Ajax forms not working for anonymous users on cached pages

Problem/Motivation

Ajax forms (e.g. product attributes in on the commerce add-to-cart form) do not work properly for anonymous users. After the form was submitted for the first time, every subsequent attempt of any other user to reuse the form fails.

Authenticated users are not affected by this problem. Authcache Form has a mechanism built-in capable of cloning the form cache entries required by Ajax forms for authenticated users (see #1946022: Problem with ajax forms for more details).

Proposed resolution

The simplest way to resolve this problem would be to also allow cloning of form cache entries for anonymous users. However this will likely increase the form cache table even further. Also this problem only affects Ajax-enabled forms and therefore this is only a problem for clients having JavaScript enabled. Therefore a better approach would be to try and tap into the Ajax framework and only clone form cache entries when necessary (e.g. only during the first form-related Ajax request). This has the benefit that no work is done unless the user triggers a callback.

Remaining tasks

• Implementation
• Tests

User interface changes

None.

API changes

None.