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// Copyright (c) 2011-present, Facebook, Inc. All rights reserved.
// This source code is licensed under both the GPLv2 (found in the
// COPYING file in the root directory) and Apache 2.0 License
// (found in the LICENSE.Apache file in the root directory).
//
// Copyright (c) 2012 The LevelDB Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file. See the AUTHORS file for names of contributors.
#pragma once
#include <cassert>
#include "port/likely.h"
#include "rocksdb/cache.h"
#include "rocksdb/cleanable.h"
namespace ROCKSDB_NAMESPACE {
// CachableEntry is a handle to an object that may or may not be in the block
// cache. It is used in a variety of ways:
//
// 1) It may refer to an object in the block cache. In this case, cache_ and
// cache_handle_ are not nullptr, and the cache handle has to be released when
// the CachableEntry is destroyed (the lifecycle of the cached object, on the
// other hand, is managed by the cache itself).
// 2) It may uniquely own the (non-cached) object it refers to (examples include
// a block read directly from file, or uncompressed blocks when there is a
// compressed block cache but no uncompressed block cache). In such cases, the
// object has to be destroyed when the CachableEntry is destroyed.
// 3) It may point to an object (cached or not) without owning it. In this case,
// no action is needed when the CachableEntry is destroyed.
// 4) Sometimes, management of a cached or owned object (see #1 and #2 above)
// is transferred to some other object. This is used for instance with iterators
// (where cleanup is performed using a chain of cleanup functions,
// see Cleanable).
//
// Because of #1 and #2 above, copying a CachableEntry is not safe (and thus not
// allowed); hence, this is a move-only type, where a move transfers the
// management responsibilities, and leaves the source object in an empty state.
template <class T>
class CachableEntry {
public:
CachableEntry() = default;
CachableEntry(T* value, Cache* cache, Cache::Handle* cache_handle,
bool own_value)
: value_(value),
cache_(cache),
cache_handle_(cache_handle),
own_value_(own_value) {
assert(value_ != nullptr ||
(cache_ == nullptr && cache_handle_ == nullptr && !own_value_));
assert(!!cache_ == !!cache_handle_);
assert(!cache_handle_ || !own_value_);
}
CachableEntry(const CachableEntry&) = delete;
CachableEntry& operator=(const CachableEntry&) = delete;
CachableEntry(CachableEntry&& rhs) noexcept
: value_(rhs.value_),
cache_(rhs.cache_),
cache_handle_(rhs.cache_handle_),
own_value_(rhs.own_value_) {
assert(value_ != nullptr ||
(cache_ == nullptr && cache_handle_ == nullptr && !own_value_));
assert(!!cache_ == !!cache_handle_);
assert(!cache_handle_ || !own_value_);
rhs.ResetFields();
}
CachableEntry& operator=(CachableEntry&& rhs) noexcept {
if (UNLIKELY(this == &rhs)) {
return *this;
}
ReleaseResource();
value_ = rhs.value_;
cache_ = rhs.cache_;
cache_handle_ = rhs.cache_handle_;
own_value_ = rhs.own_value_;
assert(value_ != nullptr ||
(cache_ == nullptr && cache_handle_ == nullptr && !own_value_));
assert(!!cache_ == !!cache_handle_);
assert(!cache_handle_ || !own_value_);
rhs.ResetFields();
return *this;
}
~CachableEntry() { ReleaseResource(); }
bool IsEmpty() const {
return value_ == nullptr && cache_ == nullptr && cache_handle_ == nullptr &&
!own_value_;
}
bool IsCached() const {
assert(!!cache_ == !!cache_handle_);
return cache_handle_ != nullptr;
}
T* GetValue() const { return value_; }
Cache* GetCache() const { return cache_; }
Cache::Handle* GetCacheHandle() const { return cache_handle_; }
bool GetOwnValue() const { return own_value_; }
void Reset() {
ReleaseResource();
ResetFields();
}
void TransferTo(Cleanable* cleanable) {
if (cleanable) {
if (cache_handle_ != nullptr) {
assert(cache_ != nullptr);
cleanable->RegisterCleanup(&ReleaseCacheHandle, cache_, cache_handle_);
} else if (own_value_) {
cleanable->RegisterCleanup(&DeleteValue, value_, nullptr);
}
}
ResetFields();
}
void SetOwnedValue(std::unique_ptr<T>&& value) {
assert(value.get() != nullptr);
if (UNLIKELY(value_ == value.get() && own_value_)) {
assert(cache_ == nullptr && cache_handle_ == nullptr);
return;
}
Reset();
value_ = value.release();
own_value_ = true;
}
void SetUnownedValue(T* value) {
assert(value != nullptr);
if (UNLIKELY(value_ == value && cache_ == nullptr &&
cache_handle_ == nullptr && !own_value_)) {
return;
}
Reset();
value_ = value;
assert(!own_value_);
}
void SetCachedValue(T* value, Cache* cache, Cache::Handle* cache_handle) {
assert(cache != nullptr);
assert(cache_handle != nullptr);
if (UNLIKELY(value_ == value && cache_ == cache &&
cache_handle_ == cache_handle && !own_value_)) {
return;
}
Reset();
value_ = value;
cache_ = cache;
cache_handle_ = cache_handle;
assert(!own_value_);
}
void UpdateCachedValue() {
assert(cache_ != nullptr);
assert(cache_handle_ != nullptr);
value_ = static_cast<T*>(cache_->Value(cache_handle_));
}
bool IsReady() {
if (!own_value_) {
assert(cache_ != nullptr);
assert(cache_handle_ != nullptr);
return cache_->IsReady(cache_handle_);
}
return true;
}
private:
void ReleaseResource() noexcept {
if (LIKELY(cache_handle_ != nullptr)) {
assert(cache_ != nullptr);
cache_->Release(cache_handle_);
} else if (own_value_) {
delete value_;
}
}
void ResetFields() noexcept {
value_ = nullptr;
cache_ = nullptr;
cache_handle_ = nullptr;
own_value_ = false;
}
static void ReleaseCacheHandle(void* arg1, void* arg2) {
Cache* const cache = static_cast<Cache*>(arg1);
assert(cache);
Cache::Handle* const cache_handle = static_cast<Cache::Handle*>(arg2);
assert(cache_handle);
cache->Release(cache_handle);
}
static void DeleteValue(void* arg1, void* /* arg2 */) {
delete static_cast<T*>(arg1);
}
private:
T* value_ = nullptr;
Cache* cache_ = nullptr;
Cache::Handle* cache_handle_ = nullptr;
bool own_value_ = false;
};
} // namespace ROCKSDB_NAMESPACE
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