Add a template specialization for ConcurrentCache storing shared_ptr

When ConcurrentCache store a shared_ptr we may have shared_ptr in used
while the ConcurrentCache has drop it.
When we "recreate" a value to put in the cache, we don't want to recreate
it, but copying the shared_ptr in use.

To do so we use a (unlimited) store of weak_ptr (aka `WeakStore`)
Every created shared_ptr added to the cache has a weak_ptr ref also stored
in the WeakStore, and we check the WeakStore before creating the value.

src/tools/concurrent_cache.h

@@ -95,6 +95,113 @@ protected: // data
   std::mutex lock_;
 };
 
+
+/**
+  WeakStore represent a thread safe store (map) of weak ptr.
+  It allows to store weak_ptr from shared_ptr and retrieve shared_ptr from
+  potential non expired weak_ptr.
+  It is not limited in size.
+*/
+template<typename Key, typename Value>
+class WeakStore {
+  private: // types
+    typedef std::weak_ptr<Value> WeakValue;
+
+  public:
+    explicit WeakStore() = default;
+
+    std::shared_ptr<Value> get(const Key& key)
+    {
+      std::lock_guard<std::mutex> l(m_lock);
+      auto it = m_weakMap.find(key);
+      if (it != m_weakMap.end()) {
+        auto shared = it->second.lock();
+        if (shared) {
+          return shared;
+        } else {
+          m_weakMap.erase(it);
+        }
+      }
+      throw std::runtime_error("No weak ptr");
+    }
+
+    void add(const Key& key, std::shared_ptr<Value> shared)
+    {
+      std::lock_guard<std::mutex> l(m_lock);
+      m_weakMap[key] = WeakValue(shared);
+    }
+
+
+  private: //data
+    std::map<Key, WeakValue> m_weakMap;
+    std::mutex m_lock;
+};
+
+template <typename Key, typename RawValue>
+class ConcurrentCache<Key, std::shared_ptr<RawValue>>
+{
+private: // types
+  typedef std::shared_ptr<RawValue> Value;
+  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 to get back the shared_ptr from the weak_ptr first.
+      try {
+        valuePromise.set_value(m_weakStore.get(key));
+      } catch(const std::runtime_error& e) {
+        try {
+          const auto value = f();
+          valuePromise.set_value(value);
+          m_weakStore.add(key, value);
+        } 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);
+  }
+
+  size_t setMaxSize(size_t new_size) {
+    std::unique_lock<std::mutex> l(lock_);
+    return  impl_.setMaxSize(new_size);
+  }
+
+protected: // data
+  std::mutex lock_;
+  Impl impl_;
+  WeakStore<Key, RawValue> m_weakStore;
+};
+
 } // namespace kiwix
 
 #endif // ZIM_CONCURRENT_CACHE_H

test/lrucache.cpp

@@ -107,3 +107,24 @@ TEST(ConcurrentCacheTest, handleException) {
     EXPECT_EQ(val, 888);
 }
 
+TEST(ConcurrentCacheTest, weakPtr) {
+    kiwix::ConcurrentCache<int, std::shared_ptr<int>> cache(1);
+    auto refValue = cache.getOrPut(7, []() { return std::make_shared<int>(777); });
+    EXPECT_EQ(*refValue, 777);
+    EXPECT_EQ(refValue.use_count(), 2);
+
+    // This will drop shared(777) from the cache
+    cache.getOrPut(8, []() { return std::make_shared<int>(888); });
+    EXPECT_EQ(refValue.use_count(), 1);
+
+    // We must get the shared value from the weakPtr we have
+    EXPECT_NO_THROW(cache.getOrPut(7, []() { throw std::runtime_error("oups"); return nullptr; }));
+    EXPECT_EQ(refValue.use_count(), 2);
+
+    // Drop all ref
+    cache.getOrPut(8, []() { return std::make_shared<int>(888); });
+    refValue.reset();
+
+    // Be sure we call the construction function
+    EXPECT_THROW(cache.getOrPut(7, []() { throw std::runtime_error("oups"); return nullptr; }), std::runtime_error);
+}