Insert new term on wp_update_term when multiple taxonomies share term being updated

[wonderboymusic]

This bug, and it is a bug, has had real-world consequences for our site management. Here is the scenario, which I think we are all familiar with:

1) I add a term for post_tag: Brooklyn
2) I add a term for taxonomy, location: Brooklyn
3) I add a term for taxonomy, radio_station: Brooklyn
4) I change the name of my radio_station: Brooklyn's Finest
5) My tag and location are now called: Brooklyn's Finest

This sucks. I have if / else'd a few lines of wp_update_term to take this scenario into account. Here is what it does:

1) checks the number of taxonomies a term belongs to
2) If that number is more than 1:

• stores reference to old_tt_id
• calls wp_insert_term (which wil handle dupe and slug checking, etc)
• updates the term_relationships for posts - old_tt_id becomes new_tt_id generated by wp_insert_term
• calls wp_delete_term on the old term_id which will delete the term cache
• changes the function internal $term_id to whatever wp_insert_term returned

3) If count is not more than one, function runs like normal

The result:
The post_tag and location remain "Brooklyn"
The radio_station is "renamed" to "Brooklyn's Finest"

Here is the code stub you can use to test this:

add_action( 'init', function () {
    
    register_taxonomy( 'radio_station', 'post', array( 
        'labels' => array( 'name' => 'Radio Stations' )
    ) );
    
} );

[johnbillion]

Dupe of #5809

This is indeed an infuriating bug when you get hit by it. Please have a read through the comments on #5809 and see if your patch fits the bill. Note that any fix won't get in without unit tests.