drupal_get_installed_schema_version() doesn't document that the $module parameter can be NULL

Modified the doc block.

And returning an array in addition to string|int.

When the first parameter is set to NULL then it gets the information about all the modules in the system.

function drupal_get_installed_schema_version($module, $reset = FALSE, $array = FALSE) {
  $versions = &drupal_static(__FUNCTION__, []);

  if ($reset) {
    $versions = [];
  }

  if (!$versions) {
    if (!$versions = \Drupal::keyValue('system.schema')->getAll()) {
      $versions = [];
    }
  }

  if ($array) {
    return $versions;
  }
  else {
    return isset($versions[$module]) ? $versions[$module] : SCHEMA_UNINSTALLED;
  }
}

No, it always gets all versions upon first call. Only when $array is TRUE it returns all. Which is why it needs the array added as return value. The $module is irrelevant/disgarded when $array is TRUE. So, it could be NULL, but its value is ignored/not used. When $array is FALSE, $module shouldn't be NULL and it returns SCHEMA_UNINSTALLED when it does.

@mmjvb, I've added the array as one of the return type and also changed the description for the first parameter.

The description of the return type should mention it returns the version of each discovered module (array), the version of module requested (string), SCHEMA_UNINSTALLED (int) when that module is uninstalled from database.

@mmjvb, I've updated the description.

Oops, I've added the wrong patches previously.

Although I am not native speaker, believe that description not to be a proper sentence. Couldn't find a proper example that quickly on explaining the @return. There might even be standards for it. Maybe somebody else knows.

Thanks everyone for working on this.

Looking into this I find that this has been addressed in #2124069: Convert schema.inc to the update.update_hook_registry service (UpdateHookRegistry). Therefor closing as a duplicate.