form['#cache'] not respected during rebuilds

Also, a fun tip: if you ever put a SimpleXML object in $form_state, be sure you set $form['#cache'] to FALSE. SimpleXML objects, they don't like to be serialized, see...

You're right -- I only just noticed the shift with the existence of drupal_rebuild_form(). This is still a pretty significant problem, though, as $form['#cache'] SHOULD be respected. Obviously, a form marked as no-cache will not be compatible with various AHAH techniques and so on.

The issue still needs attention; without this change, #cache is all but useless: it only applies to the first time a form is displayed.

Also, I notice that the tweaky handling of form caching when #process callbacks are involved is not duplicated in drupal_rebuild_form(). I assume this is intentional, to avoid double-processing of items, or something like that?

Changing the title to reflect the details.

The easiest example of where this is a problem would be listing pages where a form is clicked, and the results are displayed below the input field. This is easy to do with a combination of form_state and rebuild, but at present it causes the entire form, and the form_state, to be cached unnecessarily.

The second reason for the #cache property was to prevent the cache from bloating with entries that weren't needed. I think I need to put some time into explaining the interplay of #cache and 'rebuild' and 'storage' -- the results are counter-intuitive if you set #cache = FALSE, then rebuild the form. It'll be cached no matter what you say in the #cache flag.

Also, there's a subtle issue: ONLY things that can be serialized can be placed in $form_state, now. Otherwise you'll get errors on the next display of the form. I ran into that with a SimpleXML object -- aparently that's a known issue with the library.

This is true. However, fixing this issue is probably not worth the headaches at the moment -- it doesn't break anything, it just results in counter-intuitive behavior when the #cache flag is used. It allows $form_state['rebuild'] and $form_state['storage'] to trigger form caching regardless of the state of the #cache flag. There are good reasons for that, and good reasons why it could work differently, but for now it shouldn't complicate the D6 release.