WordPress get_post_type() doesn’t work when restoring a trashed post via undo

I have a custom post type that uses the following hook:

add_action(
  'untrash_post',
  array($this, 'custom_restore_function')
);

And the custom function looks as follows:

function custom_restore_function() {
  if(get_post_type() == 'custom_post_type') {
    $this->set_custom_post_type_display_status(1);
  }
}

I have a very similar setup for custom_save_function, custom_trash_function, and custom_delete_function, and in all instances, get_post_type() works like a charm.

When you use the Trash action link to move a post to the trash, the link looks something like this:

post.php?post=118&action=trash&_wpnonce=910f2d7f30

And if you choose to restore that post from the trash screen via the Restore action link, it looks very similar:

post.php?post=118&action=untrash&_wpnonce=ddb4c9c68b

The problem is that when you trash a post and then decide to restore it again via the Undo link in the confirmation message, the custom_restore_function will fail.

The Undo link looks something like this:

edit.php?post_type=custom_post_type&doaction=undo&action=untrash&
ids=118&_wpnonce=ea0d2a97d2

And as you can see, the format is a bit different.

After doing more research and taking a closer look inside post.php to see what get_post_type() is actually doing:

function get_post_type($the_post = false) {
  global $post;
  if (false === $the_post)
    $the_post = $post;
  else if(is_numeric($the_post))
    $the_post = get_post($the_post);
  if (is_object($the_post))
    return $the_post->post_type;
  return false;
}

It appears that $post doesn’t contain the post when using the Undo action link. I would be interested in knowing why?

Nevertheless, the easiest way to solve this problem was to modify the if statement to check for the post_type query string, which fortunately is available inside the custom_restore_function:

function custom_restore_function() {
  if(get_post_type() == 'custom_post_type'
  || $_GET['post_type'] == 'custom_post_type') {
    $this->set_custom_post_type_display_status(1);
  }
}

Featured image by Dietmar Becker.