Emit per-page Open Graph metadata

Replaces one hardcoded German description and an unrenderable SVG with
per-page title, description, canonical URL, locale and publication date,
plus the card variant or a site-wide default.

3 files changed, 157 insertions, 3 deletions

diff --git a/app/helpers/social_helper.rb b/app/helpers/social_helper.rb
new file mode 100644
index 00000000..e66884a4
--- /dev/null
+++ b/app/helpers/social_helper.rb
@@ -0,0 +1,122 @@
+module SocialHelper
+  # The club's name is a proper noun and identical in both locales.
+  OG_SITE_NAME = "Chaos Computer Club".freeze
+
+  # Facebook's og:locale wants a language_TERRITORY tag, not a bare
+  # language code. Anything not listed falls back to the default locale's
+  # tag rather than emitting something no consumer recognises.
+  OG_LOCALES = { :de => "de_DE", :en => "en_GB" }.freeze
+
+  OG_DEFAULT_IMAGE = "/images/social_default.png".freeze
+
+  # Previews render unpublished drafts behind nothing but a token in the
+  # URL, so these controllers emit no social metadata at all.
+  PREVIEW_CONTROLLERS = %w[shared_previews pages].freeze
+
+  # Search result pages are the classic noindex case. Previews join them
+  # for the reason above.
+  NOINDEX_CONTROLLERS = (PREVIEW_CONTROLLERS + %w[search]).freeze
+
+  # og:image must be an absolute URL with a scheme; a path is ignored.
+  # request.base_url rather than a routing helper, so default_url_options
+  # cannot inject a locale prefix into a static asset path.
+  def og_absolute_url(path)
+    "#{request.base_url}#{path}"
+  end
+
+  def social_meta?
+    !PREVIEW_CONTROLLERS.include?(controller_name)
+  end
+
+  def robots_directive
+    return nil unless NOINDEX_CONTROLLERS.include?(controller_name)
+
+    # nofollow on previews as well, so a crawler that reaches one does not
+    # walk out of it into whatever the draft links to. Search pages omit it,
+    # since following result links is the one useful thing a crawler can do
+    # there.
+    PREVIEW_CONTROLLERS.include?(controller_name) ? "noindex, nofollow" : "noindex"
+  end
+
+  # The headline asset's card if it has one, else the site default.
+  # has_variant?
+  #
+  # The query suffix defeats indefinite crawler caching: FileAttachment
+  # deliberately keeps public URLs stable across a file replacement, so
+  # without it Facebook would serve the superseded card forever.
+  def og_image_url
+    asset = @page&.persisted? ? @page.headline_asset : nil
+
+    if asset&.has_variant?(:og)
+      og_absolute_url("#{asset.upload.url(:og)}?v=#{asset.upload_updated_at.to_i}")
+    else
+      og_absolute_url(OG_DEFAULT_IMAGE)
+    end
+  end
+
+  # Both files are exactly 1200x630, so these are constants either way.
+  def og_image_width
+    FileAttachment::OG_WIDTH
+  end
+
+  def og_image_height
+    FileAttachment::OG_HEIGHT
+  end
+
+  def og_image_alt
+    asset = @page&.persisted? ? @page.headline_asset : nil
+    asset&.has_variant?(:og) ? asset.name.to_s : OG_SITE_NAME
+  end
+
+  def og_title
+    @page&.title.presence || OG_SITE_NAME
+  end
+
+  # Public controllers are deliberately not pinned to the default locale,
+  # so Globalize follows I18n.locale here and the abstract arrives in the
+  # language being served. Abstracts hold markup, hence strip_tags.
+  def og_description
+    text = strip_tags(@page&.abstract.to_s).squish
+    return truncate(text, :length => 200, :separator => " ") if text.present?
+
+    t("layouts.social_meta.site_description")
+  end
+
+  # A rendered page is a piece of content; the aggregate views (search,
+  # tags, gallery) have no @page and are the site itself.
+  def og_type
+    @page&.persisted? ? "article" : "website"
+  end
+
+  def og_published_time
+    @page&.published_at&.iso8601
+  end
+
+  # request.path rather than a routing helper: it is already the locale's
+  # own canonical form -- unprefixed for German, /en/ for English -- and
+  # dropping the query string is what makes it canonical.
+  def og_canonical_url
+    og_absolute_url(request.path)
+  end
+
+  def og_locale
+    OG_LOCALES.fetch((@page&.effective_lang || I18n.locale).to_sym,
+                     OG_LOCALES[I18n.default_locale.to_sym])
+  end
+
+  # Only locales in which this page genuinely has a translation, so a
+  # crawler is not told about a variant that would fall back.
+  def og_locale_alternates
+    return [] unless @page
+
+    (@page.translated_locales.map(&:to_sym) - [og_locale_key]).filter_map do |locale|
+      OG_LOCALES[locale]
+    end
+  end
+
+  private
+
+    def og_locale_key
+      (@page&.effective_lang || I18n.locale).to_sym
+    end
+end
diff --git a/app/views/layouts/_social_meta.html.erb b/app/views/layouts/_social_meta.html.erb
new file mode 100644
index 00000000..9a86d26d
--- /dev/null
+++ b/app/views/layouts/_social_meta.html.erb
@@ -0,0 +1,34 @@
+<% if (directive = robots_directive) %>
+  <meta name="robots" content="<%= directive %>"/>
+<% end %>
+
+<% if social_meta? %>
+  <meta name="description" content="<%= og_description %>"/>
+
+  <link rel="canonical" href="<%= og_canonical_url %>"/>
+
+  <meta property="og:site_name" content="<%= SocialHelper::OG_SITE_NAME %>"/>
+  <meta property="og:type" content="<%= og_type %>"/>
+  <meta property="og:url" content="<%= og_canonical_url %>"/>
+  <meta property="og:title" content="<%= og_title %>"/>
+  <meta property="og:description" content="<%= og_description %>"/>
+
+  <meta property="og:image" content="<%= og_image_url %>"/>
+  <meta property="og:image:width" content="<%= og_image_width %>"/>
+  <meta property="og:image:height" content="<%= og_image_height %>"/>
+  <meta property="og:image:alt" content="<%= og_image_alt %>"/>
+
+  <meta property="og:locale" content="<%= og_locale %>"/>
+  <% og_locale_alternates.each do |alternate| %>
+    <meta property="og:locale:alternate" content="<%= alternate %>"/>
+  <% end %>
+
+  <% if og_published_time %>
+    <meta property="article:published_time" content="<%= og_published_time %>"/>
+  <% end %>
+
+  <%# X ignores og:image unless the card type is declared, and falls back
+      to a small square thumbnail. twitter:image is deliberately absent:
+      with only twitter:card set, X reuses og:image. %>
+  <meta name="twitter:card" content="summary_large_image"/>
+<% end %>
diff --git a/app/views/layouts/application.html.erb b/app/views/layouts/application.html.erb
index 6a31104a..5717cebf 100644
--- a/app/views/layouts/application.html.erb
+++ b/app/views/layouts/application.html.erb
@@ -4,10 +4,8 @@
   <head>
     <meta http-equiv="Content-Type" content="text/html; charset=utf-8"/>
     <meta name="viewport" content="width=device-width,initial-scale=1"/>
-    <meta name="description" content="Der Chaos Computer Club ist eine galaktische Gemeinschaft von Lebewesen für Informationsfreiheit und Technikfolgenabschätzung."/>
 
-    <meta property="og:image" content="https://www.ccc.de/images/chaosknoten.svg" />
-    <meta property="og:description" content="Der Chaos Computer Club ist eine galaktische Gemeinschaft von Lebewesen für Informationsfreiheit und Technikfolgenabschätzung." />
+    <%= render :partial => "layouts/social_meta" %>
 
     <title><%= page_title %></title>
     <link rel="stylesheet" href="<%= mtime_busted_path('/stylesheets/ccc.css') %>">