Issue 315: making cereal thread safe

CMakeLists.txt

@@ -6,6 +6,11 @@ if(NOT CMAKE_VERSION VERSION_LESS 3.0) # installing cereal requires INTERFACE li
     option(JUST_INSTALL_CEREAL "Don't do anything besides installing the library" OFF)
 endif()
 
+option(THREAD_SAFE "Use mutexes to ensure thread safety" OFF)
+if(THREAD_SAFE)
+    add_definitions(-DCEREAL_THREAD_SAFE)
+endif()
+
 if(NOT MSVC)
     set(CMAKE_CXX_FLAGS "-Wall -Werror -g -Wextra -Wshadow -pedantic ${CMAKE_CXX_FLAGS}")
     if(CMAKE_VERSION VERSION_LESS 3.1)

include/cereal/details/helpers.hpp

@@ -36,6 +36,9 @@
 #include <memory>
 #include <unordered_map>
 #include <stdexcept>
+#ifdef CEREAL_THREAD_SAFE
+#include <mutex>
+#endif
 
 #include <cereal/macros.hpp>
 #include <cereal/details/static_object.hpp>
@@ -393,8 +396,15 @@ namespace cereal
     {
       std::unordered_map<std::size_t, std::uint32_t> mapping;
 
+      #ifdef CEREAL_THREAD_SAFE
+      std::mutex mutex;
+      #endif
+
       std::uint32_t find( std::size_t hash, std::uint32_t version )
       {
+        #ifdef CEREAL_THREAD_SAFE
+        std::unique_lock<std::mutex> lock(mutex);
+        #endif
         const auto result = mapping.emplace( hash, version );
         return result.first->second;
       }

include/cereal/details/polymorphic_impl.hpp

@@ -52,6 +52,9 @@
 #include <functional>
 #include <typeindex>
 #include <map>
+#ifdef CEREAL_THREAD_SAFE
+#include <mutex>
+#endif
 
 //! Binds a polymorhic type to all registered archives
 /*! This binds a polymorphic type to all compatible registered archives that
@@ -114,6 +117,9 @@ namespace cereal
     {
       //! Maps from base type index to a map from derived type index to caster
       std::map<std::type_index, std::map<std::type_index, std::vector<PolymorphicCaster const*>>> map;
+      #ifdef CEREAL_THREAD_SAFE
+      std::mutex mapMutex;
+      #endif
 
       //! Error message used for unregistered polymorphic casts
       #define UNREGISTERED_POLYMORPHIC_CAST_EXCEPTION(LoadSave)                                                                                                                \
@@ -218,7 +224,11 @@ namespace cereal
           assuming dynamic type information is available */
       PolymorphicVirtualCaster()
       {
-        auto & baseMap = StaticObject<PolymorphicCasters>::getInstance().map;
+        auto & polymorphicCasters = StaticObject<PolymorphicCasters>::getInstance();
+        #ifdef CEREAL_THREAD_SAFE
+        std::unique_lock<std::mutex> lock(polymorphicCasters.mapMutex);
+        #endif
+        auto & baseMap = polymorphicCasters.map;
         auto baseKey = std::type_index(typeid(Base));
         auto lb = baseMap.lower_bound(baseKey);