Ensure that feed renegeration restores non-zero items

[Gargron]

Fix #5398

Ordering the home timeline query by account_id meant that the first
100 items belonged to a single account. There was also no reason to
reverse-iterate over the statuses. Assuming the user accesses the
feed halfway-through, it's better to have recent statuses already
available at the top. Therefore working from newer->older is ideal.

If the algorithm ends up filtering all items out during last-mile
filtering, repeat again a page further. The algorithm terminates
when either at least one item has been added, or if the database
query returns nothing (end of data reached)

[Gargron]

Aside: Should I use the same strategy here?

https://github.com/tootsuite/mastodon/blob/894da3dcca781e27ce9c5130f1021526ac8a6887/app/models/feed.rb#L26-L30

To make sure that even on first load of inactive user, feed is not empty?

[aschmitz]

I don't have any idea why this was grouped by account_id before, but this is clearly a better way of doing it. I don't have numbers for how many times you're likely to filter out all potential statuses, but making sure there's at least one seems fine.

In practice, this seems likely to be overkill, but it's in the right direction, so sure, I say go for it. 👍

In terms of Feed#from_database, I'd be inclined to think leaving that alone is fine: it only gets called when the worker is in the process of rebuilding using exactly this code, so an extra loop over it (while usually not actually used?) would only be doing extra work that the worker is already doing. On the off chance that it's empty, the worker will fill in a status eventually, and not block the web worker while it does it. (If you do want to use the same code, I'd consider just calling FeedManager.instance.populate_feed, to avoid needlessly duplicating the code, but then I don't really see the point of a separate worker.)

[akihikodaki]

Sorry for late review, but I think the reverse-iteration made difference in case of reblogs. When adding reblogs, add_to_feed checks if there is the reblogged status before. That would not work without reverse-iteration.