Fix Optimization Detective compatibility with WooCommerce when Coming Soon page is served

[westonruter]

When testing WooCommerce, I discovered an unexpected result when attempting to access the Shop page:

In my local testing site, I imported some sample products but didn't "launch" the store yet, so WooCommerce is attempting to serve the Coming Soon page to a logged-out user. The relevant code is ComingSoonRequestHandler::handle_template_include() which filters template_include to conditionally return null on FSE themes:

		if ( $is_fse_theme ) {
			// Since we've already rendered a template, return null to ensure no other template is rendered.
			return null;
		} else {
			// In non-FSE themes, other templates will still be rendered.
			// We need to exit to prevent further processing.
			exit();
		}

This causes a fatal error in Optimization Detective:

PHP Fatal error:  Uncaught TypeError: od_buffer_output(): Argument #1 ($passthrough) must be of type string, null given, called in /var/www/html/wp-includes/class-wp-hook.php on line 324 and defined in /var/www/html/wp-content/plugins/optimization-detective/optimization.php:33
Stack trace:
#0 /var/www/html/wp-includes/class-wp-hook.php(324): od_buffer_output(NULL)
#1 /var/www/html/wp-includes/plugin.php(205): WP_Hook->apply_filters(NULL, Array)
#2 /var/www/html/wp-includes/template-loader.php(104): apply_filters('template_includ...', '/var/www/html/w...')
#3 /var/www/html/wp-blog-header.php(19): require_once('/var/www/html/w...')
#4 /var/www/html/index.php(17): require('/var/www/html/w...')
#5 {main}
  thrown in /var/www/html/wp-content/plugins/optimization-detective/optimization.php on line 33

This is because od_buffer_output() is erroneously defined with a strict string type for the method parameter. Well, I suppose WooCommerce is also erroneously returning null when it should instead return an empty string (which has the same effect) since the template_include filter is defined as passing a string and not string|null.

In any case, since we don't actually do anything with the variable we can just remove the typing. The result is the page as expected:

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Co-authored-by: westonruter <westonruter@git.wordpress.org>
Co-authored-by: felixarntz <flixos90@git.wordpress.org>

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[westonruter]

Note that Optimization Detective does not operate on this Coming Soon page because WooCommerce is outputting it inside the template_include filter at priority 10, whereas Optimization Detective starts output buffering in the template_include filter at PHP_INT_MAX. If it were changed to PHP_INT_MIN then it would apply optimizations on this Coming Soon page as well. Something to consider for the future. I had chosen PHP_INT_MAX because I expected that when merged into core it would call ob_start() after the following line in wp-includes/template-loader.php:

$template = apply_filters( 'template_include', $template );

WooCommerce's injecting a template override in the template_include filter as well as Optimization Detective starting output buffering in the template_include filter are both somewhat abuses of this filter.

[westonruter]

I also opened a Woo PR to fix this issue as it could cause problems with other plugins: woocommerce/woocommerce#51751

[felixarntz]

Would be good to include a note here explaining why that's not always a string.

[westonruter]

We do the |mixed type elsewhere for callbacks for filters. But in general the reason is that other plugins may return anything in filter callbacks, so there is no guarantee for the type.

So you want something like the following?

Suggested change

	* @param string|mixed $passthrough Value for the template_include filter which is passed through.
	* @param string|mixed $passthrough Value for the template_include filter which is passed through. (A plugin may have filtered this to return `null` or any other value type.)

[felixarntz]

Ah, nevermind then. I somehow was confused thinking this was a callback to a PHP function like ob_start(), but since it's a filter callback, it's fine without a comment.