Add site.feed.updated = latest_post setting

[Krinkle]

Two main uses cases:

1. Reduce deployment churn in repositories that hold a docsite in
   addition to source code. These currently regenerate and find
   something new commit on every change, even when nothing in the
   site has been changed. The only file changing each time is
   feed.xml.
   For example: https://github.com/qunitjs/qunit/commits/gh-pages

2. Improve reproducibility of the build, as highlighted via
   Respect SOURCE_DATE_EPOCH environment variable (if set) over Time.now jekyll#7187,
   by offering a choice that simply eliminates use of current time entirely,
   not even having to mock it.

[parkr]

Boolean config options are always restrictive. What if we went with something that allowed the user to decide from one of a list of values? Like feed.updated: latest_post would choose the publish time of the latest post, and feed.updated: site_time (default) would be site.time. Something like that, the values could change but it would grant more freedom in the future.

[parkr]

latest_post.last_modified_at

This is a field from another plug-in, jekyll-last-modified-at, I think. (It requires that Git data is available, which isn't always true.) Looking at dependents, I'm not sure it's entirely worth it to support. Why did you add it here?

Also, if you're not going by published time, then you'd need to iterate through every post to get the latest modification date. Modifying an older post wouldn't reset this time.

[Krinkle]

I don't mind supporting the jekyll-last-modified-at plugin, as it seems already supported by jekyll-feed. Good point on looping through, I'll amend the change to do that.

I think supporting it is relevant to the <updated> element. This value would otherwise be limited to original publishing dates only. I imagine people who care about this element to use my new option, would likely still want to increment it when they update existing posts, to ensure readers pick it up. Supporting this plugin, seems like a good option to offer for those people, so that at least there is a way to do that.

6000 dependents compared to 1 million for jekyll-feed is small indeed, but 6000 is a lot of sites. A loose search for both suggests there's at least 4000 sites with their source public on github.com using both. I imagine there's more usage among self-hosted sites, on other Git platforms, and in private repos.

[michaelnordmeyer]

last_modified_at is already being used by jekyll-feed:

jekyll-feed/lib/jekyll-feed/feed.xml

Line 62 in 1ac64bc

<updated>{{ post.last_modified_at | default: post.date | date_to_xmlschema }}</updated>

It was added in 2015 with a dependency to jekyll-last-modified-at, but this dependency was removed in 2016. Now it relies on manually setting last_modified_at in frontmatter:

https://github.com/jekyll/jekyll-feed/search?q=last_modified_at&type=commits

last_modified_at is also supported by jekyll-seo-tag:

https://github.com/jekyll/jekyll-seo-tag/search?q=last_modified_at&type=code

IMHO jekyll-feed shouldn't stop using it and may use it for additional features.

[parkr]

Cc @vincerubinetti and #387

[michaelnordmeyer]

That's a very good idea and I would love to have the feed's <updated> tag have the newest date of all the posts in the feed, but unfortunately a build could have been triggered because the title of the site or anything else non-post-related but feed-included has changed.

The Atom spec states (emphasis mine):

The "atom:updated" element is a Date construct indicating the most recent instant in time when an entry or feed was modified in a way the publisher considers significant.

To make sure that all the other non-entry tags haven't changed since the last build, they have to be cached and compared, which at least Jekyll doesn't do.