Possible bug in output buffering for infinite scroll

[arnowelzel]

Also see arnowelzel/lightbox-photoswipe#45 about this.

See the following code fragment:

jetpack/modules/infinite-scroll/infinity.php

Lines 1335 to 1341 in ec88783

	if ( have_posts() ) {
	// Fire wp_head to ensure that all necessary scripts are enqueued. Output isn't used, but scripts are extracted in self::action_wp_footer.
	ob_start();
	wp_head();
	while ( ob_get_length() ) {
	ob_end_clean();
	}

What this does:

1. Start a new output buffer
2. Call wp_head()
3. As long as there is anything in the output buffer (and only then!) delete the buffer and stop buffering

However this causes a lot of problems if wp_head() does not create any output (which might happen in rare cases). Also note that ob_end_clean() only needs to be called once - after calling this, there is no buffer any longer (also see https://www.php.net/manual/en/function.ob-end-clean.php). On the other hand it also must be called every time after using ob_start() and not only if there is anything in the buffer.

My plugin "Lightbox with PhotoSwipe" won't work at least on one site using infinite scroll and the fix by the user was to add ob_end_clean() after the while()-loop. The correct code should be:

 if ( have_posts() ) { 
 	// Fire wp_head to ensure that all necessary scripts are enqueued. Output isn't used, but scripts are extracted in self::action_wp_footer. 
 	ob_start(); 
 	wp_head(); 
 	ob_end_clean();