In CirculationManagerAnnotator.open_access link we use the Resource associated with the LicensePoolDeliveryMechanism as the target of the open-access link. Up to this point, circulation managers have been getting their open-access links from the content server, so this Resource represents the version of the book that has already been mirrored to S3.

But now there are circulation managers that import directly from foreign OPDS feeds. In this case the Resource associated with the LicensePoolDeliveryMechanism is the original download provided by the foreign OPDS feed. If the resource was not mirrored during the import, then that's still the URL we should use, but if it was mirrored, we should be providing patrons with the mirror URL instead: resource.representation.mirror_url.

You can see this in action at http://opds-browser-demo.herokuapp.com/collection/https%3A%2F%2Fcirculation.librarysimplified.org%2FCLASSICS%2Ffeed%2F1793/

Books like "Cattle Brands" have a "Download EPUB" link that points to Feedbooks, even though we've mirrored the book and could provide our mirrored version.