ollybach

Trying on different sites / servers , it’s all fine on *nix, but still same issues on win (which I’m using as a dev box)
Just passing on the info as it is here

there’s only this atec one installed and running and 2.3.34 is still the same
(also the same with a default theme and no other plugin enabled)

still broken

We are going round the houses it seems

>but its placement is critical
obviously. like everything else

>unconditionally in wp-config.php,
forget wp-config.php. it was just a brutal example . as we know and stated earlier, it makes no sense to put the constant in there

>If the page is already cached, then the plugin code never runs,
As already said , this is simply not true.
If I add the following to any plugin or your theme’s functions.php for that matter it will run and – on the homepage – the DONOTCACHEPAGE constant will be defined.

/* a simple / crude example */
add_action( 'init', 'dostuff' );
function dostuff( ) {
	//do some conditional logic - in this case check if it's the homepage - and if so, do not serve cached page
	if($_SERVER['REQUEST_URI'] == '/'){
		define( "DONOTCACHEPAGE", true );
	}
return;
}

>because the cached content is served early and execution stops.
As your headers() is tied to the ‘send_headers’ action hook the init hook (and all sorts of others) will fire regardless
So if your headers() function were to listen to DONOTCACHEPAGE
i.e

public static function headers()
{

	if (defined('DONOTCACHEPAGE') && DONOTCACHEPAGE) return; // Skip cache output	

.......etc 

it will stop serving the cached page

>init() calls headers().
exactly
and headers() does not respect DONOTCACHEPAGE – that’s really all I’ve been saying

We must be using a different version of you plugin or the setups differ significantly because none of your statements apply here.

on a cached page (or any page for that matter):

– all my init hooks run just fine
– there does not appear to be a plugin.php in the plugin (?)
– generally all init, plugins_loaded hooks etc seem to execute (though not checked in full detail admittedly )
– my nocache_orderpage most certainly does run in the setup I have here with your plugin
– your headers() also executes just fine

>Page caching, by design, works by skipping the execution of WordPress and PHP entirely when a cached version of the page already exists

Sorry, but that’s patently not the case. (Not in a WordPress context)
All sorts of scripts get executed before the cached page gets served (just fewer)
If it were the case than pages would also be served cached when a user is logged in (as admin or otherwise) which – according to your little green dot / console.log for easy checking ( thanks for that , very useful) – does not happen.
And – to take it to the extreme – adding something like exit(); to the wp_config would not do anything as “PHP doesn’t run”…

Back in the realms of usefulness and perhaps as a proof of concept at least (of course I may be missing things as I have not been digging through everything naturally).
Adding
if (defined('DONOTCACHEPAGE') && DONOTCACHEPAGE) {return;}

right after public static function headers(){....
in atec-wpca-pcache.php seems to do exactly what you are saying cannot be done …

>That’s safe and efficient — the plugin page will never be cached, and the rest of the site still benefits from full-page caching
Yes, I understand that and is the ideal scenario, but not always the case in all circumstances if the constant gets added after a/the page was cached for example

Again, just having a discussion here – happy to stand corrected if I’ve missed something