WordPress 4.0 upgrade caused 404 on single custom post types

[MarjWyatt]

I was alerted to an issue with viewing single custom post types. I reverted the site to WordPress v3.9.1 and this was not failing.

Today, I went through the tedious process of troubleshooting the problem. When I upgraded to WordPress 4.0, the code failed.

I used the following steps to research the problem further:
1) I enabled the WordPress default theme, the code worked.
2) Re-enabled my theme and disabled all the plugins to determine whether or not there was a plugin conflict. There was not.

I deduced the problem was in the theme code I'd written.

I started with functions.php and found this bit of code:

// Manually protest wsg-intros because WLM doesn't
add_action( 'pre_get_posts', 'protect_wsg_intros' );
function protect_wsg_intros() {
    if ( ( is_post_type_archive ( 'wsg-intros' ) ) && ( ! is_user_logged_in() ) ) {
        wp_redirect( 'http://localhost/wsg/oops-this-content-is-members-only' );
    }
    elseif ( ( is_singular ( 'wsg-intros' ) ) && ( ! is_user_logged_in() ) ) {
        wp_redirect( 'http://localhost/wsg/oops-this-content-is-members-only' );
    }
}

When I remarked out the elseif statement, the single custom post type worked on my theme with all plugins enabled.

Great! Problem solved! But the solution leaves me with a question. Why would that elseif statement cause a 404 on the single custom post type view with the upgrade to WordPress 4.0? Is this a bug?

[MarjWyatt]

I see that I added the code incorrectly and didn't supply keywords. Sorry.

Here is the code/function:

// Manually protest wsg-intros because WLM doesn't
add_action( 'pre_get_posts', 'protect_wsg_intros' );
function protect_wsg_intros() {
    if ( ( is_post_type_archive ( 'wsg-intros' ) ) && ( ! is_user_logged_in() ) ) {
        wp_redirect( 'http://localhost/wsg/oops-this-content-is-members-only' );
    }
    elseif ( ( is_singular ( 'wsg-intros' ) ) && ( ! is_user_logged_in() ) ) {
        wp_redirect( 'http://localhost/wsg/oops-this-content-is-members-only' );
    }
}

[dd32]

Calling template tags, such as is_singular() before the posts have been queried is rather unreliable - unreliable in that WordPress doesn't yet know what the current post/page is on pre_get_posts so it can't reliably tell you what the status of the request is. Running that code on something such as template_redirect should solve that problem though.

That being said, I can't reproduce this in trunk, the is_singular() does nothing since the post isn't loaded, and doesn't affect the query, the single post is still loaded (logged in, or not).

Additionally, you should check ​is_main_query() in pre_get_posts callbacks to prevent also running on other queries (such as theme sidebars listing posts, etc). You should also enable ​WP_DEBUG as that'll have indicated a PHP Notice due to calling is_singular too early.

[MarjWyatt]

Thank you for your response. I have one more question.

So nothing changed from 3.9.1 to 4.0 to cause this to fail after upgrading?

[dd32]

There is the potential something changed between 3.9 and 4.0, but my inability to duplicate the issue you're seeing on 4.1 doesn't allow me to find out if that's the case.

The code would've been broken on 3.9 - is_singular('wsg-intros') would've always returned false. Although there's the possibility it was working because you had a 2nd posts loop later in the page load (or in the sidebar) where it was referencing the main query still.