Also warn about read-only files just before saving

src/calibre/gui2/tweak_book/boss.py

@@ -107,6 +107,7 @@ class Boss(QObject):
     def __init__(self, parent, notify=None):
         QObject.__init__(self, parent)
         self.global_undo = GlobalUndoHistory()
+        self.file_was_readonly = False
         self.container_count = 0
         self.tdir = None
         self.save_manager = SaveManager(parent, notify)
@@ -318,9 +319,6 @@ def open_book(self, path=None, edit_file=None, clear_notify_data=True, open_fold
         if not os.path.exists(path):
             return error_dialog(self.gui, _('File not found'), _(
                 'The file %s does not exist.') % path, show=True)
-        if not os.access(path, os.W_OK):
-            warning_dialog(self.gui, _('Read-only file'), _(
-                'The file {} is read-only. Saving changes to it will either fail or cause its permissions to be reset.').format(path), show=True)
         isdir = os.path.isdir(path)
         ext = path.rpartition('.')[-1].upper()
         if ext not in SUPPORTED and not isdir:
@@ -332,6 +330,11 @@ def open_book(self, path=None, edit_file=None, clear_notify_data=True, open_fold
                   ' Convert your book to one of these formats first.') % _(' and ').join(sorted(SUPPORTED)),
                 show=True)
 
+        self.file_was_readonly = not os.access(path, os.W_OK)
+        if self.file_was_readonly:
+            warning_dialog(self.gui, _('Read-only file'), _(
+                'The file {} is read-only. Saving changes to it will either fail or cause its permissions to be reset.').format(path), show=True)
+
         for name in tuple(editors):
             self.close_editor(name)
         self.gui.preview.clear()
@@ -1302,6 +1305,11 @@ def save_book(self):
                 self.global_undo.update_path_to_ebook(path)
             else:
                 return
+        if os.path.exists(c.path_to_ebook) and not os.access(c.path_to_ebook, os.W_OK):
+            if not question_dialog(self.gui, _('File is read-only'), _(
+                    'The file at {} is read-only. The editor will try to reset its permissions before saving. Proceed with saving?'
+            ).format(c.path_to_ebook), override_icon='dialog_warning.png', yes_text=_('&Save'), no_text=_('&Cancel'), yes_icon='save.png'):
+                return
         self.gui.action_save.setEnabled(False)
         tdir = self.mkdtemp(prefix='save-')
         container = clone_container(c, tdir)