`wp_publish_post` does not add default category to auto-drafts

[peterwilsoncc]

It's possible to publish a post without any categories by creating an auto-draft and then calling wp_publish_post.

It looks like this can only happen by transitioning from an auto-draft via code. To reproduce:

<?php
$post = wp_insert_post( /* status: auto-draft */ );
wp_publish_post( $post );

Expected behaviour: wp_publish_post() should add the default category if no categories are set on the post.

Trac ticket: https://core.trac.wordpress.org/ticket/51292

This also includes the unit tests that will be required for scaling term counts but they probably should have existed all along and I couldn't find any that did.

[peterwilsoncc]

Moving the the 5.6 milestone as this will be required for #40351.

Does this need to account for the new default_term argument for custom taxonomies or are they not affected?

@TimothyBJacobs Nice catch, they are also affected. Further changes to come.

@TimothyBJacobs It does now, see 7c9a69b -- any further thought would be welcome.

[peterwilsoncc]

In 51292.2.diff​:

• Adds default terms for taxonomies if required in wp_publish_post()
• Unit tests to ensure default terms are added
• Unit tests to ensure default terms do not override existing terms
• Tests to ensure term counts are modified accordingly. It could be argued that these tests do not belong on this ticket. Writing these tests for another ticket led me to discover this bug so that's why they are included.

Props due to @TimothyBlynJacobs for review on pull request.

[peterwilsoncc]

51292.3.diff​ incorporates final feedback from Timothy Jacobs and Fränk Klein. Committing once I can get confirmation on how to correctly props that latter.

[peterwilsoncc]

In 49000:

Posts, Post Types: Ensure default terms are added by wp_publish_post().

Transitioning posts from auto-draft to publish via wp_publish_post() could result in published posts without the default category or custom taxonomy default terms.

Props frank-klein, TimothyBlynJacobs, peterwilsoncc.
Fixes #51292.