1 files changed, 93 insertions, 0 deletions
diff --git a/app/lib/webfinger.rb b/app/lib/webfinger.rb
new file mode 100644
index 000000000..b2374c494
--- /dev/null
+++ b/app/lib/webfinger.rb
@@ -0,0 +1,93 @@
+# frozen_string_literal: true
+
+class Webfinger
+  class Error < StandardError; end
+
+  class Response
+    def initialize(body)
+      @json = Oj.load(body, mode: :strict)
+    end
+
+    def subject
+      @json['subject']
+    end
+
+    def link(rel, attribute)
+      links.dig(rel, attribute)
+    end
+
+    private
+
+    def links
+      @links ||= @json['links'].map { |link| [link['rel'], link] }.to_h
+    end
+  end
+
+  def initialize(uri)
+    _, @domain = uri.split('@')
+
+    raise ArgumentError, 'Webfinger requested for local account' if @domain.nil?
+
+    @uri = uri
+  end
+
+  def perform
+    Response.new(body_from_webfinger)
+  rescue Oj::ParseError
+    raise Webfinger::Error, "Invalid JSON in response for #{@uri}"
+  rescue Addressable::URI::InvalidURIError
+    raise Webfinger::Error, "Invalid URI for #{@uri}"
+  end
+
+  private
+
+  def body_from_webfinger(url = standard_url, use_fallback = true)
+    webfinger_request(url).perform do |res|
+      if res.code == 200
+        res.body_with_limit
+      elsif res.code == 404 && use_fallback
+        body_from_host_meta
+      else
+        raise Webfinger::Error, "Request for #{@uri} returned HTTP #{res.code}"
+      end
+    end
+  end
+
+  def body_from_host_meta
+    host_meta_request.perform do |res|
+      if res.code == 200
+        body_from_webfinger(url_from_template(res.body_with_limit), false)
+      else
+        raise Webfinger::Error, "Request for #{@uri} returned HTTP #{res.code}"
+      end
+    end
+  end
+
+  def url_from_template(str)
+    link = Nokogiri::XML(str).at_xpath('//xmlns:Link[@rel="lrdd"]')
+
+    if link.present?
+      link['template'].gsub('{uri}', @uri)
+    else
+      raise Webfinger::Error, "Request for #{@uri} returned host-meta without link to Webfinger"
+    end
+  rescue Nokogiri::XML::XPath::SyntaxError
+    raise Webfinger::Error, "Invalid XML encountered in host-meta for #{@uri}"
+  end
+
+  def host_meta_request
+    Request.new(:get, host_meta_url).add_headers('Accept' => 'application/xrd+xml, application/xml, text/xml')
+  end
+
+  def webfinger_request(url)
+    Request.new(:get, url).add_headers('Accept' => 'application/jrd+json, application/json')
+  end
+
+  def standard_url
+    "https://#{@domain}/.well-known/webfinger?resource=#{@uri}"
+  end
+
+  def host_meta_url
+    "https://#{@domain}/.well-known/host-meta"
+  end
+end