocopy_linkbuffer.go

// Copyright 2024 CloudWeGo Authors
//
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
//
//    http://www.apache.org/licenses/LICENSE-2.0
//
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.

package netpoll

import (
	"bytes"
	"errors"
	"fmt"
	"sync"
	"sync/atomic"

	"github.com/bytedance/gopkg/lang/dirtmake"
)

// BinaryInplaceThreshold marks the minimum value of the nocopy slice length,
// which is the threshold to use copy to minimize overhead.
const BinaryInplaceThreshold = block4k

// LinkBufferCap that can be modified marks the minimum value of each node of LinkBuffer.
var LinkBufferCap = block4k

var untilErr = errors.New("link buffer read slice cannot find delim")

var (
	_ Reader = &LinkBuffer{}
	_ Writer = &LinkBuffer{}
)

// NewLinkBuffer size defines the initial capacity, but there is no readable data.
func NewLinkBuffer(size ...int) *LinkBuffer {
	buf := &LinkBuffer{}
	var l int
	if len(size) > 0 {
		l = size[0]
	}
	node := newLinkBufferNode(l)
	buf.head, buf.read, buf.flush, buf.write = node, node, node, node
	return buf
}

// UnsafeLinkBuffer implements ReadWriter.
type UnsafeLinkBuffer struct {
	length     int64
	mallocSize int

	head  *linkBufferNode // release head
	read  *linkBufferNode // read head
	flush *linkBufferNode // malloc head
	write *linkBufferNode // malloc tail

	// buf allocated by Next when cross-package, which should be freed when release
	caches [][]byte

	// for `Peek` only, avoid creating too many []byte in `caches`
	// fix the issue when we have a large buffer and we call `Peek` multiple times
	cachePeek []byte
}

// Len implements Reader.
func (b *UnsafeLinkBuffer) Len() int {
	l := atomic.LoadInt64(&b.length)
	return int(l)
}

// IsEmpty check if this LinkBuffer is empty.
func (b *UnsafeLinkBuffer) IsEmpty() (ok bool) {
	return b.Len() == 0
}

// ------------------------------------------ implement zero-copy reader ------------------------------------------

// Next implements Reader.
func (b *UnsafeLinkBuffer) Next(n int) (p []byte, err error) {
	if n <= 0 {
		return
	}
	// check whether enough or not.
	if b.Len() < n {
		return p, fmt.Errorf("link buffer next[%d] not enough", n)
	}
	b.recalLen(-n) // re-cal length

	// single node
	if b.isSingleNode(n) {
		return b.read.Next(n), nil
	}
	// multiple nodes
	var pIdx int
	if block1k < n && n <= mallocMax {
		p = malloc(n, n)
		b.caches = append(b.caches, p)
	} else {
		p = dirtmake.Bytes(n, n)
	}
	var l int
	for ack := n; ack > 0; ack = ack - l {
		l = b.read.Len()
		if l >= ack {
			pIdx += copy(p[pIdx:], b.read.Next(ack))
			break
		} else if l > 0 {
			pIdx += copy(p[pIdx:], b.read.Next(l))
		}
		b.read = b.read.next
	}
	_ = pIdx
	return p, nil
}

// Peek does not have an independent lifecycle, and there is no signal to
// indicate that Peek content can be released, so Peek will not introduce mcache for now.
func (b *UnsafeLinkBuffer) Peek(n int) (p []byte, err error) {
	if n <= 0 {
		return
	}
	// check whether enough or not.
	if b.Len() < n {
		return p, fmt.Errorf("link buffer peek[%d] not enough", n)
	}
	// single node
	if b.isSingleNode(n) {
		return b.read.Peek(n), nil
	}

	// multiple nodes

	// try to make use of the cap of b.cachePeek, if can't, free it.
	if b.cachePeek != nil && cap(b.cachePeek) < n {
		free(b.cachePeek)
		b.cachePeek = nil
	}
	if b.cachePeek == nil {
		b.cachePeek = malloc(0, n) // init with zero len, will append later
	}
	p = b.cachePeek
	if len(p) >= n {
		// in case we peek smaller than last time,
		// we can return cache data directly.
		// we will reset cachePeek when Next or Skip, no worries about stale data
		return p[:n], nil
	}

	// How it works >>>>>>
	// [ -------- node0 -------- ][ --------- node1 --------- ]  <- b.read
	// [ --------------- p --------------- ]
	//                                     ^ len(p)     ^ n here
	//                           ^ scanned
	// `scanned` var is the len of last nodes which we scanned and already copied to p
	// `len(p) - scanned` is the start pos of current node for p to copy from
	// `n - len(p)` is the len of bytes we're going to append to p
	// 		we copy `len(node1)` - `len(p) - scanned` bytes in case node1 doesn't have enough data
	for scanned, node := 0, b.read; len(p) < n; node = node.next {
		l := node.Len()
		if scanned+l <= len(p) { // already copied in p, skip
			scanned += l
			continue
		}
		start := len(p) - scanned // `start` must be smaller than l coz `scanned+l <= len(p)` is false
		copyn := n - len(p)
		if nodeLeftN := l - start; copyn > nodeLeftN {
			copyn = nodeLeftN
		}
		p = append(p, node.Peek(l)[start:start+copyn]...)
		scanned += l
	}
	b.cachePeek = p
	return p[:n], nil
}

// Skip implements Reader.
func (b *UnsafeLinkBuffer) Skip(n int) (err error) {
	if n <= 0 {
		return
	}
	// check whether enough or not.
	if b.Len() < n {
		return fmt.Errorf("link buffer skip[%d] not enough", n)
	}
	b.recalLen(-n) // re-cal length

	var l int
	for ack := n; ack > 0; ack = ack - l {
		l = b.read.Len()
		if l >= ack {
			b.read.off += ack
			break
		}
		b.read = b.read.next
	}
	return nil
}

// Release the node that has been read.
// b.flush == nil indicates that this LinkBuffer is created by LinkBuffer.Slice
func (b *UnsafeLinkBuffer) Release() (err error) {
	for b.read != b.flush && b.read.Len() == 0 {
		b.read = b.read.next
	}
	for b.head != b.read {
		node := b.head
		b.head = b.head.next
		node.Release()
	}
	for i := range b.caches {
		free(b.caches[i])
		b.caches[i] = nil
	}
	b.caches = b.caches[:0]
	if b.cachePeek != nil {
		free(b.cachePeek)
		b.cachePeek = nil
	}
	return nil
}

// ReadString implements Reader.
func (b *UnsafeLinkBuffer) ReadString(n int) (s string, err error) {
	if n <= 0 {
		return
	}
	// check whether enough or not.
	if b.Len() < n {
		return s, fmt.Errorf("link buffer read string[%d] not enough", n)
	}
	return unsafeSliceToString(b.readBinary(n)), nil
}

// ReadBinary implements Reader.
func (b *UnsafeLinkBuffer) ReadBinary(n int) (p []byte, err error) {
	if n <= 0 {
		return
	}
	// check whether enough or not.
	if b.Len() < n {
		return p, fmt.Errorf("link buffer read binary[%d] not enough", n)
	}
	return b.readBinary(n), nil
}

// readBinary cannot use mcache, because the memory allocated by readBinary will not be recycled.
func (b *UnsafeLinkBuffer) readBinary(n int) (p []byte) {
	b.recalLen(-n) // re-cal length

	// single node
	if b.isSingleNode(n) {
		// TODO: enable nocopy read mode when ensure no legacy depend on copy-read
		// we cannot nocopy read a readonly mode buffer, since readonly buffer's memory is not control by itself
		if !b.read.getMode(readonlyMask) {
			// if readBinary use no-copy mode, it will cause more memory used but get higher memory access efficiently
			// for example, if user's codec need to decode 10 strings and each have 100 bytes, here could help the codec
			// no need to malloc 10 times and the string slice could have the compact memory allocation.
			if b.read.getMode(nocopyReadMask) {
				return b.read.Next(n)
			}
			if featureAlwaysNoCopyRead && n >= minReuseBytes {
				b.read.setMode(nocopyReadMask, true)
				return b.read.Next(n)
			}
		}
		// if the underlying buffer too large, we shouldn't use no-copy mode
		p = dirtmake.Bytes(n, n)
		copy(p, b.read.Next(n))
		return p
	}
	p = dirtmake.Bytes(n, n)
	// multiple nodes
	var pIdx int
	var l int
	for ack := n; ack > 0; ack = ack - l {
		l = b.read.Len()
		if l >= ack {
			pIdx += copy(p[pIdx:], b.read.Next(ack))
			break
		} else if l > 0 {
			pIdx += copy(p[pIdx:], b.read.Next(l))
		}
		b.read = b.read.next
	}
	_ = pIdx
	return p
}

// ReadByte implements Reader.
func (b *UnsafeLinkBuffer) ReadByte() (p byte, err error) {
	// check whether enough or not.
	if b.Len() < 1 {
		return p, errors.New("link buffer read byte is empty")
	}
	b.recalLen(-1) // re-cal length
	for {
		if b.read.Len() >= 1 {
			return b.read.Next(1)[0], nil
		}
		b.read = b.read.next
	}
}

// Until returns a slice ends with the delim in the buffer.
func (b *UnsafeLinkBuffer) Until(delim byte) (line []byte, err error) {
	n := b.indexByte(delim, 0)
	if n < 0 {
		return nil, untilErr
	}
	return b.Next(n + 1)
}

// Slice returns a new LinkBuffer, which is a zero-copy slice of this LinkBuffer,
// and only holds the ability of Reader.
//
// Slice will automatically execute a Release.
func (b *UnsafeLinkBuffer) Slice(n int) (r Reader, err error) {
	if n <= 0 {
		return NewLinkBuffer(0), nil
	}
	// check whether enough or not.
	if b.Len() < n {
		return r, fmt.Errorf("link buffer readv[%d] not enough", n)
	}
	b.recalLen(-n) // re-cal length

	// just use for range
	p := new(LinkBuffer)
	p.length = int64(n)

	defer func() {
		// set to read-only
		p.flush = p.flush.next
		p.write = p.flush
	}()

	// single node
	if b.isSingleNode(n) {
		node := b.read.Refer(n)
		p.head, p.read, p.flush = node, node, node
		return p, nil
	}
	// multiple nodes
	l := b.read.Len()
	node := b.read.Refer(l)
	b.read = b.read.next

	p.head, p.read, p.flush = node, node, node
	for ack := n - l; ack > 0; ack = ack - l {
		l = b.read.Len()
		if l >= ack {
			p.flush.next = b.read.Refer(ack)
			p.flush = p.flush.next
			break
		} else if l > 0 {
			p.flush.next = b.read.Refer(l)
			p.flush = p.flush.next
		}
		b.read = b.read.next
	}
	return p, b.Release()
}

// ------------------------------------------ implement zero-copy writer ------------------------------------------

// Malloc pre-allocates memory, which is not readable, and becomes readable data after submission(e.g. Flush).
func (b *UnsafeLinkBuffer) Malloc(n int) (buf []byte, err error) {
	if n <= 0 {
		return
	}
	b.mallocSize += n
	b.growth(n)
	return b.write.Malloc(n), nil
}

// MallocLen implements Writer.
func (b *UnsafeLinkBuffer) MallocLen() (length int) {
	return b.mallocSize
}

// MallocAck will keep the first n malloc bytes and discard the rest.
func (b *UnsafeLinkBuffer) MallocAck(n int) (err error) {
	if n < 0 {
		return fmt.Errorf("link buffer malloc ack[%d] invalid", n)
	}
	b.mallocSize = n
	b.write = b.flush

	var l int
	for ack := n; ack > 0; ack = ack - l {
		l = b.write.malloc - len(b.write.buf)
		if l >= ack {
			b.write.malloc = ack + len(b.write.buf)
			break
		}
		b.write = b.write.next
	}
	// discard the rest
	for node := b.write.next; node != nil; node = node.next {
		node.off, node.malloc, node.refer, node.buf = 0, 0, 1, node.buf[:0]
	}
	return nil
}

// Flush will submit all malloc data and must confirm that the allocated bytes have been correctly assigned.
func (b *UnsafeLinkBuffer) Flush() (err error) {
	b.mallocSize = 0
	// FIXME: The tail node must not be larger than 8KB to prevent Out Of Memory.
	if cap(b.write.buf) > pagesize {
		b.write.next = newLinkBufferNode(0)
		b.write = b.write.next
	}
	var n int
	for node := b.flush; node != b.write.next; node = node.next {
		delta := node.malloc - len(node.buf)
		if delta > 0 {
			n += delta
			node.buf = node.buf[:node.malloc]
		}
	}
	b.flush = b.write
	// re-cal length
	b.recalLen(n)
	return nil
}

// Append implements Writer.
func (b *UnsafeLinkBuffer) Append(w Writer) (err error) {
	buf, ok := w.(*LinkBuffer)
	if !ok {
		return errors.New("unsupported writer which is not LinkBuffer")
	}
	return b.WriteBuffer(buf)
}

// WriteBuffer will not submit(e.g. Flush) data to ensure normal use of MallocLen.
// you must actively submit before read the data.
// The argument buf can't be used after calling WriteBuffer. (set it to nil)
func (b *UnsafeLinkBuffer) WriteBuffer(buf *LinkBuffer) (err error) {
	if buf == nil {
		return
	}
	bufLen, bufMallocLen := buf.Len(), buf.MallocLen()
	if bufLen+bufMallocLen <= 0 {
		return nil
	}
	b.write.next = buf.read
	b.write = buf.write

	// close buf, prevents reuse.
	for buf.head != buf.read {
		nd := buf.head
		buf.head = buf.head.next
		nd.Release()
	}
	for buf.write = buf.write.next; buf.write != nil; {
		nd := buf.write
		buf.write = buf.write.next
		nd.Release()
	}
	buf.length, buf.mallocSize, buf.head, buf.read, buf.flush, buf.write = 0, 0, nil, nil, nil, nil

	// DON'T MODIFY THE CODE BELOW UNLESS YOU KNOW WHAT YOU ARE DOING !
	//
	// You may encounter a chain of bugs and not be able to
	// find out within a week that they are caused by modifications here.
	//
	// After release buf, continue to adjust b.
	b.write.next = nil
	if bufLen > 0 {
		b.recalLen(bufLen)
	}
	b.mallocSize += bufMallocLen
	return nil
}

// WriteString implements Writer.
func (b *UnsafeLinkBuffer) WriteString(s string) (n int, err error) {
	if len(s) == 0 {
		return
	}
	buf := unsafeStringToSlice(s)
	return b.WriteBinary(buf)
}

// WriteBinary implements Writer.
func (b *UnsafeLinkBuffer) WriteBinary(p []byte) (n int, err error) {
	n = len(p)
	if n == 0 {
		return
	}
	b.mallocSize += n

	// TODO: Verify that all nocopy is possible under mcache.
	if n > BinaryInplaceThreshold {
		// expand buffer directly with nocopy
		b.write.next = newLinkBufferNode(0)
		b.write = b.write.next
		b.write.buf, b.write.malloc = p[:0], n
		return n, nil
	}
	// here will copy
	b.growth(n)
	buf := b.write.Malloc(n)
	return copy(buf, p), nil
}

// WriteDirect cannot be mixed with WriteString or WriteBinary functions.
func (b *UnsafeLinkBuffer) WriteDirect(extra []byte, remainLen int) error {
	n := len(extra)
	if n == 0 || remainLen < 0 {
		return nil
	}
	// find origin
	origin := b.flush
	malloc := b.mallocSize - remainLen // calculate the remaining malloc length
	for t := origin.malloc - len(origin.buf); t < malloc; t = origin.malloc - len(origin.buf) {
		malloc -= t
		origin = origin.next
	}
	// Add the buf length of the original node
	// `malloc` is the origin buffer offset that already malloced, the extra buffer should be inserted after that offset.
	malloc += len(origin.buf)

	// Create dataNode and newNode and insert them into the chain
	// dataNode wrap the user buffer extra, and newNode wrap the origin left netpoll buffer
	// - originNode{buf=origin, off=0, malloc=malloc, readonly=true} : non-reusable
	// - dataNode{buf=extra, off=0, malloc=len(extra), readonly=true} : non-reusable
	// - newNode{buf=origin, off=malloc, malloc=origin.malloc, readonly=false} : reusable
	dataNode := newLinkBufferNode(0) // zero node will be set by readonly mode
	dataNode.buf, dataNode.malloc = extra[:0], n

	if remainLen > 0 {
		// split a single buffer node to originNode and newNode
		newNode := newLinkBufferNode(0)
		newNode.off = malloc
		newNode.buf = origin.buf[:malloc]
		newNode.malloc = origin.malloc
		newNode.setMode(readonlyMask, false)
		origin.malloc = malloc
		origin.setMode(readonlyMask, true)

		// link nodes
		dataNode.next = newNode
		newNode.next = origin.next
		origin.next = dataNode
	} else {
		// link nodes
		dataNode.next = origin.next
		origin.next = dataNode
	}

	// adjust b.write
	for b.write.next != nil {
		b.write = b.write.next
	}

	b.mallocSize += n
	return nil
}

// WriteByte implements Writer.
func (b *UnsafeLinkBuffer) WriteByte(p byte) (err error) {
	dst, err := b.Malloc(1)
	if len(dst) == 1 {
		dst[0] = p
	}
	return err
}

// Close will recycle all buffer.
func (b *UnsafeLinkBuffer) Close() (err error) {
	atomic.StoreInt64(&b.length, 0)
	b.mallocSize = 0
	// just release all
	b.Release()
	for node := b.head; node != nil; {
		nd := node
		node = node.next
		nd.Release()
	}
	b.head, b.read, b.flush, b.write = nil, nil, nil, nil
	return nil
}

// ------------------------------------------ implement connection interface ------------------------------------------

// Bytes returns all the readable bytes of this LinkBuffer.
func (b *UnsafeLinkBuffer) Bytes() []byte {
	node, flush := b.read, b.flush
	if node == flush {
		return node.buf[node.off:]
	}
	n := 0
	p := dirtmake.Bytes(b.Len(), b.Len())
	for ; node != flush; node = node.next {
		if node.Len() > 0 {
			n += copy(p[n:], node.buf[node.off:])
		}
	}
	n += copy(p[n:], flush.buf[flush.off:])
	return p[:n]
}

// GetBytes will read and fill the slice p as much as possible.
// If p is not passed, return all readable bytes.
func (b *UnsafeLinkBuffer) GetBytes(p [][]byte) (vs [][]byte) {
	node, flush := b.read, b.flush
	if len(p) == 0 {
		n := 0
		for ; node != flush; node = node.next {
			n++
		}
		node = b.read
		p = make([][]byte, n)
	}
	var i int
	for i = 0; node != flush && i < len(p); node = node.next {
		if node.Len() > 0 {
			p[i] = node.buf[node.off:]
			i++
		}
	}
	if i < len(p) {
		p[i] = flush.buf[flush.off:]
		i++
	}
	return p[:i]
}

// book will grow and malloc buffer to hold data.
//
// bookSize: The size of data that can be read at once.
// maxSize: The maximum size of data between two Release(). In some cases, this can
//
//	guarantee all data allocated in one node to reduce copy.
func (b *UnsafeLinkBuffer) book(bookSize, maxSize int) (p []byte) {
	l := cap(b.write.buf) - b.write.malloc
	// grow linkBuffer
	if l == 0 {
		l = maxSize
		b.write.next = newLinkBufferNode(maxSize)
		b.write = b.write.next
	}
	if l > bookSize {
		l = bookSize
	}
	return b.write.Malloc(l)
}

// bookAck will ack the first n malloc bytes and discard the rest.
//
// length: The size of data in inputBuffer. It is used to calculate the maxSize
func (b *UnsafeLinkBuffer) bookAck(n int) (length int, err error) {
	b.write.malloc = n + len(b.write.buf)
	b.write.buf = b.write.buf[:b.write.malloc]
	b.flush = b.write

	// re-cal length
	length = b.recalLen(n)
	return length, nil
}

// calcMaxSize will calculate the data size between two Release()
func (b *UnsafeLinkBuffer) calcMaxSize() (sum int) {
	for node := b.head; node != b.read; node = node.next {
		sum += len(node.buf)
	}
	sum += len(b.read.buf)
	return sum
}

// resetTail will reset tail node or add an empty tail node to
// guarantee the tail node is not larger than 8KB
func (b *UnsafeLinkBuffer) resetTail(maxSize int) {
	if maxSize <= pagesize {
		// no need to reset a small buffer tail node
		return
	}
	// set nil tail
	b.write.next = newLinkBufferNode(0)
	b.write = b.write.next
	b.flush = b.write
}

// indexByte returns the index of the first instance of c in buffer, or -1 if c is not present in buffer.
func (b *UnsafeLinkBuffer) indexByte(c byte, skip int) int {
	size := b.Len()
	if skip >= size {
		return -1
	}
	var unread, n, l int
	node := b.read
	for unread = size; unread > 0; unread -= n {
		l = node.Len()
		if l >= unread { // last node
			n = unread
		} else { // read full node
			n = l
		}

		// skip current node
		if skip >= n {
			skip -= n
			node = node.next
			continue
		}
		i := bytes.IndexByte(node.Peek(n)[skip:], c)
		if i >= 0 {
			return (size - unread) + skip + i // past_read + skip_read + index
		}
		skip = 0 // no skip bytes
		node = node.next
	}
	return -1
}

// ------------------------------------------ private function ------------------------------------------

// recalLen re-calculate the length
func (b *UnsafeLinkBuffer) recalLen(delta int) (length int) {
	if delta < 0 && len(b.cachePeek) > 0 {
		// b.cachePeek will contain stale data if we read out even a single byte from buffer,
		// so we need to reset it or the next Peek call will return invalid bytes.
		b.cachePeek = b.cachePeek[:0]
	}
	return int(atomic.AddInt64(&b.length, int64(delta)))
}

// growth directly create the next node, when b.write is not enough.
func (b *UnsafeLinkBuffer) growth(n int) {
	if n <= 0 {
		return
	}
	// the memory of readonly node if not malloc by us so should skip them
	for b.write.getMode(readonlyMask) || cap(b.write.buf)-b.write.malloc < n {
		if b.write.next == nil {
			b.write.next = newLinkBufferNode(n)
			b.write = b.write.next
			return
		}
		b.write = b.write.next
	}
}

// isSingleNode determines whether reading needs to cross nodes.
// isSingleNode will move b.read to latest non-empty node if there is a zero-size node
// Must require b.Len() > 0
func (b *UnsafeLinkBuffer) isSingleNode(readN int) (single bool) {
	if readN <= 0 {
		return true
	}
	l := b.read.Len()
	for l == 0 && b.read != b.flush {
		b.read = b.read.next
		l = b.read.Len()
	}
	return l >= readN
}

// memorySize return the real memory size in bytes the LinkBuffer occupied
func (b *LinkBuffer) memorySize() (bytes int) {
	for node := b.head; node != nil; node = node.next {
		bytes += cap(node.buf)
	}
	for _, c := range b.caches {
		bytes += cap(c)
	}
	bytes += cap(b.cachePeek)
	return bytes
}

// ------------------------------------------ implement link node ------------------------------------------

// newLinkBufferNode create or reuse linkBufferNode.
// Nodes with size <= 0 are marked as readonly, which means the node.buf is not allocated by this mcache.
func newLinkBufferNode(size int) *linkBufferNode {
	node := linkedPool.Get().(*linkBufferNode)
	// reset node offset
	node.off, node.malloc, node.refer, node.mode = 0, 0, 1, defaultLinkBufferMode
	if size <= 0 {
		node.setMode(readonlyMask, true)
		return node
	}
	if size < LinkBufferCap {
		size = LinkBufferCap
	}
	node.buf = malloc(0, size)
	return node
}

var linkedPool = sync.Pool{
	New: func() interface{} {
		return &linkBufferNode{
			refer: 1, // 自带 1 引用
		}
	},
}

type linkBufferNode struct {
	buf    []byte          // buffer
	off    int             // read-offset
	malloc int             // write-offset
	refer  int32           // reference count
	mode   uint8           // mode store all bool bit status
	origin *linkBufferNode // the root node of the extends
	next   *linkBufferNode // the next node of the linked buffer
}

func (node *linkBufferNode) Len() (l int) {
	return len(node.buf) - node.off
}

func (node *linkBufferNode) IsEmpty() (ok bool) {
	return node.off == len(node.buf)
}

func (node *linkBufferNode) Reset() {
	if node.origin != nil || atomic.LoadInt32(&node.refer) != 1 {
		return
	}
	node.off, node.malloc = 0, 0
	node.buf = node.buf[:0]
}

func (node *linkBufferNode) Next(n int) (p []byte) {
	off := node.off
	node.off += n
	return node.buf[off:node.off:node.off]
}

func (node *linkBufferNode) Peek(n int) (p []byte) {
	return node.buf[node.off : node.off+n : node.off+n]
}

func (node *linkBufferNode) Malloc(n int) (buf []byte) {
	malloc := node.malloc
	node.malloc += n
	return node.buf[malloc:node.malloc:node.malloc]
}

// Refer holds a reference count at the same time as Next, and releases the real buffer after Release.
// The node obtained by Refer is read-only.
func (node *linkBufferNode) Refer(n int) (p *linkBufferNode) {
	p = newLinkBufferNode(0)
	p.buf = node.Next(n)

	if node.origin != nil {
		p.origin = node.origin
	} else {
		p.origin = node
	}
	atomic.AddInt32(&p.origin.refer, 1)
	return p
}

// Release consists of two parts:
// 1. reduce the reference count of itself and origin.
// 2. recycle the buf when the reference count is 0.
func (node *linkBufferNode) Release() (err error) {
	if node.origin != nil {
		node.origin.Release()
	}
	// release self
	if atomic.AddInt32(&node.refer, -1) == 0 {
		// readonly nodes cannot recycle node.buf, other node.buf are recycled to mcache.
		if node.reusable() {
			free(node.buf)
		}
		node.buf, node.origin, node.next = nil, nil, nil
		linkedPool.Put(node)
	}
	return nil
}

func (node *linkBufferNode) getMode(mask uint8) bool {
	return (node.mode & mask) > 0
}

func (node *linkBufferNode) setMode(mask uint8, enable bool) {
	if enable {
		node.mode = node.mode | mask
	} else {
		node.mode = node.mode &^ mask
	}
}

// only non-readonly and copied-read node should be reusable
func (node *linkBufferNode) reusable() bool {
	return node.mode&(readonlyMask|nocopyReadMask) == 0
}