Well, if we have some configuration set based on the presence of an extension (or the existence of a class provided by an extension), this would prevent purging the cache when needed

I know, but I also consider this an edge-case. When you enable a module, it makes sense to use a clear cache. If your feature doesn't work after enabling the module, first thing you should do: clear your cache.

Perhaps this check can be done elsewhere (WDT?) and show that the modules have changed between 2 requests.

but if the cache was created in the CLI assuming the extension was there, and there you PHP-FPM process does not have it, your project is broken too

I don't have have PHP-FPM and my project is not broken in the web because it doesn't have PCNTL. In this case, not having PCNTL in the web, is by design.

Another solution that I've discussed with colleagues here, is making it configurable via possibly a composer extra setting.

@stof, are you aware this is broken now for every setup where the CLI and Web extensions differ? (for instance, not having an apachehandler in CLI makes sense, since you do not need it).

The current behavior triggers a container rebuild forevery request for no reason.

I don't get why you say this is triggering a rebuild forevery request.
Only the first web request should do it, then the next ones should be "fresh" from this resource's pov, isn't it?

@nicolas-grekas That's what I expected, but is not the case, most likely because we run commands on the CLI during cache warmup.

But even in this case, calling the console would again trigger a rebuild, switching back to the web, another one.

@nicolas-grekas ah yes, what@iltar says.

During a request when the cache needs a warmup we run a CLI command, which triggers this bug again and rebuilds the containeragain for CLI. So at the end of that request we are back in "CLI-config" so the next request will do the same rebuild. Hence why it happens with every request.

Let's do that and maybe add another more fine grained resource in 3.4 if we want to - tracking only the actually used extensions.

Thank you@iltar.