Split 2FA login into two prompts

app/controllers/auth/sessions_controller.rb

@@ -5,18 +5,33 @@ class Auth::SessionsController < Devise::SessionsController
 
   layout 'auth'
 
-  before_action :configure_sign_in_params, only: [:create]
+  skip_before_action :require_no_authentication, only: [:create]
+  prepend_before_action :authenticate_with_two_factor, if: :two_factor_enabled?, only: [:create]
 
   def create
     super do |resource|
       remember_me(resource)
+      flash[:notice] = nil
     end
   end
 
+  def destroy
+    super
+    flash[:notice] = nil
+  end
+
   protected
 
-  def configure_sign_in_params
-    devise_parameter_sanitizer.permit(:sign_in, keys: [:otp_attempt])
+  def find_user
+    if session[:otp_user_id]
+      User.find(session[:otp_user_id])
+    elsif user_params[:email]
+      User.find_by(email: user_params[:email])
+    end
+  end
+
+  def user_params
+    params.require(:user).permit(:email, :password, :otp_attempt)
   end
 
   def after_sign_in_path_for(_resource)
@@ -28,4 +43,38 @@ class Auth::SessionsController < Devise::SessionsController
       last_url || root_path
     end
   end
+
+  def two_factor_enabled?
+    find_user.try(:otp_required_for_login?)
+  end
+
+  def valid_otp_attempt?(user)
+    user.validate_and_consume_otp!(user_params[:otp_attempt])
+  end
+
+  def authenticate_with_two_factor
+    user = self.resource = find_user
+
+    if user_params[:otp_attempt].present? && session[:otp_user_id]
+      authenticate_with_two_factor_via_otp(user)
+    elsif user && user.valid_password?(user_params[:password])
+      prompt_for_two_factor(user)
+    end
+  end
+
+  def authenticate_with_two_factor_via_otp(user)
+    if valid_otp_attempt?(user)
+      session.delete(:otp_user_id)
+      remember_me(user)
+      sign_in(user)
+    else
+      flash.now[:alert] = I18n.t('users.invalid_otp_token')
+      prompt_for_two_factor(user)
+    end
+  end
+
+  def prompt_for_two_factor(user)
+    session[:otp_user_id] = user.id
+    render :two_factor
+  end
 end