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/**
* Copyright (C) 2014 MongoDB Inc.
*
* This program is free software: you can redistribute it and/or modify
* it under the terms of the GNU Affero General Public License, version 3,
* as published by the Free Software Foundation.
*
* 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
* GNU Affero General Public License for more details.
*
* You should have received a copy of the GNU Affero General Public License
* along with this program. If not, see <http://www.gnu.org/licenses/>.
*
* 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 GNU Affero General 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"
namespace mongo {
class SharedBuffer {
public:
SharedBuffer() = default;
void swap(SharedBuffer& other) {
_holder.swap(other._holder);
}
SharedBuffer(const SharedBuffer&) = default;
SharedBuffer& operator=(const SharedBuffer&) = default;
SharedBuffer(SharedBuffer&& other) : _holder() {
swap(other);
}
SharedBuffer& operator=(SharedBuffer&& other) {
swap(other);
other._holder.reset();
return *this;
}
static SharedBuffer allocate(size_t bytes) {
return takeOwnership(static_cast<char*>(mongoMalloc(sizeof(Holder) + bytes)));
}
/**
* 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(char* holderPrefixedData) {
// Initialize the refcount to 1 so we don't need to increment it in the constructor
// (see private Holder* constructor below).
//
// TODO: Should dassert alignment of holderPrefixedData
// here if possible.
return SharedBuffer(new (holderPrefixedData) Holder(1U));
}
char* get() const {
return _holder ? _holder->data() : NULL;
}
class Holder {
public:
explicit Holder(AtomicUInt32::WordType initial = AtomicUInt32::WordType())
: _refCount(initial) {}
// 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);
}
private:
AtomicUInt32 _refCount;
};
private:
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 above. This avoids an atomic increment.
}
boost::intrusive_ptr<Holder> _holder;
};
inline void swap(SharedBuffer& one, SharedBuffer& two) {
one.swap(two);
}
}
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