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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 <boost/intrusive_ptr.hpp>
#include "mongo/platform/atomic_word.h"
#include "mongo/util/allocator.h"
#include "mongo/util/assert_util.h"
namespace mongo {
/**
* A mutable, ref-counted buffer.
*/
class SharedBuffer {
public:
SharedBuffer() = default;
void swap(SharedBuffer& other) {
_holder.swap(other._holder);
}
static SharedBuffer allocate(size_t bytes) {
return takeOwnership(mongoMalloc(sizeof(Holder) + 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 + sizeof(Holder);
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);
}
char* get() const {
return _holder ? _holder->data() : NULL;
}
explicit operator bool() const {
return bool(_holder);
}
/**
* Returns true 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;
};
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();
}
/**
* 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);
}
private:
SharedBuffer _buffer;
};
inline void swap(ConstSharedBuffer& one, ConstSharedBuffer& two) {
one.swap(two);
}
}
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