Invalid custom formengine element causing persistence issue in TYPO3 backend

TL;DR: Always make sure, that a custom TYPO3 formengine element returns valid HTML in $resultArray['html']

Some days ago I stumbled across this issue on TYPO3 forge with the title “TCA IRRE does not save, maybe if there are to many objects”. Since I also use inline elements in my extensions and I never experienced the reported problem, I was in doubt, that the problem was related to TYPO3 core, since TYPO3 has not any hard limit for the amount of inline records.

As the issue description was very detailled and the reporter even attached an extension to test the scenario, I took some time to analyse the problem.

First I could not really reproduce the problem and assumed, that it maybe was related to PHP max_input_vars limits on the server the website was running. But it came out, that this was not the reason for the problem, since the reporter increased max_input_vars to a very high value and the problem was still present.

I then again tried to reproduce the problem and this time I was successful. I did not even add many inline elements as suggested in the report, but only added a few elements. So the problem could not be related to the amount of inline elements at all. I therefore checked the TCA of the affected records and noticed, that a custom formengine element was in use in the main record. After I removed the custom formengine element, the problem was gone.

I was however interested in, why the element caused the described problem. After debugging some time, I found out, that the POST parameter doSave was not set to 1, when the custom formengine element was in the TCA. Since the value of doSave is set by JavaScript when the user clicks the save-button in TYPO3 backend, the custom formengine element must in some way be invalid. After my analysis, I found the following 2 errors with the element.

The first problem with the element was, that it used 'type' => 'select' but without actually holding an input field for a value. Formengine only has one single type, where no database field is required and that is the none field (https://docs.typo3.org/m/typo3/reference-tca/main/en-us/ColumnsConfig/Type/None/Index.html).

The second problem was somehow hard to spot, but it was the main reason why new inline elements at some point were not persisted anymore. The HTML markup in $resultArray['html'] of the custom formengine element was not valid HTML. The HTML contained too many closing div elements, which resulted in the whole structure of the TYPO3 backend form being invalid/unexpected. No error was shown in the browser console, but the JavaScript form validation part of formengine was not able to fully validate the form anymore and therefore finally did not set doSave to 1 resulting in changes not being saved.

As a result of the analysis, I created a pull request for the TYPO3 documentation to outline, that custom formengine elements must always return valid HTML to avoid unexpected behavior in TYPO3 backend.