Thx for the debate on this guys. So to summarise the state of play as I understand it...
- Find Duplicates can be easily sorted with Kovids additions today.
- Count Pages and Modify ePub I should be able to change without drama.
- With Extract ISBN there could be a large number of files to copy up front (if the user selects several thousand books as they have in the past it will result in many more thousands of book formats being copied up front) but it's the "right" thing to do anyway as a background job.
- Quality Check could potentially be changed to use the SpooledTempFile approach since it runs in process. But will hold off making changes until the db API settles in case some other idea crops up
- Open With remains a sticky problem, but again some bright idea may come in down the track.