ErrorException with start_session on wp_cron

[herrvigg]

With WP5.1 i had some new ErrorException raised on calls to wp-cron.php:

session_start(): Cannot start session when headers already sent

Part of my code in functions.php (and other third-party components) was doing this:

<?php
if(!session_id()) {
    session_start(); // worked before WP5.1 (so it is handled before headers sent)
}

I actually did not need that session handling for my CRON so i could disable all this part when DOING_CRON is defined, solving the problem. But it's a workaround.

Peter Wilson is aware of this problem:

This is related to #18738, with further details in the associated dev-note, as sessions can not be created after fastcgi_finish_request() runs.

So he asked me to create this ticket for follow-up. It looks like a small regression but it can be problematic in some cases.

[peterwilsoncc]

A similar error will occur for setting cookies and any other code that attempts to send an HTTP header.

Sending headers on cron tasks is, arguably, best avoided but probably happens on more than one site. Potential solutions I see are:

• close this as wontfix
• constant or filter defaulting to false to prevent unintentional fastcgi_finish_request() calls

[jackennilsen]

I'm facing the same problem with a contact form plugin called VFB Pro. And the famous wpDiscuz has the same problem as well. Please don't close this as wontfix, there a probably many more plugins out there causing similar problems.

[peterwilsoncc]

Replying to jackennilsen:

I'm facing the same problem with a contact form plugin called VFB Pro. And the famous wpDiscuz has the same problem as well.

Thanks for confirming my suspicion this is happening on more than one site. The plugin repo shows ​wpDiscuz as active on 50,000+ sites.

The 5.1.1 RC is due out shortly so I'm afraid this will need to wait until the following minor release. I'll move it to the milestone once it's created.

[peterwilsoncc]

Having considered this, I've decided to close the ticket as wontfix.

The warning is thrown as a result of plugins failing to check the outcome header_sent() prior to starting a session. fastcgi_finish_request() blocks the further sending of headers within PHP.

WordPress's plugin architecture requires such a check before setting headers, a practice followed by WordPress Core's code.

Rather than checking DOING_CRON the recommended approach is to use:

<?php
if ( ! headers_sent() ) {
        header( 'X-Ticket: 46417' );
        // or redirects, starting sessions, setting cookies, etc.
}

[herrvigg]

Yes i think it makes sense, header_sent() should be a better check indeed.
Fine with wontfix for me.

[peterwilsoncc]

I've ​logged a support issue in the wpDiscuz forum, please log a similar ticket in the forums of any other plugins you notice with this problem.

Discussion on this ticket can continue without reopening it, I'll continue to get notifications.

[peterwilsoncc]

#46719 was marked as a duplicate.