[bugfix] Tidy up rss feed serving; don't error on empty feed

[tsmethurst]

Description

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If this is a documentation change, please briefly describe what you've changed and why.

This PR tidies up some of the very messy rss feed logic and adds comments for clarity.

Also allows RSS feeds to be served even when the account owner hasn't posted anything yet (closes #1951). In such a case, the result will be a feed that just doesn't have any items in it.

Example with items:

<?xml version="1.0" encoding="UTF-8"?><rss version="2.0" xmlns:content="http://purl.org/rss/1.0/modules/content/">
  <channel>
    <title>Posts from @the_mighty_zork@localhost:8080</title>
    <link>http://localhost:8080/@the_mighty_zork</link>
    <description>Posts from @the_mighty_zork@localhost:8080</description>
    <pubDate>Wed, 20 Oct 2021 10:40:37 +0000</pubDate>
    <lastBuildDate>Wed, 20 Oct 2021 10:40:37 +0000</lastBuildDate>
    <image>
      <url>http://localhost:8080/fileserver/01F8MH1H7YV1Z7D2C8K2730QBF/avatar/small/01F8MH58A357CV5K7R7TJMSH6S.jpg</url>
      <title>Avatar for @the_mighty_zork@localhost:8080</title>
      <link>http://localhost:8080/@the_mighty_zork</link>
    </image>
    <item>
      <title>introduction post</title>
      <link>http://localhost:8080/@the_mighty_zork/statuses/01F8MHAMCHF6Y650WCRSCP4WMY</link>
      <description>@the_mighty_zork@localhost:8080 made a new post: &#34;hello everyone!&#34;</description>
      <content:encoded><![CDATA[hello everyone!]]></content:encoded>
      <author>@the_mighty_zork@localhost:8080</author>
      <guid>http://localhost:8080/@the_mighty_zork/statuses/01F8MHAMCHF6Y650WCRSCP4WMY</guid>
      <pubDate>Wed, 20 Oct 2021 10:40:37 +0000</pubDate>
      <source>http://localhost:8080/@the_mighty_zork/feed.rss</source>
    </item>
  </channel>
</rss>

Example with no items:

<?xml version="1.0" encoding="UTF-8"?><rss version="2.0" xmlns:content="http://purl.org/rss/1.0/modules/content/">
  <channel>
    <title>Posts from @the_mighty_zork@localhost:8080</title>
    <link>http://localhost:8080/@the_mighty_zork</link>
    <description>Posts from @the_mighty_zork@localhost:8080</description>
    <pubDate>Fri, 20 May 2022 11:09:18 +0000</pubDate>
    <lastBuildDate>Fri, 20 May 2022 11:09:18 +0000</lastBuildDate>
    <image>
      <url>http://localhost:8080/fileserver/01F8MH1H7YV1Z7D2C8K2730QBF/avatar/small/01F8MH58A357CV5K7R7TJMSH6S.jpg</url>
      <title>Avatar for @the_mighty_zork@localhost:8080</title>
      <link>http://localhost:8080/@the_mighty_zork</link>
    </image>
  </channel>
</rss>

Checklist

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• I/we have read the GoToSocial contribution guidelines.
• I/we have discussed the proposed changes already, either in an issue on the repository, or in the Matrix chat.
• I/we have performed a self-review of added code.
• I/we have written code that is legible and maintainable by others.
• I/we have commented the added code, particularly in hard-to-understand areas.
• I/we have made any necessary changes to documentation.
• I/we have added tests that cover new code.
• I/we have run tests and they pass locally with the changes.
• I/we have run go fmt ./... and golangci-lint run.

[daenney]

I wonder if this whole section, up to the feed.ToRss(), could be made part of the else block? When lastPostAt.IsZero() is true, we can skip doing the database call entirely in that case. The query GetAccountWebStatuses generates is kinda beefy, although it should be really quick I suppose if there's nothing in it.

I'm fine with either honestly.

[tsmethurst]

Good point! Fixed it now by returning early.

[daenney]

I'm curious as to why we're comparing the .Unix() values instead of t1.After(t2)? Looking at the Go playground, I think using the time functions directly always do what we want: https://go.dev/play/p/z35W9c5ch7M

[tsmethurst]

It's timezone related, basically. I found that doing After wasn't always reliable when you're comparing dates from different timezones, but casting them to unix time always seemed to get the expected result.

edit: there's probably a nicer, go-approved way of doing this, i just found it easier to cast them to unix because it makes sense to me

[daenney]