/
shared_buffer.h
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/
shared_buffer.h
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/**
* Copyright (C) 2018-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 <algorithm>
#include <boost/intrusive_ptr.hpp>
#include <boost/smart_ptr.hpp>
#include <boost/smart_ptr/intrusive_ptr.hpp>
#include <cstdint>
#include <cstdlib>
#include <cstring>
#include <new>
#include <type_traits>
#include <utility>
#include "mongo/base/data_view.h"
#include "mongo/base/static_assert.h"
#include "mongo/platform/atomic_word.h"
#include "mongo/util/allocator.h"
#include "mongo/util/assert_util.h"
namespace mongo {
class UniqueBuffer;
/**
* A mutable, ref-counted buffer.
*/
class SharedBuffer {
public:
SharedBuffer() = default;
explicit SharedBuffer(UniqueBuffer&& uniqueBuf);
void swap(SharedBuffer& other) {
_holder.swap(other._holder);
}
static SharedBuffer allocate(size_t bytes) {
return takeOwnership(mongoMalloc(kHolderSize + bytes), bytes);
}
/**
* Resizes the buffer, copying the current contents.
*
* Like ::realloc() this can be called on a null SharedBuffer.
*
* This method is illegal to call if any other SharedBuffer instances share this buffer since
* they wouldn't be updated and would still try to delete the original buffer.
*/
void realloc(size_t size) {
invariant(!_holder || !_holder->isShared());
const size_t realSize = size + kHolderSize;
void* newPtr = mongoRealloc(_holder.get(), realSize);
// Get newPtr into _holder with a ref-count of 1 without touching the current pointee of
// _holder which is now invalid.
auto tmp = SharedBuffer::takeOwnership(newPtr, size);
_holder.detach();
_holder = std::move(tmp._holder);
}
/**
* Resizes the buffer, copying the current contents. If shared, an exclusive copy is made.
*/
void reallocOrCopy(size_t size) {
if (isShared()) {
auto tmp = SharedBuffer::allocate(size);
memcpy(tmp._holder->data(),
_holder->data(),
std::min(size, static_cast<size_t>(_holder->_capacity)));
swap(tmp);
} else if (_holder) {
realloc(size);
} else {
*this = SharedBuffer::allocate(size);
}
}
char* get() const {
return _holder ? _holder->data() : nullptr;
}
explicit operator bool() const {
return bool(_holder);
}
/**
* Returns false if this object has exclusive access to the underlying buffer.
* (That is, reference count == 1).
*/
bool isShared() const {
return _holder && _holder->isShared();
}
/**
* Returns the allocation size of the underlying buffer.
* Users of this type must maintain the "used" size separately.
*/
size_t capacity() const {
return _holder ? _holder->_capacity : 0;
}
private:
class Holder {
public:
explicit Holder(unsigned initial, size_t capacity)
: _refCount(initial), _capacity(capacity) {
invariant(capacity == _capacity);
}
// these are called automatically by boost::intrusive_ptr
friend void intrusive_ptr_add_ref(Holder* h) {
h->_refCount.fetchAndAdd(1);
}
friend void intrusive_ptr_release(Holder* h) {
if (h->_refCount.subtractAndFetch(1) == 0) {
// We placement new'ed a Holder in takeOwnership above,
// so we must destroy the object here.
h->~Holder();
free(h);
}
}
char* data() {
return reinterpret_cast<char*>(this + 1);
}
const char* data() const {
return reinterpret_cast<const char*>(this + 1);
}
bool isShared() const {
return _refCount.load() > 1;
}
AtomicWord<unsigned> _refCount;
uint32_t _capacity;
};
explicit SharedBuffer(Holder* holder) : _holder(holder, /*add_ref=*/false) {
// NOTE: The 'false' above is because we have already initialized the Holder with a
// refcount of '1' in takeOwnership below. This avoids an atomic increment.
}
/**
* Given a pointer to a region of un-owned data, prefixed by sufficient space for a
* SharedBuffer::Holder object, return an SharedBuffer that owns the memory.
*
* This class will call free(holderPrefixedData), so it must have been allocated in a way
* that makes that valid.
*/
static SharedBuffer takeOwnership(void* holderPrefixedData, size_t capacity) {
// Initialize the refcount to 1 so we don't need to increment it in the constructor
// (see private Holder* constructor above).
//
// TODO: Should dassert alignment of holderPrefixedData here if possible.
return SharedBuffer(new (holderPrefixedData) Holder(1U, capacity));
}
boost::intrusive_ptr<Holder> _holder;
public:
// Declared here so definition of 'Holder' is available.
static constexpr size_t kHolderSize = sizeof(Holder);
};
MONGO_STATIC_ASSERT(std::is_nothrow_move_constructible_v<SharedBuffer>);
MONGO_STATIC_ASSERT(std::is_nothrow_move_assignable_v<SharedBuffer>);
inline void swap(SharedBuffer& one, SharedBuffer& two) {
one.swap(two);
}
/**
* A constant view into a ref-counted buffer.
*
* Use SharedBuffer to allocate since allocating a const buffer is useless.
*/
class ConstSharedBuffer {
public:
ConstSharedBuffer() = default;
/*implicit*/ ConstSharedBuffer(SharedBuffer source) : _buffer(std::move(source)) {}
void swap(ConstSharedBuffer& other) {
_buffer.swap(other._buffer);
}
const char* get() const {
return _buffer.get();
}
explicit operator bool() const {
return bool(_buffer);
}
bool isShared() const {
return _buffer.isShared();
}
size_t capacity() const {
return _buffer.capacity();
}
/**
* Converts to a mutable SharedBuffer.
* This is only legal to call if you have exclusive access to the underlying buffer.
*/
SharedBuffer constCast() && {
invariant(!isShared());
return std::move(_buffer);
}
// The buffer holder size for 'ConstSharedBuffer' is the same as the one for 'SharedBuffer'
static constexpr size_t kHolderSize = SharedBuffer::kHolderSize;
private:
SharedBuffer _buffer;
};
inline void swap(ConstSharedBuffer& one, ConstSharedBuffer& two) {
one.swap(two);
}
/**
* A uniquely owned buffer. Has the same memory layout as SharedBuffer so that it
* can be easily converted into a SharedBuffer.
*
* Layout:
* | <size (4 bytes)> <unused (4 bytes)> | <data> |
*
* When converting to SharedBuffer, the entire prefix region is turned into a Holder.
*/
class UniqueBuffer {
public:
static UniqueBuffer allocate(uint32_t sz) {
return UniqueBuffer(mongoMalloc(SharedBuffer::kHolderSize + sz), sz);
}
/**
* Given memory which was released from a UniqueBuffer using the release() method,
* returns a UniqueBuffer owning that memory.
*/
static UniqueBuffer reclaim(char* data) {
return UniqueBuffer(data - SharedBuffer::kHolderSize);
}
UniqueBuffer() = default;
UniqueBuffer(const UniqueBuffer&) = delete;
UniqueBuffer(UniqueBuffer&& other) : _data(other._data) {
other._data = nullptr;
}
~UniqueBuffer() {
free(_data);
}
UniqueBuffer& operator=(const UniqueBuffer&) = delete;
UniqueBuffer& operator=(UniqueBuffer&& other) {
UniqueBuffer temp(std::move(other));
swap(*this, temp);
return *this;
}
friend void swap(UniqueBuffer& lhs, UniqueBuffer& rhs) {
using std::swap;
swap(lhs._data, rhs._data);
}
void realloc(uint32_t size) {
size_t realSize = size + SharedBuffer::kHolderSize;
_data = reinterpret_cast<char*>(mongoRealloc(_data, realSize));
DataView(_data).write<uint32_t>(size);
}
char* get() const {
return _data ? _data + SharedBuffer::kHolderSize : nullptr;
}
explicit operator bool() const {
return _data != nullptr;
}
size_t capacity() const {
return _data ? ConstDataView(_data).read<uint32_t>() : 0;
}
/**
* Releases the buffer to the caller. The caller may not free the buffer themselves,
* and must eventually turn it back into a UniqueBuffer using the reclaim() method.
*/
char* release() {
auto ret = _data;
_data = nullptr;
return ret + SharedBuffer::kHolderSize;
}
// The buffer holder size for 'UniqueBuffer' is the same as the one for 'SharedBuffer'
static constexpr size_t kHolderSize = SharedBuffer::kHolderSize;
private:
friend class SharedBuffer;
// Assumes the size has already been initialized.
UniqueBuffer(void* buffer) : _data(static_cast<char*>(buffer)) {}
UniqueBuffer(void* buffer, uint32_t sz) : _data(static_cast<char*>(buffer)) {
DataView(_data).write<uint32_t>(sz);
}
char* _data = nullptr;
};
inline SharedBuffer::SharedBuffer(UniqueBuffer&& other) {
*this = takeOwnership(other._data, other.capacity());
other._data = nullptr;
}
} // namespace mongo