[wp-trac] [WordPress Trac] #56171: Unexpected behaviour and data loss with a post type called 'link' and Link Manager plugin active

[WordPress Trac]

#56171: Unexpected behaviour and data loss with a post type called 'link' and Link
Manager plugin active
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 Reporter:  manfcarlo     |      Owner:  (none)
     Type:  defect (bug)  |     Status:  new
 Priority:  normal        |  Milestone:  Awaiting Review
Component:  Taxonomy      |    Version:
 Severity:  critical      |   Keywords:
  Focuses:                |
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 Steps to reproduce:

 0. Install a fresh site (this will be easier to reproduce if the posts
 table has a low `auto_increment` value)
 1. Install the Link Manager plugin (prior to version 3.5 this plugin was
 not required)
 2. Register a post type called `link` (you can use the default args)
 **3. Observe that "Link Categories" is added under the newly registered
 post type in the admin menu**
 4. Publish a post through the `link` post type and take note of its
 numeric ID
 5. Add enough links through the Link Manager until you reach the same
 numeric ID observed in step 4
 6. Assign a link category to the link with the same numeric ID observed in
 step 4 (if it's a fresh site it shouldn't be too many)
 **7. Go back to the `link` post type and observe that the published post
 has received the link category that was assigned to the link in step 6**
 8. Now delete the post permanently
 **9. Go back to the Link Manager and observe that the link has lost its
 category that was assigned in step 6**

 This bug is caused because the taxonomy `link_category` is registered for
 the string `link`. [https://github.com/WordPress/WordPress/blob/6.0/wp-
 includes/taxonomy.php#L136 (See code)]

 Interestingly, the function `register_taxonomy_for_object_type`
 [https://github.com/WordPress/WordPress/blob/6.0/wp-
 includes/taxonomy.php#L734 rejects any string that is not registered as a
 post type.] Of course, if the same check were included for immediate
 registration through `register_taxonomy`, link categories could not exist
 at all.

 I discovered this while trying to get a clear answer on whether taxonomies
 were actually supported for non-post entities. I was nearly convinced they
 were not, until I noticed that link categories exist, and now it looks to
 me that no one knows the answer.

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Ticket URL: <https://core.trac.wordpress.org/ticket/56171>
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