Introduce getValidBookById in Library

Raise an exception the book does not exist in the file system. Since the change to directory monitoring alters the behavior of the default getBookById, the function should be called if you need the book to actually exist.

src/contentmanager.cpp

@@ -361,7 +361,7 @@ ContentManager::BookInfo ContentManager::getBookInfos(QString id, const QStringL
 {
     const kiwix::Book* b = nullptr;
     try {
-        b = &mp_library->getBookById(id);
+        b = &mp_library->getValidBookById(id);
         if ( ! b->getDownloadId().empty() ) {
             // The book is still being downloaded and has been entered into the
             // local library for technical reasons only. Get the book info from
@@ -395,7 +395,7 @@ ContentManager::BookState ContentManager::getBookState(QString bookId)
     }
 
     try {
-        const kiwix::Book& b = mp_library->getBookById(bookId);
+        const kiwix::Book& b = mp_library->getValidBookById(bookId);
         return b.getDownloadId().empty()
              ? getStateOfLocalBook(b)
              : BookState::DOWNLOADING;
@@ -469,7 +469,7 @@ void ContentManager::downloadDisappeared(QString bookId)
     removeDownload(bookId);
     kiwix::Book bCopy;
     try {
-        bCopy = mp_library->getBookById(bookId);
+        bCopy = mp_library->getValidBookById(bookId);
     } catch ( const std::out_of_range& ) {
         // If the download has disappeared as a result of some
         // obscure chain of events, the book may have disappeared too.

src/library.cpp

@@ -213,3 +213,11 @@ const kiwix::Book &Library::getBookById(QString id) const
 {
     return mp_library->getBookById(id.toStdString());
 }
+
+const kiwix::Book &Library::getValidBookById(QString id) const
+{
+    auto& book = mp_library->getBookById(id.toStdString());
+    if (book.isPathValid())
+        return book;
+    throw std::out_of_range ("Book is invalid or does not exist.");
+}

src/library.h

@@ -49,6 +49,7 @@ class Library : public QObject
     kiwix::LibraryPtr getKiwixLibrary() { return mp_library; }
 public slots:
     const kiwix::Book& getBookById(QString id) const;
+    const kiwix::Book& getValidBookById(QString id) const;
 
 signals:
     void booksChanged();

src/urlschemehandler.cpp

@@ -79,7 +79,7 @@ UrlSchemeHandler::handleMetaRequest(QWebEngineUrlRequestJob* request)
     if (metaName == "favicon") {
         try {
           auto library = KiwixApp::instance()->getLibrary();
-          auto book = library->getBookById(zimId);
+          auto book = library->getValidBookById(zimId);
           auto illustration = book.getIllustration(48);
           std::string content = illustration->getData();
           std::string mimeType = illustration->mimeType;
@@ -184,7 +184,7 @@ UrlSchemeHandler::replyZimNotFoundPage(QWebEngineUrlRequestJob *request,
     QString path = "N/A", name = "N/A";
     try 
     {
-        auto& book = KiwixApp::instance()->getLibrary()->getBookById(zimId);
+        auto& book = KiwixApp::instance()->getLibrary()->getValidBookById(zimId);
         path = QString::fromStdString(book.getPath());
         name = QString::fromStdString(book.getName());
     }