Weird issue with redis object cache, transients, css parsed cache and output

[fidoboy]

Bug Description

Sometimes the AMP pages are displayed without any CSS but restarting or flushing the object cache solves the issue. I suspect that the aproach wich caches the parsed CSS is not working well with redis object cache.

Expected Behaviour

Expected behaviour is that the pages are being displayed with the CSS styles without issues.

Screenshots

No response

PHP Version

8.3

Plugin Version

Latest one

AMP plugin template mode

Transitional

WordPress Version

Latest one

Site Health

No response

Gutenberg Version

No response

OS(s) Affected

No response

Browser(s) Affected

All

Device(s) Affected

All

Acceptance Criteria

No response

Implementation Brief

No response

QA Testing Instructions

Try to test it with a wordpress redis object cache plugin installed

Demo

No response

Changelog Entry

No response

[milindmore22]

Hello @fidoboy

Thank you for reaching out to us, Can you please send your site health information so we can further analyze the issues, please send your site health information using this form

[fidoboy]

It's just a matter about how the AMP plugin works by caching in some way the CSS applied. I'm not sure why, but after A LOT OF TRIAL AND ERROR it works stable... until something breaks that. That's why I'm suggesting you to test it with redis object cache because there are issues. May be a special setup is needed for transients or something like that. What I know is that it is a combination of opcache + object cache + AMP. When it happens, ALL amp pages are being displayed without any CSS applied, and it's needed to restart several times, flushing the caches, etc. It works fine until you install a new plugin or when you update a plugin. Then it breaks and the site starts to throw broken AMP pages without CSS applied

[fidoboy]

Please test it because it's not easy to reproduce it. Sometimes it breaks for no reason and it's needed to restart redis, php and database many times until it works fine again. And I'm almost sure that it's an issue with the CSS cache used by AMP or with the transients (cached when there is a object cache plugin enabled in WP). May be it's that cached data is staled and the AMP plugin doesn't refresh it or it fails with some verification. I don't know about the inner working of the AMP plugin so I can only speculate.

So I don't need any support for this, I'm trying to help by reporting an issue for its verification. It's needed a AMP plugin update with a more solid method to manage the output being served. That's why I'm giving you the clues to point you into the right direction to look for

[milindmore22]

Thanks for the update and for looking into this!

We tried to reproduce the CSS caching problems you've been seeing with the AMP plugin, Redis Object Cache, and PHP OPcache, but we haven't been able to replicate them yet. We understand you're not looking for direct support, and we really appreciate you sharing your observations to help us track down this tricky issue.

Since it's tough to nail down these kinds of intermittent bugs, getting your exact environment details would be a huge help. Could you please tell us:

• Which Redis Object Cache plugin are you using for WordPress?
• What's the version of Redis running on your server?
• What's your precise PHP version?

Knowing these specifics will let us set up a testing environment that closely matches yours, giving us a much better chance at finding the root cause.

[fidoboy]

Thanks for the update and for looking into this!

We tried to reproduce the CSS caching problems you've been seeing with the AMP plugin, Redis Object Cache, and PHP OPcache, but we haven't been able to replicate them yet. We understand you're not looking for direct support, and we really appreciate you sharing your observations to help us track down this tricky issue.

Since it's tough to nail down these kinds of intermittent bugs, getting your exact environment details would be a huge help. Could you please tell us:

• Which Redis Object Cache plugin are you using for WordPress?
• What's the version of Redis running on your server?
• What's your precise PHP version?

Knowing these specifics will let us set up a testing environment that closely matches yours, giving us a much better chance at finding the root cause.

1. The Redis Object Cache. This one: https://wordpress.org/plugins/redis-cache/ with Relay -> https://github.com/cachewerk/relay (but I've also tested it without Relay and it's the same. So I believe that you can test without it, there is no changes)
2. It's 7.2.5 version (output of redis-server --version)
3. It's PHP 8.3.21 (running with php-fpm) and OPCache enabled.

I'll help all that I can, because it's very annoying. And it doesn't happens inmediately but after some time running. I'm not sure why. And it starts to work fine again if I manually restart php-fpm and also the Redis server (also losing all the cached content in Redis, so all transients are being reset and of course all the CSS cached by the AMP plugin). I should say that this doesn't happened with old versions of WP. So may be it's not a matter of the AMP plugin alone but a combination of multiple factors. But who knows, may be that from the AMP plugin side could be posible to enhance the process of managing the cached CSS.

[fidoboy]

The issue happens after some time working the server alone, with no changes or installing anything in WP. There is a point where, suddenly, is starts to return all the AMP pages without the CSS, so all looks ugly and weird. I solve it just with restarting (sometimes I need two or more attempts) the Redis server to force a object cache flush and restarting also the php-fpm service. So I believe that the issue is with the CSS cache method that the AMP plugin is using. May be it becomes outdated or corrupted or anything. I don't know what it is. But what I can see is that restarting services all works again with no issues and the AMP pages are displayed again with the right CSS styles applied.

The PHP log doesn't show anything unusual when this happens.

[fidoboy]

Thanks for the update and for looking into this!

We tried to reproduce the CSS caching problems you've been seeing with the AMP plugin, Redis Object Cache, and PHP OPcache, but we haven't been able to replicate them yet. We understand you're not looking for direct support, and we really appreciate you sharing your observations to help us track down this tricky issue.

Since it's tough to nail down these kinds of intermittent bugs, getting your exact environment details would be a huge help. Could you please tell us:

• Which Redis Object Cache plugin are you using for WordPress?
• What's the version of Redis running on your server?
• What's your precise PHP version?

Knowing these specifics will let us set up a testing environment that closely matches yours, giving us a much better chance at finding the root cause.

Is there any news about this issue? The AMP plugin seems to have troubles when using among Redis and object cache plugins in Wordpress. I have just tested it on a clean Wordpress installation and it happens the same when activating the redis object cache. CSS broken when displaying AMP pages