fix: stop mutating the shared PagePresenter when wrapping the index block

lib/activeadmin_table_footer/index_as_table_extension.rb

@@ -7,17 +7,20 @@ module ActiveadminTableFooter
   module IndexAsTableExtension
     def build(page_presenter, collection)
       footer_proc = page_presenter[:footer_data]
+      return super unless footer_proc && page_presenter.block
 
-      if footer_proc && page_presenter.block
-        original_block = page_presenter.block
-        wrapped = lambda do |table|
-          table.instance_variable_set(:@footer_data_proc, footer_proc)
-          instance_exec(table, &original_block)
-        end
-        page_presenter.instance_variable_set(:@block, wrapped)
+      original_block = page_presenter.block
+      wrapped = lambda do |table|
+        table.instance_variable_set(:@footer_data_proc, footer_proc)
+        instance_exec(table, &original_block)
       end
 
-      super
+      # Render against a copy. ActiveAdmin builds one PagePresenter per
+      # `index do ... end` when it loads the resource and hands that same
+      # object to every render, so writing the wrapper back into the presenter
+      # would wrap the previous wrapper once per request — growing retained
+      # closures and block call depth until a render raises SystemStackError.
+      super(ActiveAdmin::PagePresenter.new(page_presenter.options, &wrapped), collection)
     end
   end
 end

spec/features/index_presenter_mutation_spec.rb

@@ -0,0 +1,47 @@
+# frozen_string_literal: true
+
+require "rails_helper"
+
+# Regression guard for a shared-state bug in IndexAsTableExtension.
+#
+# ActiveAdmin builds one PagePresenter per `index do ... end` when it loads the
+# resource, and `get_page_presenter` hands that same object to every render
+# (`ActiveAdmin::Views::Pages::Index#main_content`). Writing the footer wrapper
+# back into that presenter therefore wrapped the previous wrapper on each
+# render: retained closures and block call depth grew by one per request, until
+# rendering an index raised SystemStackError in a long-lived process (~6k
+# renders). Keep the presenter untouched — render against a copy.
+RSpec.describe "Index page presenter across repeated renders", type: :feature do
+  let!(:plan)         { Plan.create!(name: "Basic", monthly_price: 10) }
+  let!(:customer)     { Customer.create!(name: "Acme") }
+  let!(:subscription) { Subscription.create!(customer: customer, plan: plan, seats: 4) }
+
+  def index_page_presenter
+    ActiveAdmin.application.load!
+    ActiveAdmin.application.namespaces[:admin]
+               .resources
+               .find { |resource| resource.resource_name.to_s == "Subscription" }
+               .get_page_presenter(:index)
+  end
+
+  it "leaves PagePresenter#block the very same object after repeated renders" do
+    presenter = index_page_presenter
+    original_block = presenter.block
+
+    5.times { visit "/admin/subscriptions" }
+
+    expect(presenter.block).to equal(original_block)
+  end
+
+  it "still renders the footer on every render" do
+    3.times do
+      visit "/admin/subscriptions"
+
+      within_table_for("subscriptions") do
+        within_table_footer do
+          expect(page).to have_table_cell(column: "Seats", exact_text: "4")
+        end
+      end
+    end
+  end
+end