Error with the function Perflab_Server_Timing::send_header

[felixxoes]

Bug Description

"Die Funktion Perflab_Server_Timing::send_header wurde fehlerhaft aufgerufen." This error appears on mobile and tablet versions of the website www.mozaikzeitung.ch, possibly in connection with the "PDF Embedder" Plug-in.

Steps to reproduce

1. On your mobile phone or tablet go to 'https://mozaikzeitung.ch/mobilitaet/'
2. Scroll down
3. See error beneath the embedded pdf

Screenshots

Additional Context

• PHP Version: 8.1.32
• WP Version: 6.8.1
• OS: [e.g. iOS] iOS
• Browser: [e.g. chrome, safari] Safari
• Plugin Version: [e.g. 22] 3.9.0
• Device: [e.g. iPhone6] IPhone 12

[kishanjasani]

@felixxoes If you're loading a PDF or rendering a file via readfile(), fpassthru(), or outputting raw binary data, that output flushes headers immediately. So, by the time send_header() is called, it’s too late to send headers.

[westonruter]

@felixxoes In the admin under Tools > Server-Timing, do you have anything entered there?

[felixxoes]

No. Everything is empty.

[b1ink0]

This is most likely happening because the PDF Embedder plugin could be using the template_include filter to render the PDF. However, in Performance Lab, server timing is also using the same filter with PHP_INT_MAX as the priority. As a result, send_header will always execute last. If some plugin starts echoing something, like custom HTML, then headers_sent will return true, causing the error.

performance/plugins/performance-lab/includes/server-timing/class-perflab-server-timing.php

Lines 239 to 244 in ee86977

	public function add_hooks(): void {
	if ( $this->use_output_buffer() ) {
	add_action( 'template_redirect', array( $this, 'start_output_buffer' ), PHP_INT_MIN );
	} else {
	add_filter( 'template_include', array( $this, 'on_template_include' ), PHP_INT_MAX );
	}

performance/plugins/performance-lab/includes/server-timing/class-perflab-server-timing.php

Lines 143 to 151 in ee86977

	public function send_header(): void {
	if ( headers_sent() ) {
	_doing_it_wrong(
	__METHOD__,
	esc_html__( 'The method must be called before headers have been sent.', 'performance-lab' ),
	''
	);
	return;
	}

I also tested the PDF Embedder plugin from the WordPress Plugin Directory (https://wordpress.org/plugins/pdf-embedder/), and this plugin does not cause any errors. The premium version of PDF Embedder seems to have this issue. The premium version appears to use an IFRAME for rendering PDFs for extra security, based on inspecting the provided site. https://mozaikzeitung.ch/mobilitaet/ the PDF iframe has url https://mozaikzeitung.ch/?pdfemb-data=eyJwZGZJRCI6MTAyODIsInVybCI6Imh0dHBzOlwvXC9tb3phaWt6ZWl0dW5nLmNoXC93cC1jb250ZW50XC91cGxvYWRzXC8yMDI1XC8wNlwvbW96YWlrXzIwMjUtMDJfcGFnZXMucGRmIiwiaW5kZXgiOjF9

It could be using a custom query var to render a custom HTML page for displaying the PDF, and doing this would likely require the use of the template_include filter.

---

@felixxoes Is there any chance you might be using the premium version of PDF Embedder?