conversion: Add sanity check for page count

Add a sanity check at the end of the conversion from doc to pixels, to
ensure that the resulting document will have the same number of pages as
the original one.

Refs #560

dangerzone/conversion/doc_to_pixels.py

@@ -349,16 +349,22 @@ async def pdftoppm_progress_callback(line: bytes) -> None:
             timeout=timeout,
         )
 
-        self.update_progress("Converted document to pixels")
-
-        # Move converted files into /tmp/dangerzone
-        for filename in (
+        final_files = (
             glob.glob("/tmp/page-*.rgb")
             + glob.glob("/tmp/page-*.width")
             + glob.glob("/tmp/page-*.height")
-        ):
+        )
+
+        # XXX: Sanity check to avoid situations like #560.
+        if not running_on_qubes() and len(final_files) != 3 * num_pages:
+            raise errors.PageCountMismatch()
+
+        # Move converted files into /tmp/dangerzone
+        for filename in final_files:
             shutil.move(filename, "/tmp/dangerzone")
 
+        self.update_progress("Converted document to pixels")
+
     async def install_libreoffice_ext(self, libreoffice_ext: str) -> None:
         self.update_progress(f"Installing LibreOffice extension '{libreoffice_ext}'")
         unzip_args = [

dangerzone/conversion/errors.py

@@ -82,6 +82,13 @@ class MaxPageHeightException(PagesException):
     error_message = f"A page exceeded the maximum height."
 
 
+class PageCountMismatch(PagesException):
+    error_code = ERROR_SHIFT + 46
+    error_message = (
+        "The final document does not have the same page count as the original one"
+    )
+
+
 class PDFtoPPMException(ConversionException):
     error_code = ERROR_SHIFT + 50
     error_message = "Error converting PDF to Pixels (pdftoppm)"