Reject requests without mandatory custom fields

We need to make sure subscribers cannot be created without custom fields
Users require GDPR consent and we need to make sure there are no way to
create a subscriber without mandatory custom fields

[MAILPOET-1405]

lib/Subscribers/RequiredCustomFieldValidator.php

@@ -0,0 +1,55 @@
+<?php
+
+namespace MailPoet\Subscribers;
+
+use MailPoet\Models\CustomField;
+
+class RequiredCustomFieldValidator {
+
+  /**
+   * @param array $data
+   *
+   * @throws \Exception
+   */
+  public function validate(array $data) {
+    $all_custom_fields = $this->getCustomFields();
+    foreach($all_custom_fields as $custom_field_id => $custom_field_name) {
+      if($this->isCustomFieldMissing($custom_field_id, $data)) {
+        throw new \Exception(
+          __(sprintf('Missing value for custom field "%s"', $custom_field_name), 'mailpoet')
+        );
+      }
+    }
+  }
+
+  private function isCustomFieldMissing($custom_field_id, $data) {
+    if(!array_key_exists($custom_field_id, $data) && !array_key_exists('cf_' . $custom_field_id, $data)) {
+      return true;
+    }
+    if(isset($data[$custom_field_id]) && !$data[$custom_field_id]) {
+      return true;
+    }
+    if(isset($data['cf_' . $custom_field_id]) && !$data['cf_' . $custom_field_id]) {
+      return true;
+    }
+    return false;
+  }
+
+  private function getCustomFields() {
+    $result = [];
+
+    $required_custom_fields = CustomField::findMany();
+
+    foreach($required_custom_fields as $custom_field) {
+      if(is_serialized($custom_field->params)) {
+        $params = unserialize($custom_field->params);
+        if(is_array($params) && isset($params['required']) && $params['required']) {
+          $result[$custom_field->id] = $custom_field->name;
+        }
+      }
+    }
+
+    return $result;
+  }
+
+}