Reuse authorized read pages without stale presentation or CSRF masks

Transfer the C completed-response cache lesson into Rails, keeping authentication, room checks and cookies per request. A persistent read-only SQLite observer detects local and foreign commits and rejects racing admission. Whole-page misses render fresh to avoid stale nested fragments; message ETags reflect token-neutral presentation.

test/models/response_cache_test.rb

@@ -0,0 +1,57 @@
+require "test_helper"
+require "tmpdir"
+
+class ResponseCacheTest < ActiveSupport::TestCase
+  setup do
+    @directory = Dir.mktmpdir
+    @database = File.join(@directory, "cache.sqlite3")
+    SQLite3::Database.new(@database) do |database|
+      database.execute("PRAGMA journal_mode=WAL")
+      database.execute("CREATE TABLE values_for_test (value TEXT)")
+    end
+    ActiveRecord::Base.stubs(:connection_db_config).returns(Struct.new(:database).new(@database))
+    @cache = ResponseCache.new
+    @cache.stubs(:budget).returns(1024)
+  end
+
+  teardown do
+    @cache.clear
+    FileUtils.remove_entry(@directory)
+  end
+
+  test "foreign commits expire entries and reject old render admission" do
+    version = @cache.version
+    @cache.write("page", version, entry("first"))
+    assert_equal "first", @cache.read("page", version)[:body]
+    SQLite3::Database.new(@database) { |database| database.execute("INSERT INTO values_for_test VALUES ('committed')") }
+    assert_nil @cache.read("page", version)
+    @cache.write("page", version, entry("old in-flight render"))
+    assert_nil @cache.read("page", @cache.version)
+    @cache.write("page", @cache.version, entry("fresh"))
+    assert_equal "fresh", @cache.read("page", @cache.version)[:body]
+  end
+
+  test "byte budget evicts oldest and bypasses oversized entries and keys" do
+    version = @cache.version
+    @cache.stubs(:budget).returns(512)
+    @cache.write("first", version, entry("a" * 100))
+    @cache.write("second", version, entry("b" * 100))
+    assert_nil @cache.read("first", version)
+    assert_equal "b" * 100, @cache.read("second", version)[:body]
+    @cache.write("oversized", version, entry("c" * 1000))
+    assert_nil @cache.read("oversized", version)
+    @cache.write("k" * 2049, version, entry("small"))
+    assert_nil @cache.read("k" * 2049, version)
+  end
+
+  test "observer namespaces cannot reuse old shared fragment keys after restart" do
+    version = @cache.version
+    @cache.clear
+    assert_not_equal version, @cache.version
+  end
+
+  private
+    def entry(body)
+      { body: body, marker: "unexposed-marker", headers: {} }
+    end
+end