Canonical redirect and user_trailingslashit()

[mbis]

Hi,

this bug is quite difficult to spot and it appears only when redirect_canonical() function is used to force the canonical redirect on front page and the trailing slashes are not added to the permalinks. Happily it is nothing serious.

How to reproduce this bug?

Step 1. Remove the trailing slashes from WP permalinks:
First of all, you need to use "Custom structure" mode in Permalinks settings and remove the trailing slash (/%postname%/ => /%postname%) in the adjacent input field. Therefore, the value of $wp_rewrite->use_trailing_slashes is changed to false.

Step 2. Trigger the canonical redirect on front page
Now let's disable the feed URL by redirecting the visitors trying to access it:

<?php
function disable_feed() {
        if(is_feed() && is_front_page()) {
                redirect_canonical();
        }
}
add_action('template_redirect', 'disable_feed', 1);

Now, when one tries to access the feed URL linked to front page, (eg. ​http://example.com/feed), the above hook will be triggered and visitor redirected to the canonical URL (​http://example.com/feed => ​http://example.com).

The problem is that the PHP notice is displayed then:
<b>Notice</b>: Undefined index: path in <b>/var/www/vhosts/maciejbis.net/example.com/wp-includes/canonical.php</b> on line <b>576</b><br />

I guess that the source of the problem is here (line 501):
https://core.trac.wordpress.org/browser/tags/5.5.1/src/wp-includes/canonical.php#L501

$redirect['path'] = user_trailingslashit( $redirect['path'] );

Unfortunately because of user_trailingslashit() function, the slash is removed and value is set to null. The value of $redirect['path'] should not be changed in this particular case (value '/' should be preserved).

My first suggestion is to exclude user_trailingslashit() function there and not use it when value of $redirect['path'] variable is just a single slash '/':

$redirect['path'] = ( $redirect['path'] === '/' ) ? $redirect['path'] : user_trailingslashit( $redirect['path'] );

Another idea is to do the same but directly in user_trailingslashit() function:

https://core.trac.wordpress.org/browser/tags/5.5.1/src/wp-includes/link-template.php#L51

Basically this piece of code:

if ( $wp_rewrite->use_trailing_slashes ) {
        $string = trailingslashit( $string );
} else {
        $string = untrailingslashit( $string );
}

can be replaced with:

if ( $wp_rewrite->use_trailing_slashes ) {
        $string = trailingslashit( $string );
} else if ( $string !== '/' ) {
        $string = untrailingslashit( $string );
}

[mbis]

Permalink settings

Both code changes should prevent malformed output of _user_trailingslashit()_ function when the input value is a single slash:
$foo = user_trailingslashit('/');

This is the case, when the trailing slashes are removed from WP permalinks:
$wp_rewrite->use_trailing_slashes = false

and _redirect_canonical()_ function is used to force the canonical redirect on front page.

Trac ticket: https://core.trac.wordpress.org/ticket/51725

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[SergeyBiryukov]

Hi there, welcome to WordPress Trac! Thanks for the report and the PR.

Just linking to some related tickets here: #34353, #39827, #41577.

[bobz]

This was the fix in my case, I'm using relative permalinks without trailing slash.

<?php
add_filter( 'redirect_canonical', function( $redirect_url, $requested_url ) {

    $redirect  = parse_url( $redirect_url );
    $requested = parse_url( $requested_url );

    // Prevent redirect to exact same page
    if ( strtolower($redirect['path']) == strtolower($requested['path']) )
    {
        return false;
    }

}, 101, 2 );

[lopo]

#55653 was marked as a duplicate.