Comment 2 for bug 1443809

I may have misfiled this, but not quite sure which project is responsible. I also note that when adding gnome-desktop to the list of affected projects, it said "gnome-desktop does not use Launchpad to track bugs. Nobody will be notified about this issue."

Added a screenshot of an image not being thumbnailed.

The relevant function seems to be "gnome_desktop_thumbnail_factory_has_valid_failed_thumbnail ()"
https://developer.gnome.org/gnome-desktop/stable/GnomeDesktopThumbnailFactory.html#gnome-desktop-thumbnail-factory-has-valid-failed-thumbnail

This is old code because I don't know how to reference the latest code:
http://sourcecodebrowser.com/gnome-desktop/2.32.0/gnome-desktop-thumbnail_8h.html#ab9946e6856418572e55147637a1493ab

It includes this:

  pixbuf = gdk_pixbuf_new_from_file (path, NULL);
  g_free (path);

  if (pixbuf)
    {
      res = gnome_desktop_thumbnail_is_valid (pixbuf, uri, mtime);
      g_object_unref (pixbuf);
    }

  g_checksum_free (checksum);

It mentions mtime, yet evidently that is not being checked if the process described in this report can cause a new thumbnail to fail to be generated.

As implied in the original report, if there is a valid thumbnail and also a failed thumbnail for the same path to a file, if the valid thumbnail doesn't match the mtime of the actual file, a new thumbnail will be generated. So it seems to work unless no 'normal' thumbnail (whether normal or large-sized) already exists.