WP_Term_Query returns an empty array instead of 0

[xParham]

When running a term query using the parent or child_of query parameters, and count as the term fields to query for, if the parent has no descendants the query doesn't respect the fields parameter and returns an empty array instead of 0.

[xParham]

I have attached a patch.

This in particular for me, is causing a fatal error on the edit-tags.php screen, when a filter that I have on the main query for the terms list sets the parent parameter to one with no descendants.

Having an array returned instead of 0 will make the WP_List_Table::set_pagination_args() to fail because the pagination args set in WP_Terms_List_Table::prepare_items() uses wp_count_terms() which returns an array instead of integer in this case, which is then passed as the total_items for the pagination args:

Fatal error: Unsupported operand types in /wp-admin/includes/class-wp-list-table.php on line 284

[westonruter]

@xParham is this a new issue in 4.9 or is it also present in 4.8 and before?

[xParham]

@westonruter this is also present in 4.8 and before.

[westonruter]

According to git-blame it looks like this was introduced in 4.9 via #35381 from [37572].

[xParham]

@westonruter That's when the WP_Term_Query was introduced which borrowed the code from get_terms(). Avoiding the query and returning an empty array if the queried parent/child_of term has no descendants had been in get_terms() since [6118] but there was no count option back then. The bug was actually introduced in [13491] via #10746 when the count option for the fields parameter was added but the part which handled the early bailing of the function wasn't updated to return 0 instead of an empty array if a count is the expected return value.

[birgire]

I think the terms property should also be set as an empty array, so I slightly modified the patch from 42327.patch​ to:

if ( 'count' == $args['fields'] ) {
	return 0;
} else {
	$this->terms = array();
	return $this->terms;
}

to avoid the mismatch between the query() method returning an empty array and the terms property being null, for the case when fields is not count.

42327.2.diff​ contains tests.

[boonebgorges]

Thanks for the patch and especially for the excellent tests.

[boonebgorges]

In 42209:

Improve data types returned from empty hierarchical term queries.

When querying for 'count', ensure that 0 is returned. Otherwise,
ensure that it's an array.

Props xParham, birgire.
Fixes #42327.