Library got a yet unused mutex

Introducing a mutex in `Library` necessitates manually implementing the
move constructor and assignment operator. It's better to still delegate
that work to the compiler to eliminate any possibility of bugs when new
data members are added to `Library`. The trick is to move the data into
an auxiliary class `LibraryBase` and derive `Library` from it.

include/library.h

@@ -24,6 +24,7 @@
 #include <vector>
 #include <map>
 #include <memory>
+#include <mutex>
 #include <zim/archive.h>
 
 #include "book.h"
@@ -139,12 +140,14 @@ private: // functions
     bool accept(const Book& book) const;
 };
 
-
 /**
- * A Library store several books.
+ * This class is not part of the libkiwix API. Its only purpose is
+ * to simplify the implementation of the Library's move operations
+ * and avoid bugs should new data members be added to Library.
  */
-class Library
+class LibraryBase
 {
+protected: // data
   std::map<std::string, kiwix::Book> m_books;
   std::map<std::string, std::shared_ptr<Reader>> m_readers;
   std::map<std::string, std::shared_ptr<zim::Archive>> m_archives;
@@ -152,6 +155,22 @@ class Library
   class BookDB;
   std::unique_ptr<BookDB> m_bookDB;
 
+protected: // functions
+  LibraryBase();
+  ~LibraryBase();
+
+  LibraryBase(LibraryBase&& );
+  LibraryBase& operator=(LibraryBase&& );
+};
+
+/**
+ * A Library store several books.
+ */
+class Library : private LibraryBase
+{
+  // all data fields must be added in LibraryBase
+  mutable std::mutex m_mutex;
+
  public:
   typedef std::vector<std::string> BookIdCollection;
   typedef std::map<std::string, int> AttributeCounts;

src/library.cpp

@@ -58,21 +58,39 @@ bool booksReferToTheSameArchive(const Book& book1, const Book& book2)
 
 } // unnamed namespace
 
-class Library::BookDB : public Xapian::WritableDatabase
+class LibraryBase::BookDB : public Xapian::WritableDatabase
 {
 public:
   BookDB() : Xapian::WritableDatabase("", Xapian::DB_BACKEND_INMEMORY) {}
 };
 
-/* Constructor */
-Library::Library()
+LibraryBase::LibraryBase()
   : m_bookDB(new BookDB)
 {
 }
 
-Library::Library(Library&& ) = default;
+LibraryBase::~LibraryBase()
+{
+}
 
-Library& Library::operator=(Library&& ) = default;
+LibraryBase::LibraryBase(LibraryBase&& ) = default;
+LibraryBase& LibraryBase::operator=(LibraryBase&& ) = default;
+
+/* Constructor */
+Library::Library()
+{
+}
+
+Library::Library(Library&& other)
+  : LibraryBase(std::move(other))
+{
+}
+
+Library& Library::operator=(Library&& other)
+{
+  LibraryBase::operator=(std::move(other));
+  return *this;
+}
 
 /* Destructor */
 Library::~Library()