Check library: Fix popup about check being performed not being rendered

The vacuum locks the db and there are various events on the GUI thread
that can cause db access which causes the GUI thread to hang. Find and
fix the most common of these. It isnt a complete solution, but its
better than nothing.

src/calibre/db/cache.py

@@ -142,6 +142,7 @@ class Cache:
 
     def __init__(self, backend, library_database_instance=None):
         self.shutting_down = False
+        self.is_doing_rebuild_or_vacuum = False
         self.backend = backend
         self.library_database_instance = (None if library_database_instance is None else
                                           weakref.ref(library_database_instance))
@@ -2429,7 +2430,11 @@ class Cache:
 
     @write_api
     def vacuum(self):
-        self.backend.vacuum()
+        self.is_doing_rebuild_or_vacuum = True
+        try:
+            self.backend.vacuum()
+        finally:
+            self.is_doing_rebuild_or_vacuum = False
 
     def __del__(self):
         self.close()

src/calibre/gui2/actions/choose_library.py

@@ -659,7 +659,9 @@ class ChooseLibraryAction(InterfaceAction):
         d = DBCheck(self.gui, db)
         try:
             d.start()
-            d.exec()
+            from calibre.gui2.widgets import BusyCursor
+            with BusyCursor():
+                d.exec()
             try:
                 m.close()
             except Exception:

src/calibre/gui2/dialogs/scheduler.py

@@ -612,7 +612,9 @@ class Scheduler(QObject):
         from calibre.gui2.ui import get_gui
         gui = get_gui()
         if gui is not None:
-            return gui.current_db
+            ans = gui.current_db
+            if not ans.new_api.is_doing_rebuild_or_vacuum:
+                return ans
 
     def oldest_check(self):
         if self.oldest > 0:

src/calibre/gui2/library/models.py

@@ -966,6 +966,8 @@ class BooksModel(QAbstractTableModel):  # {{{
         self.column_to_dc_decorator_map = [self.dc_decorator.get(col, None) for col in self.column_map]
 
     def data(self, index, role):
+        if self.db.new_api.is_doing_rebuild_or_vacuum:
+            return None
         col = index.column()
         # in obscure cases where custom columns are both edited and added, for a time
         # the column map does not accurately represent the screen. In these cases,