Issue #2953246 by abramm, max-kuzomko, greggles, jnettik: Add a migrate...

Issue #2953246 by abramm, max-kuzomko, greggles, jnettik: Add a migrate process plugin to turn email into username for source without username

src/Plugin/migrate/process/EmailRegistrationUserName.php

+<?php
+
+namespace Drupal\email_registration\Plugin\migrate\process;
+
+use Drupal\migrate\ProcessPluginBase;
+use Drupal\migrate\MigrateExecutableInterface;
+use Drupal\migrate\Row;
+
+/**
+ * Transforms the user email to a user name.
+ *
+ * @code
+ * process:
+ *   name:
+ *     plugin: email_registration_user_name
+ *     source: username
+ * @endcode
+ *
+ * Note: unlike normal Email Registration module, this plugin does NOT ensure
+ * the username is unique, so be sure to use 'make_unique_entity_field'
+ * processor like this:
+ *
+ * @code
+ * process:
+ *   format:
+ *   -
+ *     plugin: email_registration_user_name
+ *     source: username
+ *   -
+ *     plugin: make_unique_entity_field
+ *     entity_type: user
+ *     field: name
+ *     postfix: _
+ * @endcode
+ *
+ * @see email_registration_user_insert().
+ *
+ * @MigrateProcessPlugin(
+ *   id = "email_registration_user_name"
+ * )
+ */
+class EmailRegistrationUserName extends ProcessPluginBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  public function transform($value, MigrateExecutableInterface $migrate_executable, Row $row, $destination_property) {
+    // Strip off everything after the @ sign.
+    $new_name = preg_replace('/@.*$/', '', $value);
+    // Clean up the username.
+    return email_registration_cleanup_username($new_name);
+  }
+
+}