1// Copyright 2014 The Go Authors. All rights reserved.
2// Use of this source code is governed by a BSD-style
3// license that can be found in the LICENSE file.
4
5package http2
6
7import (
8	"errors"
9	"fmt"
10	"sync"
11)
12
13// Buffer chunks are allocated from a pool to reduce pressure on GC.
14// The maximum wasted space per dataBuffer is 2x the largest size class,
15// which happens when the dataBuffer has multiple chunks and there is
16// one unread byte in both the first and last chunks. We use a few size
17// classes to minimize overheads for servers that typically receive very
18// small request bodies.
19//
20// TODO: Benchmark to determine if the pools are necessary. The GC may have
21// improved enough that we can instead allocate chunks like this:
22// make([]byte, max(16<<10, expectedBytesRemaining))
23var (
24	dataChunkSizeClasses = []int{
25		1 << 10,
26		2 << 10,
27		4 << 10,
28		8 << 10,
29		16 << 10,
30	}
31	dataChunkPools = [...]sync.Pool{
32		{New: func() interface{} { return make([]byte, 1<<10) }},
33		{New: func() interface{} { return make([]byte, 2<<10) }},
34		{New: func() interface{} { return make([]byte, 4<<10) }},
35		{New: func() interface{} { return make([]byte, 8<<10) }},
36		{New: func() interface{} { return make([]byte, 16<<10) }},
37	}
38)
39
40func getDataBufferChunk(size int64) []byte {
41	i := 0
42	for ; i < len(dataChunkSizeClasses)-1; i++ {
43		if size <= int64(dataChunkSizeClasses[i]) {
44			break
45		}
46	}
47	return dataChunkPools[i].Get().([]byte)
48}
49
50func putDataBufferChunk(p []byte) {
51	for i, n := range dataChunkSizeClasses {
52		if len(p) == n {
53			dataChunkPools[i].Put(p)
54			return
55		}
56	}
57	panic(fmt.Sprintf("unexpected buffer len=%v", len(p)))
58}
59
60// dataBuffer is an io.ReadWriter backed by a list of data chunks.
61// Each dataBuffer is used to read DATA frames on a single stream.
62// The buffer is divided into chunks so the server can limit the
63// total memory used by a single connection without limiting the
64// request body size on any single stream.
65type dataBuffer struct {
66	chunks   [][]byte
67	r        int   // next byte to read is chunks[0][r]
68	w        int   // next byte to write is chunks[len(chunks)-1][w]
69	size     int   // total buffered bytes
70	expected int64 // we expect at least this many bytes in future Write calls (ignored if <= 0)
71}
72
73var errReadEmpty = errors.New("read from empty dataBuffer")
74
75// Read copies bytes from the buffer into p.
76// It is an error to read when no data is available.
77func (b *dataBuffer) Read(p []byte) (int, error) {
78	if b.size == 0 {
79		return 0, errReadEmpty
80	}
81	var ntotal int
82	for len(p) > 0 && b.size > 0 {
83		readFrom := b.bytesFromFirstChunk()
84		n := copy(p, readFrom)
85		p = p[n:]
86		ntotal += n
87		b.r += n
88		b.size -= n
89		// If the first chunk has been consumed, advance to the next chunk.
90		if b.r == len(b.chunks[0]) {
91			putDataBufferChunk(b.chunks[0])
92			end := len(b.chunks) - 1
93			copy(b.chunks[:end], b.chunks[1:])
94			b.chunks[end] = nil
95			b.chunks = b.chunks[:end]
96			b.r = 0
97		}
98	}
99	return ntotal, nil
100}
101
102func (b *dataBuffer) bytesFromFirstChunk() []byte {
103	if len(b.chunks) == 1 {
104		return b.chunks[0][b.r:b.w]
105	}
106	return b.chunks[0][b.r:]
107}
108
109// Len returns the number of bytes of the unread portion of the buffer.
110func (b *dataBuffer) Len() int {
111	return b.size
112}
113
114// Write appends p to the buffer.
115func (b *dataBuffer) Write(p []byte) (int, error) {
116	ntotal := len(p)
117	for len(p) > 0 {
118		// If the last chunk is empty, allocate a new chunk. Try to allocate
119		// enough to fully copy p plus any additional bytes we expect to
120		// receive. However, this may allocate less than len(p).
121		want := int64(len(p))
122		if b.expected > want {
123			want = b.expected
124		}
125		chunk := b.lastChunkOrAlloc(want)
126		n := copy(chunk[b.w:], p)
127		p = p[n:]
128		b.w += n
129		b.size += n
130		b.expected -= int64(n)
131	}
132	return ntotal, nil
133}
134
135func (b *dataBuffer) lastChunkOrAlloc(want int64) []byte {
136	if len(b.chunks) != 0 {
137		last := b.chunks[len(b.chunks)-1]
138		if b.w < len(last) {
139			return last
140		}
141	}
142	chunk := getDataBufferChunk(want)
143	b.chunks = append(b.chunks, chunk)
144	b.w = 0
145	return chunk
146}
147