How to validate max post size on custom form - drupal

Question

I have a custom module with a multiple managed file upload.
$form['attachments'] = [
'#type' => 'managed_file',
'#multiple' => true,
'#upload_validators' => [
'file_validate_extensions' => ['jpg docx pdf xlsx jpeg png gif'],
'file_validate_size' => [10485760],
],
];
When uploading one file that is more then 10MB it gives via AJAX the correct error that a file can not be larger then 10MB. That works out of the box.
How can I limit the total MB of all files that can be uploaded via this form?
For example:
3 files of 3MB = fine
4 files of 3MB = error.
I have managed to show a message when that happens
public function validateForm(array &$form, FormStateInterface $form_state) {
$max_size = 10485760;
$total_size = 0;
$triggered_element = $form_state->getTriggeringElement();
if($triggered_element['#name'] == 'attachments_upload_button') {
$fids = (array) $form_state->getValue('attachments', []);
if(!empty($fids)) {
$files = File::loadMultiple($fids);
foreach ($files as $key => $uploadedFile) {
$total_size += $uploadedFile->getSize();
if($total_size > $max_size) {
$form_state->setErrorByName('attachments', $this->t('The total maximum size of your file sizes can not be more than 10MB.'));
$form_state->set('attachments',array_pop($fids));
return;
}
}
}
}
}
But I can not seem to remove the last uploaded file. It is still there and it is submitted on form submit. A part from the message, the code does not hold the form from submitting.
I want the last submitted file where the total of all files is > 10M to be removed from the form_state and also the tmp server folder. And ideally via AJAX without loss of field input.
I can't find a solution.
Thanks in advance.
How can I achieve this via ajax.
Ok, turns out it was not that difficult.
With upload validators it is possible to add variables. So I added $form_state as a variable to a custom upload validator.
$form['attachments'] = [
'#type' => 'managed_file',
'#multiple' => true,
'#upload_validators' => [
'file_validate_extensions' => ['jpg docx pdf xlsx jpeg png gif'],
'file_validate_size' => [10485760],
'size_max_upload' => [$form_state],
],
];
And wrote a custom validator function
/**
* Validate maximum size upload.
*
* #param \Drupal\file\FileInterface $file
* File object.
*
* #param \Drupal\Core\Form\FormStateInterface $form_state
* Form state object.
*
* #return array
* Errors array.
*/
function size_max_upload(File $file, &$form_state) {
$errors = [];
$max_size = 10485760;
$total_size = $file->getSize();
$triggered_element = $form_state->getTriggeringElement();
if($triggered_element['#name'] == 'attachments_upload_button') {
$fids = (array) $form_state->getValue('attachments', []);
if(!empty($fids)) {
$files = File::loadMultiple($fids);
foreach ($files as $key => $uploadedFile) {
$total_size += $uploadedFile->getSize();
if($total_size > $max_size) {
$errors[] = t("The total maximum size of your file sizes can not be more than 10MB.");
break;
}
}
}
}
return $errors;
}