Add optional hCaptcha support

Fixes #1649

This requires setting `HCAPTCHA_SECRET_KEY` and `HCAPTCHA_SITE_KEY`, then
enabling the admin setting at
`/admin/settings/edit#form_admin_settings_captcha_enabled`

Subsequently, a hCaptcha widget will be displayed on `/about` and
`/auth/sign_up` unless:
- the user is already signed-up already
- the user has used an invite link
- the user has already solved the captcha (and registration failed for another
  reason)

The Content-Security-Policy headers are altered automatically to allow the
third-party hCaptcha scripts on `/about` and `/auth/sign_up` following the same
rules as above.

app/controllers/about_controller.rb

@@ -2,6 +2,7 @@
 
 class AboutController < ApplicationController
   include RegistrationSpamConcern
+  include CaptchaConcern
 
   before_action :set_pack
 
@@ -12,6 +13,7 @@ class AboutController < ApplicationController
   before_action :set_instance_presenter
   before_action :set_expires_in, only: [:more, :terms]
   before_action :set_registration_form_time, only: :show
+  before_action :extend_csp_for_captcha!, only: :show
 
   skip_before_action :require_functional!, only: [:more, :terms]
 

app/controllers/api/v1/accounts_controller.rb

@@ -1,6 +1,8 @@
 # frozen_string_literal: true
 
 class Api::V1::AccountsController < Api::BaseController
+  include CaptchaConcern
+
   before_action -> { authorize_if_got_token! :read, :'read:accounts' }, except: [:create, :follow, :unfollow, :remove_from_followers, :block, :unblock, :mute, :unmute]
   before_action -> { doorkeeper_authorize! :follow, :'write:follows' }, only: [:follow, :unfollow, :remove_from_followers]
   before_action -> { doorkeeper_authorize! :follow, :'write:mutes' }, only: [:mute, :unmute]
@@ -83,7 +85,7 @@ class Api::V1::AccountsController < Api::BaseController
   end
 
   def check_enabled_registrations
-    forbidden if single_user_mode? || omniauth_only? || !allowed_registrations?
+    forbidden if single_user_mode? || omniauth_only? || !allowed_registrations? || captcha_enabled?
   end
 
   def allowed_registrations?

app/controllers/auth/registrations_controller.rb

@@ -2,6 +2,7 @@
 
 class Auth::RegistrationsController < Devise::RegistrationsController
   include RegistrationSpamConcern
+  include CaptchaConcern
 
   layout :determine_layout
 
@@ -15,6 +16,8 @@ class Auth::RegistrationsController < Devise::RegistrationsController
   before_action :require_not_suspended!, only: [:update]
   before_action :set_cache_headers, only: [:edit, :update]
   before_action :set_registration_form_time, only: :new
+  before_action :extend_csp_for_captcha!, only: [:new, :create]
+  before_action :check_captcha!, only: :create
 
   skip_before_action :require_functional!, only: [:edit, :update]
 
@@ -135,4 +138,18 @@ class Auth::RegistrationsController < Devise::RegistrationsController
   def set_cache_headers
     response.headers['Cache-Control'] = 'no-cache, no-store, max-age=0, must-revalidate'
   end
+
+  def sign_up(resource_name, resource)
+    clear_captcha!
+    super
+  end
+
+  def check_captcha!
+    super do |error|
+      build_resource(sign_up_params)
+      resource.validate
+      resource.errors.add(:base, error)
+      respond_with resource
+    end
+  end
 end

app/controllers/concerns/captcha_concern.rb

@@ -0,0 +1,66 @@
+# frozen_string_literal: true
+
+module CaptchaConcern
+  extend ActiveSupport::Concern
+  include Hcaptcha::Adapters::ViewMethods
+
+  CAPTCHA_TIMEOUT = 2.hours.freeze
+
+  included do
+    helper_method :render_captcha_if_needed
+  end
+
+  def captcha_available?
+    ENV['HCAPTCHA_SECRET_KEY'].present? && ENV['HCAPTCHA_SITE_KEY'].present?
+  end
+
+  def captcha_enabled?
+    captcha_available? && Setting.captcha_enabled
+  end
+
+  def captcha_recently_passed?
+    session[:captcha_passed_at].present? && session[:captcha_passed_at] >= CAPTCHA_TIMEOUT.ago
+  end
+
+  def captcha_required?
+    captcha_enabled? && !current_user && !(@invite.present? && @invite.valid_for_use? && !@invite.max_uses.nil?) && !captcha_recently_passed?
+  end
+
+  def clear_captcha!
+    session.delete(:captcha_passed_at)
+  end
+
+  def check_captcha!
+    return true unless captcha_required?
+
+    if verify_hcaptcha
+      session[:captcha_passed_at] = Time.now.utc
+      return true
+    else
+      if block_given?
+        message = flash[:hcaptcha_error]
+        flash.delete(:hcaptcha_error)
+        yield message
+      end
+      return false
+    end
+  end
+
+  def extend_csp_for_captcha!
+    policy = request.content_security_policy
+    return unless captcha_required? && policy.present?
+
+    %w(script_src frame_src style_src connect_src).each do |directive|
+      values = policy.send(directive)
+      values << 'https://hcaptcha.com' unless values.include?('https://hcaptcha.com') || values.include?('https:')
+      values << 'https://*.hcaptcha.com' unless values.include?('https://*.hcaptcha.com') || values.include?('https:')
+      policy.send(directive, *values)
+    end
+  end
+
+  def render_captcha_if_needed
+    return unless captcha_required?
+
+    hcaptcha_tags
+  end
+end