DEV: Modifier to add params to TopicsController redirect url (#26470)

app/controllers/topics_controller.rb

@@ -1255,7 +1255,19 @@ class TopicsController < ApplicationController
       raise(SiteSetting.detailed_404 ? ex : Discourse::NotFound)
     end
 
-    opts = params.slice(:page, :print, :filter_top_level_replies, :preview_theme_id)
+    # Allow plugins to append allowed query parameters, so they aren't scrubbed on redirect to proper topic URL
+    additional_allowed_query_parameters =
+      DiscoursePluginRegistry.apply_modifier(
+        :redirect_to_correct_topic_additional_query_parameters,
+        [],
+      )
+
+    opts =
+      params.slice(
+        *%i[page print filter_top_level_replies preview_theme_id].concat(
+          additional_allowed_query_parameters,
+        ),
+      )
     opts.delete(:page) if params[:page] == 0
 
     url = topic.relative_url

spec/requests/topics_controller_spec.rb

@@ -2317,6 +2317,12 @@ RSpec.describe TopicsController do
       expect(response).to redirect_to("#{topic.relative_url}/42?page=1")
     end
 
+    it "scrubs invalid query parameters when redirecting" do
+      get "/t/#{topic.slug}", params: { silly_param: "hehe" }
+
+      expect(response).to redirect_to(topic.relative_url)
+    end
+
     it "returns 404 when an invalid slug is given and no id" do
       get "/t/nope-nope.json"
 
@@ -2386,6 +2392,28 @@ RSpec.describe TopicsController do
       expect(second_request_queries.count).to eq(first_request_queries.count)
     end
 
+    context "with registered redirect_to_correct_topic_additional_query_parameters" do
+      let(:modifier_block) { Proc.new { |allowed_params| allowed_params << :silly_param } }
+
+      it "retains the permitted query param when redirecting" do
+        plugin_instance = Plugin::Instance.new
+        plugin_instance.register_modifier(
+          :redirect_to_correct_topic_additional_query_parameters,
+          &modifier_block
+        )
+
+        get "/t/#{topic.slug}", params: { silly_param: "hehe" }
+
+        expect(response).to redirect_to("#{topic.relative_url}?silly_param=hehe")
+      ensure
+        DiscoursePluginRegistry.unregister_modifier(
+          plugin_instance,
+          :redirect_to_correct_topic_additional_query_parameters,
+          &modifier_block
+        )
+      end
+    end
+
     context "when a topic with nil slug exists" do
       before do
         nil_slug_topic = Fabricate(:topic)