Avoid notices in `get_post_comments_feed_link()`

[dd32]

When an atom feed request is made to a non-existent page, get_post_comments_feed_link() produces PHP Notices such as the following:

E_NOTICE: Trying to get property 'post_type' of non-object in wp-includes/link-template.php:752

Example of a URLs which can trigger this is:

OPTIONS /2021/01/feed/atom/atom/
GET /404-page/feed/atom/

(Options bypasses the canonical redirect, and outputs a post-comments feed for the page "2021/01/feed/atom")

While the code path that results in this function being called without a valid post is unexpected and the resulting url generated in the atom feed is invalid, it silences these notices, which is good for sites that log/monitor these things (Such as WordPress.org)

[mukesh27]

Hi there!

I checked the 52814.diff​ and found one doc block change.

* @return string|false The permalink for the comments feed for the given post.

Replace to

* @return string|false The permalink for the comments feed for the given post. False if the post does not exist.

Trac ticket: https://core.trac.wordpress.org/ticket/52814

• Adds unit test
• Applies 52814.diff props to @dd32
• Replaces implicit loose comparison in guard clause to bail out if not an instance of WP_Post. Why? WP_Post is the expected data type _if_ the post exists.

[hellofromTonya]

Testing before and after applying the patch. Before: Warning in PHP 8. After: No notice or warning.

[hellofromTonya]

Testing Instructions

Setup:

Create a must-use file in wp-content/mu-plugins/test.php and paste this code into the file:

<?php

add_action( 'loop_start', function() {
    if ( is_admin() ) {
        return;
    }

    $post_id = PHP_INT_MAX;
    $link    = get_post_comments_feed_link( $post_id );
} );

Test

• Load the home page
• In less than PHP 8, a notice is generated. In PHP 8, a warning is generated like this one:

[08-Jun-2021 20:51:04 UTC] PHP Warning:  Attempt to read property "post_type" on null in /[path to your project]/wp-includes/link-template.php on line 752
[08-Jun-2021 20:51:04 UTC] PHP Stack trace:
[08-Jun-2021 20:51:04 UTC] PHP   1. {main}() /[path to your project]/index.php:0
[08-Jun-2021 20:51:04 UTC] PHP   2. require() /[path to your project]/index.php:17
[08-Jun-2021 20:51:04 UTC] PHP   3. require_once() /U[path to your project]/wp-blog-header.php:19
[08-Jun-2021 20:51:04 UTC] PHP   4. include() /[path to your project]/wp-includes/template-loader.php:106
[08-Jun-2021 20:51:04 UTC] PHP   5. the_post() /[path to your project]/wp-content/themes/twentytwenty/index.php:88
[08-Jun-2021 20:51:04 UTC] PHP   6. WP_Query->the_post() /[path to your project]/wp-includes/query.php:975
[08-Jun-2021 20:51:04 UTC] PHP   7. do_action_ref_array() /[path to your project]/wp-includes/class-wp-query.php:3326
[08-Jun-2021 20:51:04 UTC] PHP   8. WP_Hook->do_action() /[path to your project]/wp-includes/plugin.php:518
[08-Jun-2021 20:51:04 UTC] PHP   9. WP_Hook->apply_filters() /[path to your project]/wp-includes/class-wp-hook.php:327
[08-Jun-2021 20:51:04 UTC] PHP  10. {closure:/[path to your project]/wp-content/mu-plugins/test.php:3-10}() /[path to your project]/wp-includes/class-wp-hook.php:303
[08-Jun-2021 20:51:04 UTC] PHP  11. get_post_comments_feed_link() /[path to your project]/wp-content/mu-plugins/test.php:9

• Apply the PR
• Retest => Result: Page should render and no warning/notice generated

[hellofromTonya]

Marking for commit.

A few enhancements to Dion's patch:

• Unit test added
• Checks for the instance of WP_Post which is the expected data type if the post exists
• Retains the return signature of string data type for backwards compatibility, documentation, etc by returning an empty string if the post does not exist

[antpb]

In 51121:

Feeds: Avoid notices in get_post_comments_feed_link().

When an feed request is made to a non-existent page, surpress the notice.

Props dd32, SergeyBiryukov, mukesh27, hellofromTonya.
Fixes #52814.

Merged via changeset https://core.trac.wordpress.org/changeset/51121