Add an account endpoint for ActivityPub and link to it on HTML profile pages

app/controllers/api/activitypub/accounts_controller.rb

+# frozen_string_literal: true
+
+class Api::Activitypub::AccountsController < ApiController
+  before_action -> { doorkeeper_authorize! :read }, except: [:show]
+  before_action :require_user!, except: [:show]
+  before_action :set_account, except: [:verify_credentials, :suggestions, :search]
+
+  respond_to :'application/activity+json'
+  respond_to :'application/ld+json; profile="https://www.w3.org/ns/activitystreams#"'
+
+  def show
+    render content_type: :'application/ld+json; profile="https://www.w3.org/ns/activitystreams#"'
+  end
+
+  private
+
+  def set_account
+    @account = Account.find(params[:id])
+  end
+end

app/views/accounts/show.html.haml

@@ -4,6 +4,7 @@
 - content_for :header_tags do
   %link{ rel: 'salmon', href: api_salmon_url(@account.id) }/
   %link{ rel: 'alternate', type: 'application/atom+xml', href: account_url(@account, format: 'atom') }/
+  %link{ rel: 'alternate', type: 'application/ld+json; profile="https://www.w3.org/ns/activitystreams#"', href: "http://mastodon.dev/api/v1/activitypub/accounts/#{@account.username}" }/
 
   %meta{ property: 'og:site_name', content: 'Mastodon' }/
   %meta{ property: 'og:type', content: 'profile' }/

app/views/api/activitypub/accounts/show.rabl

+object @account
+
+node(:'@context')         { 'https://www.w3.org/ns/activitystreams' }
+node(:type)               { 'Person' }
+node(:id)                 { request.original_url }
+node(:url)                { |account| TagManager.instance.url_for(account) }
+node(:name)               { |account| account.display_name }
+node(:preferredUsername)  { |account| account.username }
+node(:summary)            { |account| account.note }
+node(:icon)               { |account| full_asset_url(account.avatar.url(:original)) }

config/routes.rb

@@ -153,6 +153,12 @@ Rails.application.routes.draw do
       end
     end
 
+    namespace :activitypub do
+      resources :accounts do
+        get '/:id', to: 'accounts#show'
+      end
+    end
+
     namespace :web do
       resource :settings, only: [:update]
     end