Protect search from multi threading race condition.

libzim's search is not thread safe (mainly because xapian is not).
So we must protect our search objects from multi thread calls.

The best way to do this is to associate a mutex to the `zim::Searcher`
and lock the searcher each time we access object derivated from the
searcher (search, results, iterator, ...)

include/library.h

@@ -141,6 +141,22 @@ private: // functions
     bool accept(const Book& book) const;
 };
 
+
+class ZimSearcher : public zim::Searcher
+{
+  public:
+    explicit ZimSearcher(zim::Searcher&& searcher)
+      : zim::Searcher(searcher)
+    {}
+
+    std::unique_lock<std::mutex> getLock() {
+      return std::unique_lock<std::mutex>(m_mutex);
+    }
+    virtual ~ZimSearcher() = default;
+  private:
+    std::mutex m_mutex;
+};
+
 /**
  * A Library store several books.
  */
@@ -209,10 +225,10 @@ class Library
 
   DEPRECATED std::shared_ptr<Reader> getReaderById(const std::string& id);
   std::shared_ptr<zim::Archive> getArchiveById(const std::string& id);
-  std::shared_ptr<zim::Searcher> getSearcherById(const std::string& id) {
+  std::shared_ptr<ZimSearcher> getSearcherById(const std::string& id) {
     return getSearcherByIds(BookIdSet{id});
   }
-  std::shared_ptr<zim::Searcher> getSearcherByIds(const BookIdSet& ids);
+  std::shared_ptr<ZimSearcher> getSearcherByIds(const BookIdSet& ids);
 
   /**
    * Remove a book from the library.

src/library.cpp

@@ -93,7 +93,7 @@ struct Library::Impl
   std::map<std::string, Entry> m_books;
   using ArchiveCache = ConcurrentCache<std::string, std::shared_ptr<zim::Archive>>;
   std::unique_ptr<ArchiveCache> mp_archiveCache;
-  using SearcherCache = MultiKeyCache<std::string, std::shared_ptr<zim::Searcher>>;
+  using SearcherCache = MultiKeyCache<std::string, std::shared_ptr<ZimSearcher>>;
   std::unique_ptr<SearcherCache> mp_searcherCache;
   std::vector<kiwix::Bookmark> m_bookmarks;
   Xapian::WritableDatabase m_bookDB;
@@ -304,7 +304,7 @@ std::shared_ptr<zim::Archive> Library::getArchiveById(const std::string& id)
   }
 }
 
-std::shared_ptr<zim::Searcher> Library::getSearcherByIds(const BookIdSet& ids)
+std::shared_ptr<ZimSearcher> Library::getSearcherByIds(const BookIdSet& ids)
 {
   assert(!ids.empty());
   try {
@@ -318,7 +318,7 @@ std::shared_ptr<zim::Searcher> Library::getSearcherByIds(const BookIdSet& ids)
         }
         archives.push_back(*archive);
       }
-      return std::make_shared<zim::Searcher>(archives);
+      return std::make_shared<ZimSearcher>(zim::Searcher(archives));
     });
   } catch (std::invalid_argument&) {
     return nullptr;

src/server/internalServer.cpp

@@ -704,11 +704,13 @@ std::unique_ptr<Response> InternalServer::handle_search(const RequestContext& re
 
     /* Make the search */
     // Try to get a search from the searchInfo, else build it
+    auto searcher = mp_library->getSearcherByIds(bookIds);
+    auto lock(searcher->getLock());
+
     std::shared_ptr<zim::Search> search;
     try {
       search = searchCache.getOrPut(searchInfo,
         [=](){
-          auto searcher = mp_library->getSearcherByIds(bookIds);
           return make_shared<zim::Search>(searcher->search(searchInfo.getZimQuery(m_verbose.load())));
         }
       );