No tagging when masking user's elfeed-feeds with another list and running elfeed-update.

[kakafarm]

I want to write a function that would update a different feed list each time it is called.

(defun update-a-different-feed-list-each-call ()
  (let ((elfeed-feeds (make-a-random-feed-list)))
    (elfeed-update)))

The function make-a-random-feed-list behaves correctly, producing a legal feed list each time, feed lists like:

'(("https://example.com/blog/rss" blog example)
  ("https://example.com/podcast/rss" podcast example))

The function elfeed-update adds new entries to the elfeed database with only the unread tag, without blog, example, or podcast.

If I define the function in this way:

(defun update-a-different-feed-list-each-call ()
  (setq elfeed-feeds (make-a-random-feed-list))
  (elfeed-update))

each of the new entries has the expected tags, but the user defined list of feeds is overwritten.

How do I provide a dynamically generated feed list for elfeed from which to fetch entries and properly tag them while also not overwriting the user configuration?

[kakafarm]

This one also only adds the "unread" tag:

(defun update-a-different-feed-list-each-call ()
  (let ((old-elfeed-feeds elfeed-feeds))
    (setq elfeed-feeds (make-a-random-feed-list))
    (elfeed-update)
    (setq elfeed-feeds old-elfeed-feeds)))