override omniauth register

app/overrides/omniauth_registrations_controller_override.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+# A command with all the business logic to create a user from omniauth
+module OmniauthRegistrationsControllerOverride
+  extend ActiveSupport::Concern
+
+  included do
+    before_action :ensure_user_exists, only: :google_oauth2
+
+    def ensure_user_exists
+      form_params = user_params_from_oauth_hash || params[:user]
+      @form = form(Decidim::OmniauthRegistrationForm).from_params(form_params)
+      email = @form.email || verified_email
+
+      return if email.present? && Decidim::User.exists?(email: email, organization: current_organization)
+
+      # No registration allowed
+      Rails.logger.info "WARNING: Attempt to register via OAuth: #{email}"
+      flash[:alert] = I18n.t("email_not_registered", email: email)
+      redirect_to decidim.root_path
+    end
+  end
+end

config/initializers/omniauth_blocker.rb

@@ -0,0 +1,3 @@
+# frozen_string_literal: true
+
+Decidim::Devise::OmniauthRegistrationsController.include(OmniauthRegistrationsControllerOverride)

config/locales/en.yml

@@ -1,2 +1,3 @@
 ---
-en:
+en:
+  email_not_registered: Sorry, the email %{email} is not registered. Only existing users are allowed to login.

config/locales/fr.yml

@@ -0,0 +1,3 @@
+---
+fr:
+  email_not_registered: Désolé, l'email %{email} n'est pas enregistré. Seuls les utilisateurs existants sont autorisés à se connecter.

config/secrets.yml

@@ -51,6 +51,11 @@ development:
     developer:
       enabled: true
       icon: phone
+    google_oauth2:
+      enabled: true
+      icon_path: decidim/brands/google.svg
+      client_id: <%= ENV["OMNIAUTH_GOOGLE_CLIENT_ID"] %>
+      client_secret: <%= ENV["OMNIAUTH_GOOGLE_CLIENT_SECRET"] %>
 
 test:
   <<: *default

spec/lib/overrides_spec.rb

@@ -9,6 +9,8 @@
   {
     package: "decidim-core",
     files: {
+      # controllers
+      "/app/controllers/decidim/devise/omniauth_registrations_controller.rb" => "05bc35af4b5f855736f14efbd22e439b",
       # layouts
       "/app/views/layouts/decidim/_head_extra.html.erb" => "1b8237357754cf519f4e418135f78440",
       # cells