Strange behavior with thumbnails on preview in 4.6

[jbenton]

Since upgrading my site to 4.6, we've had strange behavior regarding featured images on preview. Whenever a preview_nonce appears in a preview URL, an additional &_thumbnail_id=xxxxxxx appears. And at that point, all featured-image thumbnails on the page become that specific image — even when they appear in a different loop or are called through other means. @swissspidy says on Twitter this may be related to this change in 4.6: [38118]

Adding a wp_reset_postdata() does not seem to fix the problem. Screenshots and code available in this tweets in this Twitter thread:

​https://twitter.com/swissspidy/status/766004453807882244

[joemcgill]

@jbenton Thanks for the ticket. I noticed this bug at almost the same time you did, I think. The issue is that the new _wp_preview_post_thumbnail_filter() function fails to check if the $post_id is from the post we're previewing, so it can affect any secondary loops on the preview page.

37697.diff​ adds an additional check to make sure we're only affecting the main post object from the preview page and updates the unit test for this function so it includes the check as well.

[joemcgill]

For those wanting to test this issue, you'll need to add something like the below code after the main loop on a custom page template and then preview a page using that template.

<?php
$test_args = array(
        'post_type'  => 'post',
        'posts_per_page' => 4,
        'meta_query' => array(
                array(
                        'key' => '_thumbnail_id',
                        'compare' => 'EXISTS'
                ),
        ),
        'ignore_sticky_posts' => true,
);

$stories = new WP_Query( $test_args );

while ( $stories->have_posts() ) {
        $stories->the_post();
        if ( has_post_thumbnail() ) {
                the_post_thumbnail();
        }
}

wp_reset_postdata();
?>

[swissspidy]

There needs to be a unit test showing the effect of the additional $post_id != $_REQUEST['preview_id'] check.

In 37697.2.diff​ I added second unit test to demonstrate the fixed behaviour. I also added an empty( $_REQUEST['preview_id'] ) check to _wp_preview_post_thumbnail_filter() and simplified the tests a bit.

• resets inside the tests should happen before the assertions. If an assertion fails, the following could won't be called
• simply doing $GLOBALS['post'] = $old_post; should be enough there

[swissspidy]

Remove file upload

[joemcgill]

37697.4.diff​ is similar to 37697.3.diff​, but adds some additional cleanup to the unit tests to avoid potential test spillage.

[swissspidy]

#37711 was marked as a duplicate.

[swissspidy]

@jbenton do you perhaps have time to test this patch on your site to see if it properly fixes the bug?

[joemcgill]

#37729 was marked as a duplicate.

[swissspidy]

We should get this into trunk to get more feedback.

[joemcgill]

In 38433:

Post Thumbnails: Prevent post thumbnail previews from spilling into other images.

After [38118], when previewing a page with a secondary loop, all post
thumbnails would be filtered to display the post thumbnail for the
page being previewed. This ensures _wp_preview_post_thumbnail_filter()
is only applied if the $post_id of the post meta being filtered is
equal to the post or page being previewed.

Props swisspidy, joemcgill.
Fixes #37697.

[joemcgill]

Fixed in trunk. Reopening for minor.

[jeremyfelt]

I've reproduced this locally and [38433] resolves the issue. It might be nice to break the conditional up a bit in the future so that the properties of $post are checked separately from the existence and values of _thumbnail_id and preview_id, but that's only a readability preference, not a functional concern.

Standard and multisite tests are passing.

[jeremyfelt]

In 38476:

Post Thumbnails: Prevent post thumbnail previews from spilling into other images.

After [38118], when previewing a page with a secondary loop, all post
thumbnails would be filtered to display the post thumbnail for the
page being previewed. This ensures _wp_preview_post_thumbnail_filter()
is only applied if the $post_id of the post meta being filtered is
equal to the post or page being previewed.

Merge of [38433] to the 4.6 branch.

Props swissspidy, joemcgill.
Fixes #37697.