Incompatible with PHP 8.4 (occasional fatal error)

[indigoxela]

On PHP 8.4, whenever flushing caches, I get a fatal error:

Error: Class "RulesEventSet" not found in rules_get_cache() (line 418 of .../modules/rules/rules.module).

Switching back to PHP 8.1 and the site works again, another try with 8.4 and flushing caches - another fatal.

Edit: However, above fatal is tricky to reproduce. If it happens, it happens constantly, if not ... everything works as expected.
Unclear, what's going on there.

Additionally, there a several deprecation notices (if commenting out the line that causes the fatal):

    Deprecated function: rules_log(): Implicitly marking parameter $element as nullable is deprecated, the explicit nullable type must be used instead in include_once() (line 1595 of ...core/includes/bootstrap.inc).
    Deprecated function: rules_get_event_handler(): Implicitly marking parameter $settings as nullable is deprecated, the explicit nullable type must be used instead in include_once() (line 1595 of ...core/includes/bootstrap.inc).
    Deprecated function: rules_log(): Implicitly marking parameter $element as nullable is deprecated, the explicit nullable type must be used instead in include_once() (line 1595 of ...core/includes/bootstrap.inc).
    Deprecated function: rules_get_event_handler(): Implicitly marking parameter $settings as nullable is deprecated, the explicit nullable type must be used instead in include_once() (line 1595 of ...core/includes/bootstrap.inc).
    Deprecated function: RulesAction::executeCallback(): Implicitly marking parameter $state as nullable is deprecated, the explicit nullable type must be used instead in require_once() (line 4157 of ...core/includes/bootstrap.inc).
    Deprecated function: RulesCondition::executeCallback(): Implicitly marking parameter $state as nullable is deprecated, the explicit nullable type must be used instead in require_once() (line 4157 of ...core/includes/bootstrap.inc).
    Deprecated function: RulesReactionRule::event(): Implicitly marking parameter $settings as nullable is deprecated, the explicit nullable type must be used instead in require_once() (line 4157 of ...core/includes/bootstrap.inc).
    Deprecated function: RulesPluginUI::path(): Implicitly marking parameter $element as nullable is deprecated, the explicit nullable type must be used instead in require_once() (line 4157 of ...core/includes/bootstrap.inc).

In order to actually trigger the fatal, it might be necessary to add a rule on event Backdrop is initializing - the fatal also occurs, if the rule is inactive (disabled), though.

The deprecation nagging's reproducible reliably, so let's fix these.

[indigoxela]

Some more findings. Fixing the deprecation notices is easy, but fixing the fatal is not.

Weird: if I add in a debug($cid); right before RulesEventSet::rebuildEventCache(); the fatal when flushing caches goes away. 🤪 Removing the debug() and the error's back. Heisenbug?

[argiepiano]

Thanks, @indigoxela. I'm trying to reproduce. I do get all the php8.4 notices, but I'm unable to reproduce the fatal error.

This may have to do with other installed modules. Can you please try to reproduce on a clean install?

EDIT: how I'm testing:

1. Clean install. Enable Rules and Rules UI and dependencies (with the dev version of Entity Plus)
2. Created a simple rule that shows a message when new content is created.
3. Cleared caches

[argiepiano]

A quick note: I've seen this fatal in other situations regardless of PHP version. It's hard to reproduce. My hunch is that this has to do with cached objects. If you have a chance, can you manually truncate all your cache tables and try again?

[indigoxela]

I'd like to note that I saw the problem on two different dev sites, with a totally different set of modules installed. Rules was the only (obvious) similarity, but there might be more. I'll try to further dig into that.

All happened on Backdrop 1.31.0 (no D7 upgrade involved), with PHP 8.4.10.
On PHP 8.1 - all good, problem never occurs, and I don't remember to ever have seen it before. On 8.4 fatal on every cache flush.
I'm using a pretty common hosting setup with php-fpm and Apache.

As I already noted - as soon as I sneak in a "debug()", the error doesn't occur anymore. That indicates, that this might be a Heisenbug with some autoloading glitch...

[argiepiano]

PR #249 provided for the PHP 8.4 nullable warnings. Can you please test?

[argiepiano]

I'd like to note that I saw the problem on two different dev sites, with a totally different set of modules installed. Rules was the only (obvious) similarity, but there might be more. I'll try to further dig into that.

Got it. Since I can't reproduce it myself, I'll wait for further details from you. In the meantime. I'm hoping that @bugfolder gives me the green light to merge this and do a new release.

[indigoxela]

Some digging, which doesn't provide much help...

On a totally fresh install the very same problem occurs.

Part of debug info related to contrib:

Contrib
-------
devel                           1.x-1.8.2
entity_plus                     1.x-1.0.22
entity_token                    1.x-2.0.5
entity_ui                       1.x-1.0.2
rules                           1.x-2.14.3
rules_admin                     1.x-2.14.3

The problem can also get triggered with saving a node. Selectively flushing single caches via admin menu doesn't cause a fatal. Flushing "all" caches does.

Fun fact - if I reload the fpm process, the problem goes away for a while. Definitely a Heisenbug. Eventually even a PHP bug?
But a fatal is a fatal...

[argiepiano]

Ouch! This will be a hard one to fix. I enabled all of the above, created two rules (one with "Backdrop is initializing") in PHP 8.4 without any luck in reproducing the fatal.

[indigoxela]

Yeah, let's postpone the fatal and concentrate on the deprecated notices for now.

Will do PR testing tomorrow, many thanks for providing it. 👍