atom.xml: don't assume <updated> to be last updated post

The user may have triggered an manual regeneration, so it may not be
accurate to assume that the feed's last-modified date is the same as
that of the last updated post.

We change <updated> and <id>, but leave <rights> alone, since the
copyright year should rely on the post's update date.

generators.py

@@ -242,12 +242,15 @@ def generate_resource(cls, post, resource):
     q = models.BlogPost.all().order('-updated')
     # Fetch the 10 most recently updated non-draft posts
     posts = list(itertools.islice((x for x in q if x.path), 10))
+    now = datetime.datetime.now().replace(second=0, microsecond=0)
     template_vals = {
         'posts': posts,
+        'updated': now,
     }
     rendered = utils.render_template("atom.xml", template_vals)
     static.set('/feeds/atom.xml', rendered,
-               'application/atom+xml; charset=utf-8', indexed=False)
+               'application/atom+xml; charset=utf-8', indexed=False,
+               last_modified=now)
     if config.hubbub_hub_url:
       cls.send_hubbub_ping(config.hubbub_hub_url)
 

themes/default/atom.xml

@@ -3,8 +3,8 @@
 <feed xmlns="http://www.w3.org/2005/Atom">
     <title type="text">{{config.blog_name}}</title>
     <subtitle type="html">{{config.slogan}}</subtitle>
-    <updated>{{posts.0.updated|date:"Y-m-d\TH:i:s\Z"}}</updated>
-    <id>tag:{{config.host}},{{posts.0.updated|date:"Y-m-d"}}:atom.xml</id>
+    <updated>{{updated|date:"Y-m-d\TH:i:s\Z"}}</updated>
+    <id>tag:{{config.host}},{{updated|date:"Y-m-d"}}:atom.xml</id>
     <link rel="alternate" type="text/html" hreflang="en" href="http://{{config.host}}{{config.url_prefix}}/" />
     <link rel="self" type="application/atom+xml" href="http://{{config.host}}{{config.url_prefix}}/feeds/atom.xml" />
     <link rel="hub" href="{{config.hubbub_hub_url}}" />