Add an option to automatically sign-in with an Omniauth provider without showing the GitLab sign-in page

This is useful when integrating with existing SSO environments and we want to use a single Omniauth provider for
all user authentication.

CHANGELOG

@@ -34,6 +34,7 @@ v 7.12.0 (unreleased)
   - You can not remove user if he/she is an only owner of group
   - User should be able to leave group. If not - show him proper message
   - User has ability to leave project
+  - Add an option to automatically sign-in with an Omniauth provider
 
 v 7.11.4
   - Fix missing bullets when creating lists

app/controllers/sessions_controller.rb

@@ -2,6 +2,7 @@ class SessionsController < Devise::SessionsController
   include AuthenticatesWithTwoFactor
 
   prepend_before_action :authenticate_with_two_factor, only: [:create]
+  before_action :auto_sign_in_with_provider, only: [:new]
 
   def new
     redirect_path =
@@ -75,6 +76,21 @@ class SessionsController < Devise::SessionsController
     end
   end
 
+  def auto_sign_in_with_provider
+    provider = Gitlab.config.omniauth.auto_sign_in_with_provider
+    return unless provider.present?
+
+    # Auto sign in with an Omniauth provider only if the standard "you need to sign-in" alert is 
+    # registered or no alert at all. In case of another alert (such as a blocked user), it is safer  
+    # to do nothing to prevent redirection loops with certain Omniauth providers.
+    return unless flash[:alert].blank? || flash[:alert] == I18n.t('devise.failure.unauthenticated')
+    
+    # Prevent alert from popping up on the first page shown after authentication.
+    flash[:alert] = nil 
+    
+    redirect_to omniauth_authorize_path(:user, provider.to_sym)
+  end
+
   def valid_otp_attempt?(user)
     user.valid_otp?(user_params[:otp_attempt]) ||
     user.invalidate_otp_backup_code!(user_params[:otp_attempt])

config/gitlab.yml.example

@@ -182,6 +182,10 @@ production: &base
     # Allow login via Twitter, Google, etc. using OmniAuth providers
     enabled: false
 
+    # Uncomment this to automatically sign in with a specific omniauth provider's without
+    # showing GitLab's sign-in page (default: show the GitLab sign-in page)
+    # auto_sign_in_with_provider: saml
+
     # CAUTION!
     # This allows users to login without having a user account first (default: false).
     # User accounts will be created automatically when authentication was successful.

config/initializers/1_settings.rb

@@ -87,6 +87,8 @@ end
 
 Settings['omniauth'] ||= Settingslogic.new({})
 Settings.omniauth['enabled']      = false if Settings.omniauth['enabled'].nil?
+Settings.omniauth['auto_sign_in_with_provider'] = false if Settings.omniauth['auto_sign_in_with_provider'].nil?
+
 Settings.omniauth['providers']  ||= []
 
 Settings['issues_tracker']  ||= {}

config/initializers/7_omniauth.rb

@@ -12,6 +12,8 @@ if Gitlab::LDAP::Config.enabled?
 end
 
 OmniAuth.config.allowed_request_methods = [:post]
+#In case of auto sign-in, the GET method is used (users don't get to click on a button)
+OmniAuth.config.allowed_request_methods << :get if Gitlab.config.omniauth.auto_sign_in_with_provider.present?
 OmniAuth.config.before_request_phase do |env|
   OmniAuth::RequestForgeryProtection.new(env).call
 end