Make Server-Timing header output more robust

[swissspidy]

Summary

I activated the plugin on my local WP core Docker environment and filtered perflab_server_timing_use_output_buffer to enable output buffering, yet I did not see the Server-Timing header.

The shutdown hook is too late to send the header, but using the ob_start() callback for this works.

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[felixarntz]

@swissspidy Can you share a bit more context? What leads to it being "too late"? I have never encountered the problem, so would like to understand better.

[swissspidy]

When I run this in WP core‘s Docker environment, the callback in the shutdown hook is never fired, so it‘s not sending the header. I assume it‘s because the shutdown handler is called long after the page has been served.
Weirdly it‘s not an issue when using this with LocalWP, just with the core env.

[felixarntz]

@swissspidy That's odd. I have been using this in the WP core Docker env several times without problems 🤔 Maybe it's something more specific than that?

[swissspidy]

IDK 🤷 Maybe I can debug at contributor day or so

[westonruter]

This is the approach that the output buffering approach that the AMP plugin. It has been robust.

The AMP plugin also sends Server-Timing response headers.

[westonruter]

Curious why this action was configured to run early. Wouldn't it be better to run as late as possible?

[felixarntz]

It is run early based on the idea that the template output was already completed when shutdown runs. If shutdown did a bunch of other things (extremely unlikely but possible), then it would inflate the time in the wp-template metric.

[felixarntz]

Will the removal of the ob_get_clean() call in this PR lead to issues? Not very familiar with how output buffers work under the hood, but I always assumed you should also "end" an output buffer that you start.

[westonruter]

Any open buffers are automatically flushed when the page response is finished, per docs:

If output buffering is still active when the script ends, PHP outputs the contents automatically.

[felixarntz]

Tested this in my core dev environment, and it works as expected, also no change in outcomes. LGTM, thanks for the fix!