1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
|
/**
* Copyright (C) 2020-present MongoDB, Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the Server Side Public License, version 1,
* as published by MongoDB, Inc.
*
* This program is distributed in the hope that it will be useful,
* but WITHOUT ANY WARRANTY; without even the implied warranty of
* MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
* Server Side Public License for more details.
*
* You should have received a copy of the Server Side Public License
* along with this program. If not, see
* <http://www.mongodb.com/licensing/server-side-public-license>.
*
* As a special exception, the copyright holders give permission to link the
* code of portions of this program with the OpenSSL library under certain
* conditions as described in each individual source file and distribute
* linked combinations including the program with the OpenSSL library. You
* must comply with the Server Side Public License in all respects for
* all of the code used other than as permitted herein. If you modify file(s)
* with this exception, you may extend this exception to your version of the
* file(s), but you are not obligated to do so. If you do not wish to do so,
* delete this exception statement from your version. If you delete this
* exception statement from all source files in the program, then also delete
* it in the license file.
*/
#pragma once
#include "mongo/util/shared_buffer.h"
#include <functional>
namespace mongo {
/**
* Immutable view of a fragment of a ref-counted buffer.
* Shares reference count with the underlying buffer.
*/
class SharedBufferFragment {
public:
SharedBufferFragment() : _offset(0), _size(0) {}
explicit SharedBufferFragment(SharedBuffer buffer, size_t size)
: _buffer(std::move(buffer)), _offset(0), _size(size) {}
explicit SharedBufferFragment(SharedBuffer buffer, ptrdiff_t offset, size_t size)
: _buffer(std::move(buffer)), _offset(offset), _size(size) {}
void swap(SharedBufferFragment& other) {
_buffer.swap(other._buffer);
std::swap(_offset, other._offset);
std::swap(_size, other._size);
}
const char* get() const {
return _buffer.get() + _offset;
}
size_t size() const {
return _size;
}
explicit operator bool() const {
return (bool)_buffer;
}
/**
* Returns true if this object has exclusive access to the underlying buffer.
* (That is, reference count == 1).
*/
bool isShared() const {
return _buffer.isShared();
}
/**
* Returns the allocation size of the underlying buffer.
*/
size_t underlyingCapacity() const {
return _buffer.capacity();
}
private:
SharedBuffer _buffer;
ptrdiff_t _offset;
size_t _size;
};
/**
* Builder of SharedBufferFragment where multiple fragments are using different parts of the same
* underlying buffer. Can only build one fragment at a time
*/
class SharedBufferFragmentBuilder {
public:
static constexpr size_t kDefaultMaxBlockSize = 1024 * 1024; // 1MB
using GrowStrategy = std::function<size_t(size_t)>;
SharedBufferFragmentBuilder(
size_t blockSize, GrowStrategy growStrategy = DoubleGrowStrategy(kDefaultMaxBlockSize))
: _offset(0), _blockSize(blockSize), _growStrategy(growStrategy) {}
struct ConstantGrowStrategy {
size_t operator()(size_t current) const {
return current;
}
};
struct DoubleGrowStrategy {
DoubleGrowStrategy(size_t maxBlockSize) : _maxBlockSize(maxBlockSize) {}
size_t operator()(size_t current) const {
return std::min(current * 2, _maxBlockSize);
}
private:
size_t _maxBlockSize;
};
// Starts building a memory fragment with at least 'initialSize' capacity.
// May only be called if we are not currently building a fragment
SharedBufferFragmentBuilder& start(size_t initialSize) {
invariant(!_inUse);
if (_buffer.capacity() < (_offset + initialSize)) {
// If capacity is 0, then this is our initial allocation and we should not use the grow
// strategy
if (_buffer.capacity() > 0)
_blockSize = _growStrategy(_blockSize);
size_t allocSize = std::max(_blockSize, initialSize);
_buffer = SharedBuffer::allocate(allocSize);
_offset = 0;
}
_inUse = true;
return *this;
}
// Grows the currently building memory fragment so it will fit at least 'size' bytes.
// May only be called when building a fragment
void grow(size_t size) {
invariant(_inUse);
auto currentCapacity = capacity();
if (currentCapacity < size) {
_blockSize = _growStrategy(_blockSize);
size_t allocSize = std::max(_blockSize, size);
// If nothing else is using the internal buffer it would be safe to use realloc. But as
// this potentially is a large buffer realloc would need copy all of it as it doesn't
// know how much is actually used. So we create a new buffer in all cases and reset the
// offset to 0. We only need to copy the memory of the fragment we are currently
// building.
auto newBuffer = SharedBuffer::allocate(allocSize);
if (_buffer)
memcpy(newBuffer.get(), _buffer.get() + _offset, currentCapacity);
_buffer = std::move(newBuffer);
_offset = 0;
}
}
// Finishes building a memory fragment. 'totalSize' should indicate total of bytes used.
// Returns a reference counted memory fragment
// May only be called when building a fragment and will put the builder back into a 'not
// building' state.
SharedBufferFragment finish(size_t totalSize) {
invariant(_inUse);
SharedBufferFragment fragment(_buffer, _offset, totalSize);
_offset += totalSize;
_inUse = false;
return fragment;
}
// Discards the memory fragment currently building and puts the builder back into a 'not
// building' state. May only be called when building a fragment
void discard() {
invariant(_inUse);
_inUse = false;
}
// Returns the available capacity that may be used for building a memory fragment.
// If more capacity is needed the user needs to call grow()
size_t capacity() const {
return _buffer.capacity() - _offset;
}
// Returns the beginning of the memory fragment we are currently building
// May only be called when building a fragment
char* get() const {
invariant(_inUse);
return _buffer.get() + _offset;
}
// Returns whether or not a memory fragment is currently being built.
bool building() const {
return _inUse;
}
private:
SharedBuffer _buffer;
ptrdiff_t _offset;
size_t _blockSize;
GrowStrategy _growStrategy;
bool _inUse{false};
};
} // namespace mongo
|
