Ajax submission leads to => An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (2 MB) that this server supports

Question

So i'm trying to submit a form using ajax, in my form i have the following:

        $form['actions']['#type'] = 'actions';
        $form['actions']['submit'] = array(
        '#type' => 'submit',
        '#value' => $this->t('OK'),
        '#button_type' => 'primary',
        '#prefix' => '<div class="tc btn_pay relative">',
        '#suffix' => '</div>
                        </div>',
        '#attributes' => array(
            'class' => array('proximabold', 'tc', 'transition', 'no-underline', 'ph3', 'pv2', 'mb2', 'dib', 'white'),
        ),
        '#ajax' => [
            'event' => 'click',
            'callback' => '::updateTable',
            'wrapper' => 'table-wrapper',
            'progress' => [
                'type' => 'throbber',
                'message' => t('Verifying entry...'),
            ],
        ],
    );

and a simple updateTable function:

public function updateTable(array $form, FormStateInterface $form_state) {
    return "Hello World";
}

But when i click on submit i get the error: An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (2 MB) that this server supports.

I knew that the error is server side (Maybe i'm wrong), so i went to php.ini (i use PHP7) and update the following lines:

post_max_size = 6144M
memory_limit = 6144M
upload_max_filesize = 6144M
max_file_uploads = 1000

but without luck, this configuration shouldn't be necessary as the request size is only 1.5KB

Answer 1

By debugging the code I found the solution. The error was triggered from core/lib/Drupal/Core/Form/EventSubscriber/FormAjaxSubscriber.php

if ($exception instanceof BrokenPostRequestException && $request->query->has(FormBuilderInterface::AJAX_FORM_REQUEST)) {
  $this->drupalSetMessage($this->t('An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size (@size) that this server supports.', ['@size' => $this->formatSize($exception->getSize())]), 'error');
  $response = new AjaxResponse();
  $status_messages = ['#type' => 'status_messages'];
  $response->addCommand(new ReplaceCommand(NULL, $status_messages));
  $response->headers->set('X-Status-Code', 200);
  $event->setResponse($response);
  return;
}

The error triggered if the BrokenPostRequestException exception is thrown.

Now if we got to core/lib/Drupal/Core/Form/FormBuilder.php we find that this exception is thrown if the post request is bigger than what is allowed by the server and also if the request doesn't carry the form id.

if ($ajax_form_request && !$request->request->has('form_id')) {
  throw new BrokenPostRequestException($this->getFileUploadMaxSize());
}

It would be better if there was another exception for this case rather than getting the same error for when the post request exceeds the configured allowed size (post_max_size)

In my case it was not a matter of post size request, rather I didn't include the form id in my twig file.

I added the lines containing form_id and form_build_id:

<div class="fr relative filtrehold">
        <div class="filtr_btn  br-pill shadow-2 ttu dark-gray f5 proximabold">Filtres</div>
        <form> //Added this one
            {{ McRefund_build_data.filterform.form_build_id }} // Added
            {{ McRefund_build_data.filterform.form_id }} // Added 
            {{ McRefund_build_data.filterform.RelevesMensuels }}
            {{ McRefund_build_data.filterform.from }}
            {{ McRefund_build_data.filterform.to }}
            {{ McRefund_build_data.filterform.example_select }}
            {{ McRefund_build_data.filterform.actions.submit }}
        </form> //Added this one
</div>
<div class="table_hold bg-white br2">

    <div id="table-wrapper">

        {% for Num_refund,refund in McRefund_build_data.filterform.RefundsContainer['Refunds']['#Refunds_data'] %}
                   ............
                   ............
                   ............
        {% endfor %}
    </div>
</div>

Answer 2

Due to an identical error message ("An unrecoverable error occurred. The uploaded file likely exceeded the maximum file size that this server supports")in a project based on Drupal 8, I just discovered a third use of the same exception discussed above. In my case, the exception was thrown by reaching limit of allowed variables in the php.ini file.

1. Open the file -> vim /etc/php/7.2/apache2/php.ini
2. Locate the line -> /max_input_vars/n
3. Uncomment the line and put -> max_input_vars = 3000
4. Restart Apache -> sudo /etc/init.d/apache2 restart

My two Cents.