Implements https://meta.discourse.org/t/issue-user-changed-google-account-and-cant-connect-thru-his-profile/35028/18?u=supermathie

config/locales/server.en.yml

@@ -139,6 +139,7 @@ en:
       max_username_length_range: "You cannot set the maximum below the minimum."
       default_categories_already_selected: "You cannot select a category used in another list."
       s3_upload_bucket_is_required: "You cannot enable uploads to S3 unless you've provided the 's3_upload_bucket'."
+    conflicting_google_user_id: 'The Google Account ID for this account has changed, for protection this requires manual intervention. Please contact the site administrator with the following reference:<br><a href="https://meta.discourse.org/t/76575">https://meta.discourse.org/t/76575</a>'
 
   activemodel:
     errors:

lib/auth/google_oauth2_authenticator.rb

@@ -21,6 +21,19 @@ class Auth::GoogleOAuth2Authenticator < Auth::Authenticator
     if !result.user && !result.email.blank? && result.email_valid
       result.user = User.find_by_email(result.email)
       if result.user
+        # we've matched an existing user to this login attempt...
+        if result.user.google_user_info && result.user.google_user_info.google_user_id != google_hash[:google_user_id]
+          # but the user has changed the google account used to log in...
+          if result.user.google_user_info.email != google_hash[:email]
+            # the user changed their email, go ahead and scrub the old record
+            result.user.google_user_info.destroy!
+          else
+            # same email address but different account? likely a takeover scenario
+            result.failed = true
+            result.failed_reason = I18n.t('errors.conflicting_google_user_id')
+            return result
+          end
+        end
         ::GoogleUserInfo.create({ user_id: result.user.id }.merge(google_hash))
       end
     end