[fterra]

Hey, your add_action’s second parameter is different from your function’s name!

[fterra]

This would be more appropriate:

function my_pre_post_update_hook($post_ID) {
	if (bbp_is_forum($post_ID)) {
		global $my_forum_old_status;
		$my_forum_old_status = bbp_get_forum_visibility($post_ID);
	}
}
add_action( 'pre_post_update', 'my_pre_post_update_hook' );

[fterra]

This is funny.
I found out that pre_post_update hook – which at first sight seemed to fire only when updating – will fire if the post (or forum) has already been auto-saved AS A DRAFT.
Similarly I have also been surprised that “save_post_{$post->post_type}” third parameter $update will always be true after it’s first auto-saved AS A DRAFT.
So to know if it’s a new forum (or post), I guess you have to check its previous status. For new posts the previous status will be either ‘auto-draft’, ‘draft’ or I guess it’s also possible there will be no previous status yet.

So you can do something like hook to pre_post_update and store its current (which soon in the same request will become “previous”) status. Like this:

function my_pre_post_update_hook($post_ID) {
	global $my_forum_old_status;
	$my_forum_old_status = bbp_get_forum_visibility($post_ID);
}
add_action( 'pre_post_update', 'my_pre_post_update_hook', 10 );

So in your bbp_forum_attributes_metabox_save action you can check if it’s a new forum with something like:

	global $my_forum_old_status;
	if (in_array($my_forum_old_status, array(NULL, 'auto-draft', 'draft')))

Warning – this condition is enough because when bbp_forum_attributes_metabox_save action is fired we know the forum’s current status will not be any of these.

I guess you could also do it with the transition_post_status hook, whose arguments are: $new_status, $old_status, $post.

[fterra]

Besides that I just noticed that both "save_post_{$post->post_type}" and wp_insert_post are fired automaticaly during editing, to keep the forum saved as a draft.
So maybe you will benefit more from bbp_forum_attributes_metabox_save really.
Otherwise you will need some checks to see if it’s not a draft auto-save like in bbPress following code – also note the aforementioned hook being fired at the end:
(/includes/admin/forums.php)

  /** 
   * Pass the forum attributes for processing 
   * 
   * @since bbPress (r2746) 
   * 
   * @param int $forum_id Forum id 
   * @uses current_user_can() To check if the current user is capable of 
   *                           editing the forum 
   * @uses bbp_get_forum() To get the forum 
   * @uses bbp_is_forum_closed() To check if the forum is closed 
   * @uses bbp_is_forum_category() To check if the forum is a category 
   * @uses bbp_is_forum_private() To check if the forum is private 
   * @uses bbp_close_forum() To close the forum 
   * @uses bbp_open_forum() To open the forum 
   * @uses bbp_categorize_forum() To make the forum a category 
   * @uses bbp_normalize_forum() To make the forum normal (not category) 
   * @uses bbp_privatize_forum() To mark the forum as private 
   * @uses bbp_publicize_forum() To mark the forum as public 
   * @uses do_action() Calls 'bbp_forum_attributes_metabox_save' with the 
   *                    forum id 
   * @return int Forum id 
   */ 
  public function attributes_metabox_save( $forum_id ) { 
 
      if ( $this->bail() ) return $forum_id; 
 
      // Bail if doing an autosave 
      if ( defined( 'DOING_AUTOSAVE' ) && DOING_AUTOSAVE ) 
          return $forum_id; 
 
      // Bail if not a post request 
      if ( ! bbp_is_post_request() ) 
          return $forum_id; 
 
      // Nonce check 
      if ( empty( $_POST['bbp_forum_metabox'] ) || !wp_verify_nonce( $_POST['bbp_forum_metabox'], 'bbp_forum_metabox_save' ) ) 
          return $forum_id; 
 
      // Only save for forum post-types 
      if ( ! bbp_is_forum( $forum_id ) ) 
          return $forum_id; 
 
      // Bail if current user cannot edit this forum 
      if ( !current_user_can( 'edit_forum', $forum_id ) ) 
          return $forum_id; 
 
      // Parent ID 
      $parent_id = ( !empty( $_POST['parent_id'] ) && is_numeric( $_POST['parent_id'] ) ) ? (int) $_POST['parent_id'] : 0; 
 
      // Update the forum meta bidness 
      bbp_update_forum( array( 
          'forum_id' => $forum_id,  
          'post_parent' => (int) $parent_id 
 ) ); 
 
      do_action( 'bbp_forum_attributes_metabox_save', $forum_id ); 
 
      return $forum_id; 
  } 

[fterra]

If you use wp_insert_post, remember to check if the post being saved is a forum.
Default forum type value is 'forum'.
But you should use bbp_get_forum_post_type() to use its current value in case it has been altered by some filter.
You may also take advantage of the following wordpress dynamic hook:
"save_post_{$post->post_type}"
which you would hook to with something like:
add_action("save_post_" . bbp_get_forum_post_type(), "my_hook");

[fterra]

Yes, @antipole,
after I wrote to you I tried the bbp_forum_attributes_metabox_save hook and it has worked for me.
But I guess it’s also possible to use wp_insert_post hook as suggested by John James Jacoby.
bbp_forum_attributes_metabox_save will most likely fire only in admin page.
On the other hand wp_insert_post will most likely fire both in admin page and front-end. Seems like front-end forum creation requires the use of [bbp-forum-form] shortcode, which I have never tried.

[fterra]

Hi, antipole,
I’m trying to do the same, subscribe all users to new forums and to new topics also.
I’ve been working on it for a while, mainly figuring out how bbPress itself works.
In the case you want to run something when someone creates a new forum from the admin page, my bet is you will want to hook to bbp_forum_attributes_metabox_save action, whose argument is $forum_id.
Hope it helps.