Recheck captured snapshot before native fragment lookup

app/controllers/concerns/cached_responses.rb

@@ -9,7 +9,14 @@ module CachedResponses
   end
 
   def perform_caching
-    super && @response_cache_version.present? && !ActiveRecord::Base.connection.transaction_open?
+    return false unless super && @response_cache_version.present? && !ActiveRecord::Base.connection.transaction_open?
+
+    # Check again after authentication, before the first native fragment lookup.
+    # Later renders retain this immutable namespace and cannot poison a new epoch.
+    unless defined?(@fragment_cache_snapshot_valid)
+      @fragment_cache_snapshot_valid = @response_cache_version == ResponseCache.instance.version
+    end
+    @fragment_cache_snapshot_valid
   end
 
   # Keep the class store (including shared rate limits) and Rails.cache unchanged.

test/controllers/fragment_cache_test.rb

@@ -61,6 +61,19 @@ class FragmentRenderingTest < ActionDispatch::IntegrationTest
     assert_includes response.body, "foreign conditional body"
   end
 
+  test "a foreign commit after capture bypasses old native fragment lookups" do
+    ResponseCache.instance.stubs(:budget).returns(0)
+    get room_messages_url(@room)
+    assert_response :success
+    MessagesController.any_instance.stubs(:set_version_headers).with do
+      foreign_write("UPDATE action_text_rich_texts SET body = ? WHERE record_type = 'Message' AND record_id = ?", "foreign before fragments", @message.id)
+      true
+    end
+    get room_messages_url(@room), headers: { "If-None-Match" => "unmatched" }
+    assert_response :success
+    assert_includes response.body, "foreign before fragments"
+  end
+
   test "refresh streams recheck native cached creator body and boost presentation" do
     message = messages(:first)
     room = message.room