Check for DRM when preparing KEPUB for viewing

src/calibre/ebooks/conversion/plugins/epub_input.py

@@ -284,11 +284,18 @@ class EPUBInput(InputFormatPlugin):
         if opf is None:
             raise ValueError(f'{path} is not a valid EPUB file (could not find opf)')
 
-        if is_kepub and not self.for_viewer:
-            log('Removing Kobo markup...')
+        if is_kepub:
             from calibre.ebooks.oeb.polish.container import Container
-            from calibre.ebooks.oeb.polish.kepubify import unkepubify_container
+            from calibre.ebooks.oeb.polish.errors import drm_message
+            from calibre.ebooks.oeb.polish.kepubify import check_for_kobo_drm, unkepubify_container
             container = Container(os.getcwd(), opf, log)
+            if self.for_viewer:
+                log('Checking for Kobo DRM...')
+                with drm_message(_('The file {} is locked with DRM. It cannot be viewed').format(path)):
+                    check_for_kobo_drm(container)
+            else:
+                log('Removing Kobo markup...')
+                with drm_message(_('The file {} is locked with DRM. It cannot be converted').format(path)):
                     unkepubify_container(container)
                     container.commit()
             del container

src/calibre/ebooks/oeb/polish/errors.py

@@ -5,6 +5,8 @@ __license__   = 'GPL v3'
 __copyright__ = '2013, Kovid Goyal <kovid at kovidgoyal.net>'
 __docformat__ = 'restructuredtext en'
 
+from contextlib import contextmanager
+
 from calibre.ebooks import DRMError as _DRMError
 
 
@@ -12,10 +14,23 @@ class InvalidBook(ValueError):
     pass
 
 
+_drm_message = ''
+
+
+@contextmanager
+def drm_message(msg: str) -> None:
+    global _drm_message
+    orig, _drm_message = _drm_message, msg
+    try:
+        yield
+    finally:
+        _drm_message = orig
+
+
 class DRMError(_DRMError):
 
     def __init__(self):
-        super().__init__(_('This file is locked with DRM. It cannot be edited.'))
+        super().__init__(_drm_message or _('This file is locked with DRM. It cannot be edited.'))
 
 
 class MalformedMarkup(ValueError):

src/calibre/ebooks/oeb/polish/kepubify.py

@@ -473,6 +473,7 @@ def remove_kobo_files(container):
 
 
 def unkepubify_container(container: Container, max_workers: int = 0) -> None:
+    check_for_kobo_drm(container)
     remove_dummy_cover_image(container)
     remove_dummy_title_page(container)
     remove_kobo_files(container)
@@ -537,7 +538,6 @@ def check_for_kobo_drm(container: Container) -> None:
 
 def unkepubify_path(path, outpath='', max_workers=0, allow_overwrite=False):
     container = get_container(path, tweak_mode=True, ebook_cls=EpubContainer)
-    check_for_kobo_drm(container)
     unkepubify_container(container, max_workers)
     base, ext = os.path.splitext(path)
     outpath = outpath or base + '.epub'