Fix #2058115 [[Enhancement] Report compacted database size during Check Library](https://bugs.launchpad.net/calibre/+bug/2058115)

src/calibre/db/backend.py

@@ -2655,4 +2655,16 @@ class DB:
 
     def backup_notes_database(self, path):
         self._backup_database(path, 'notes_db')
+
+    def size_stats(self):
+        main_size = notes_size = fts_size = 0
+        with suppress(OSError):
+            main_size = os.path.getsize(self.dbpath)
+        if self.conn.notes_dbpath:
+            with suppress(OSError):
+                notes_size = os.path.getsize(self.conn.notes_dbpath)
+        if self.conn.fts_dbpath:
+            with suppress(OSError):
+                fts_size = os.path.getsize(self.conn.fts_dbpath)
+        return {'main': main_size, 'fts': fts_size, 'notes': notes_size}
     # }}}

src/calibre/db/cache.py

@@ -1405,6 +1405,10 @@ class Cache:
         ids_to_sort.sort(reverse=True)
         return ids_to_sort[:count]
 
+    @read_api
+    def size_stats(self) -> dict[str, int]:
+        return self.backend.size_stats()
+
     @read_api
     def multisort(self, fields, ids_to_sort=None, virtual_fields=None):
         '''

src/calibre/gui2/actions/choose_library.py

@@ -678,6 +678,7 @@ class ChooseLibraryAction(InterfaceAction):
         db = m.db
         db.prefs.disable_setting = True
         library_path = db.library_path
+        before = db.new_api.size_stats()
 
         d = DBCheck(self.gui, db)
         try:
@@ -692,10 +693,22 @@ class ChooseLibraryAction(InterfaceAction):
         if d.rejected:
             return
         if d.error is None:
+            after = self.gui.current_db.new_api.size_stats()
+            det_msg = ''
+            from calibre import human_readable
+            for which, title in {'main': _('books'), 'fts': _('full text search'), 'notes': _('notes')}.items():
+                if which != 'main' and not getattr(d, which).isChecked():
+                    continue
+                det_msg += '\n'
+                if before[which] == after[which]:
+                    det_msg += _('Size of the {} database was unchanged.').format(title)
+                else:
+                    det_msg += _('Size of the {0} database reduced from {1} to {2}.').format(
+                            title, human_readable(before[which]), human_readable(after[which]))
             if not question_dialog(self.gui, _('Success'),
                     _('Found no errors in your calibre library database.'
                         ' Do you want calibre to check if the files in your'
-                        ' library match the information in the database?')):
+                        ' library match the information in the database?'), det_msg=det_msg.strip()):
                 return
         else:
             return error_dialog(self.gui, _('Failed'),