FIX: system persona state leaking between sites

System personas leaned on reused classes, this was a problem
in a multisite environement cause state, such as "enabled"
ended up being reused between sites.

New implementation ensures state is pristine between sites in
a multisite

Author: SamSaffron

app/models/ai_persona.rb

@@ -226,12 +226,19 @@ def class_instance
 
     persona_class = DiscourseAi::Personas::Persona.system_personas_by_id[self.id]
     if persona_class
-      instance_attributes.each do |key, value|
-        # description/name are localized
-        persona_class.define_singleton_method(key) { value } if key != :description && key != :name
-      end
-      persona_class.define_method(:options) { options }
-      return persona_class
+      return(
+        # we need a new copy so we don't leak information
+        # across sites
+        Class.new(persona_class) do
+          # required for localization
+          define_singleton_method(:to_s) { persona_class.to_s }
+          instance_attributes.each do |key, value|
+            # description/name are localized
+            define_singleton_method(key) { value } if key != :description && key != :name
+          end
+          define_method(:options) { options }
+        end
+      )
     end
 
     ai_persona_id = self.id

spec/models/ai_persona_multisite_spec.rb

@@ -0,0 +1,24 @@
+# frozen_string_literal: true
+
+RSpec.describe AiPersona, type: :multisite do
+  it "is able to amend settings on system personas on multisite" do
+    persona = AiPersona.find_by(name: "Designer")
+    expect(persona.allow_personal_messages).to eq(true)
+    persona.update!(allow_personal_messages: false)
+
+    instance = persona.class_instance
+    expect(instance.allow_personal_messages).to eq(false)
+
+    test_multisite_connection("second") do
+      persona = AiPersona.find_by(name: "Designer")
+      expect(persona.allow_personal_messages).to eq(true)
+      instance = persona.class_instance
+      expect(instance.name).to eq("Designer")
+      expect(instance.allow_personal_messages).to eq(true)
+    end
+
+    persona = AiPersona.find_by(name: "Designer")
+    instance = persona.class_instance
+    expect(instance.allow_personal_messages).to eq(false)
+  end
+end