FIX: fallback to `anonymous` not `user` when suggesting usernames for anonymous users (#15354)

app/services/anonymous_shadow_creator.rb

@@ -42,7 +42,7 @@ class AnonymousShadowCreator
   private
 
   def create_shadow!
-    username = UserNameSuggester.suggest(I18n.t(:anonymous).downcase)
+    username = resolve_username
 
     User.transaction do
       shadow = User.create!(
@@ -76,4 +76,11 @@ class AnonymousShadowCreator
       shadow
     end
   end
+
+  def resolve_username
+    username = I18n.t("anonymous").downcase
+    username = "anonymous" unless UserNameSuggester.sanitize_username(username).present?
+
+    UserNameSuggester.suggest(username)
+  end
 end

spec/services/anonymous_shadow_creator_spec.rb

@@ -78,6 +78,16 @@ describe AnonymousShadowCreator do
 
       expect { AnonymousShadowCreator.get(user) }.to_not raise_error
     end
-  end
 
+    it "falls back to username 'anonymous' if the translation for 'anonymous' consists entirely of disallowed characters" do
+      # use russian locale but do not allow russian characters:
+      I18n.locale = :ru
+      SiteSetting.unicode_usernames = true
+      SiteSetting.allowed_unicode_username_characters = "[äöü]"
+
+      shadow = AnonymousShadowCreator.get(user)
+
+      expect(shadow.username).to eq("anonymous")
+    end
+  end
 end