FEATURE: Experimental support for group membership via google auth (#14835)

This commit introduces a new site setting "google_oauth2_hd_groups". If enabled, group information will be fetched from Google during authentication, and stored in the Discourse database. These 'associated groups' can be connected to a Discourse group via the "Membership" tab of the group preferences UI. 

The majority of the implementation is generic, so we will be able to add support to more authentication methods in the near future.

https://meta.discourse.org/t/managing-group-membership-via-authentication/175950

lib/auth/result.rb

@@ -21,7 +21,8 @@ class Auth::Result
     :omniauth_disallow_totp,
     :failed,
     :failed_reason,
-    :failed_code
+    :failed_code,
+    :associated_groups
   ]
 
   attr_accessor *ATTRIBUTES
@@ -36,7 +37,8 @@ class Auth::Result
     :name,
     :authenticator_name,
     :extra_data,
-    :skip_email_validation
+    :skip_email_validation,
+    :associated_groups
   ]
 
   def [](key)
@@ -94,6 +96,29 @@ class Auth::Result
     change_made
   end
 
+  def apply_associated_attributes!
+    if authenticator&.provides_groups? && !associated_groups.nil?
+      associated_group_ids = []
+
+      associated_groups.uniq.each do |associated_group|
+        begin
+          associated_group = AssociatedGroup.find_or_create_by(
+            name: associated_group[:name],
+            provider_id: associated_group[:id],
+            provider_name: extra_data[:provider]
+          )
+        rescue ActiveRecord::RecordNotUnique
+          retry
+        end
+
+        associated_group_ids.push(associated_group.id)
+      end
+
+      user.update(associated_group_ids: associated_group_ids)
+      AssociatedGroup.where(id: associated_group_ids).update_all("last_used = CURRENT_TIMESTAMP")
+    end
+  end
+
   def can_edit_name
     !SiteSetting.auth_overrides_name
   end
@@ -167,6 +192,10 @@ class Auth::Result
     username || name || email
   end
 
+  def authenticator
+    @authenticator ||= Discourse.enabled_authenticators.find { |a| a.name == authenticator_name }
+  end
+
   def resolve_username
     if staged_user
       if !username.present? || UserNameSuggester.fix_username(username) == staged_user.username