Introduce a LRU Cache and concurrent cache

The cache is copied from libzim project : https://github.com/openzim/libzim
The exact file as been copied from commit 27f5e70

src/tools/concurrent_cache.h

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+/*
+ * Copyright (C) 2021 Matthieu Gautier <mgautier@kymeria.fr>
+ * Copyright (C) 2020 Veloman Yunkan
+ *
+ * This program is free software; you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as
+ * published by the Free Software Foundation; either version 2 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful, but
+ * is provided AS IS, WITHOUT ANY WARRANTY; without even the implied
+ * warranty of MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE, and
+ * NON-INFRINGEMENT.  See the GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with this program; if not, write to the Free Software
+ * Foundation, Inc., 51 Franklin St, Fifth Floor, Boston, MA  02110-1301 USA
+ *
+ */
+
+#ifndef ZIM_CONCURRENT_CACHE_H
+#define ZIM_CONCURRENT_CACHE_H
+
+#include "lrucache.h"
+
+#include <future>
+#include <mutex>
+
+namespace kiwix
+{
+
+/**
+   ConcurrentCache implements a concurrent thread-safe cache
+
+   Compared to kiwix::lru_cache, each access operation is slightly more expensive.
+   However, different slots of the cache can be safely accessed concurrently
+   with minimal blocking. Concurrent access to the same element is also
+   safe, and, in case of a cache miss, will block until that element becomes
+   available.
+ */
+template <typename Key, typename Value>
+class ConcurrentCache
+{
+private: // types
+  typedef std::shared_future<Value> ValuePlaceholder;
+  typedef lru_cache<Key, ValuePlaceholder> Impl;
+
+public: // types
+  explicit ConcurrentCache(size_t maxEntries)
+    : impl_(maxEntries)
+  {}
+
+  // Gets the entry corresponding to the given key. If the entry is not in the
+  // cache, it is obtained by calling f() (without any arguments) and the
+  // result is put into the cache.
+  //
+  // The cache as a whole is locked only for the duration of accessing
+  // the respective slot. If, in the case of the a cache miss, the generation
+  // of the missing element takes a long time, only attempts to access that
+  // element will block - the rest of the cache remains open to concurrent
+  // access.
+  template<class F>
+  Value getOrPut(const Key& key, F f)
+  {
+    std::promise<Value> valuePromise;
+    std::unique_lock<std::mutex> l(lock_);
+    const auto x = impl_.getOrPut(key, valuePromise.get_future().share());
+    l.unlock();
+    if ( x.miss() ) {
+      try {
+        valuePromise.set_value(f());
+      } catch (std::exception& e) {
+        drop(key);
+        throw;
+      }
+    }
+
+    return x.value().get();
+  }
+
+  bool drop(const Key& key)
+  {
+    std::unique_lock<std::mutex> l(lock_);
+    return impl_.drop(key);
+  }
+
+private: // data
+  Impl impl_;
+  std::mutex lock_;
+};
+
+} // namespace kiwix
+
+#endif // ZIM_CONCURRENT_CACHE_H
+

src/tools/lrucache.h

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+/*
+ * Copyrigth (c) 2021, Matthieu Gautier <mgautier@kymeria.fr>
+ * Copyright (c) 2020, Veloman Yunkan
+ * Copyright (c) 2014, lamerman
+ * All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions are met:
+ *
+ * * Redistributions of source code must retain the above copyright notice, this
+ *   list of conditions and the following disclaimer.
+ *
+ * * Redistributions in binary form must reproduce the above copyright notice,
+ *   this list of conditions and the following disclaimer in the documentation
+ *   and/or other materials provided with the distribution.
+ *
+ * * Neither the name of lamerman nor the names of its
+ *   contributors may be used to endorse or promote products derived from
+ *   this software without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+ * AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+ * IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+ * DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+ * FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+ * DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+ * SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+ * CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+ * OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+ * OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+ *
+ * File:   lrucache.hpp
+ * Author: Alexander Ponomarev
+ *
+ * Created on June 20, 2013, 5:09 PM
+ */
+
+#ifndef _LRUCACHE_HPP_INCLUDED_
+#define _LRUCACHE_HPP_INCLUDED_
+
+#include <map>
+#include <list>
+#include <cstddef>
+#include <stdexcept>
+#include <cassert>
+
+namespace kiwix {
+
+template<typename key_t, typename value_t>
+class lru_cache {
+public: // types
+  typedef typename std::pair<key_t, value_t> key_value_pair_t;
+  typedef typename std::list<key_value_pair_t>::iterator list_iterator_t;
+
+  enum AccessStatus {
+    HIT, // key was found in the cache
+    PUT, // key was not in the cache but was created by the getOrPut() access
+    MISS // key was not in the cache; get() access failed
+  };
+
+  class AccessResult
+  {
+    const AccessStatus status_;
+    const value_t val_;
+  public:
+    AccessResult(const value_t& val, AccessStatus status)
+      : status_(status), val_(val)
+    {}
+    AccessResult() : status_(MISS), val_() {}
+
+    bool hit() const { return status_ == HIT; }
+    bool miss() const { return !hit(); }
+    const value_t& value() const
+    {
+      if ( status_ == MISS )
+        throw std::range_error("There is no such key in cache");
+      return val_;
+    }
+
+    operator const value_t& () const { return value(); }
+  };
+
+public: // functions
+  explicit lru_cache(size_t max_size) :
+    _max_size(max_size) {
+  }
+
+  // If 'key' is present in the cache, returns the associated value,
+  // otherwise puts the given value into the cache (and returns it with
+  // a status of a cache miss).
+  AccessResult getOrPut(const key_t& key, const value_t& value) {
+    auto it = _cache_items_map.find(key);
+    if (it != _cache_items_map.end()) {
+      _cache_items_list.splice(_cache_items_list.begin(), _cache_items_list, it->second);
+      return AccessResult(it->second->second, HIT);
+    } else {
+      putMissing(key, value);
+      return AccessResult(value, PUT);
+    }
+  }
+
+  void put(const key_t& key, const value_t& value) {
+    auto it = _cache_items_map.find(key);
+    if (it != _cache_items_map.end()) {
+      _cache_items_list.splice(_cache_items_list.begin(), _cache_items_list, it->second);
+      it->second->second = value;
+    } else {
+      putMissing(key, value);
+    }
+  }
+
+  AccessResult get(const key_t& key) {
+    auto it = _cache_items_map.find(key);
+    if (it == _cache_items_map.end()) {
+      return AccessResult();
+    } else {
+      _cache_items_list.splice(_cache_items_list.begin(), _cache_items_list, it->second);
+      return AccessResult(it->second->second, HIT);
+    }
+  }
+
+  bool drop(const key_t& key) {
+    try {
+      auto list_it = _cache_items_map.at(key);
+      _cache_items_list.erase(list_it);
+      _cache_items_map.erase(key);
+      return true;
+    } catch (std::out_of_range& e) {
+      return false;
+    }
+  }
+
+  bool exists(const key_t& key) const {
+    return _cache_items_map.find(key) != _cache_items_map.end();
+  }
+
+  size_t size() const {
+    return _cache_items_map.size();
+  }
+
+private: // functions
+  void putMissing(const key_t& key, const value_t& value) {
+    assert(_cache_items_map.find(key) == _cache_items_map.end());
+    _cache_items_list.push_front(key_value_pair_t(key, value));
+    _cache_items_map[key] = _cache_items_list.begin();
+    if (_cache_items_map.size() > _max_size) {
+      _cache_items_map.erase(_cache_items_list.back().first);
+      _cache_items_list.pop_back();
+    }
+  }
+
+private: // data
+  std::list<key_value_pair_t> _cache_items_list;
+  std::map<key_t, list_iterator_t> _cache_items_map;
+  size_t _max_size;
+};
+
+} // namespace kiwix
+
+#endif  /* _LRUCACHE_HPP_INCLUDED_ */