Feature: #94402 - Generate error pages via TYPO3-internal sub-request

See Issue #94402

Description

Error pages (such as 404 - not found, or 403 - access denied) may now be generated via a TYPO3-internal sub-request instead of an external HTTP request (cURL over Guzzle).

This feature is disabled by default, as there are some cases where stateful information is not correctly reset for the subrequest. It may be enabled on an experimental basis via a feature flag called subrequestPageErrors in the “Settings” module.

This change will default to enabled in a future version once all stateful services are identified and removed.

Impact

Generating error pages internally reduces the server load drastically, and solves various issues when dealing with load-balanced systems where the hostname of the server might not match the public-facing server (“front server”).

However, in some cases there might be problems with third-party extensions that override super globals (e.g. $_GET and $_POST), where the option could be disabled. There are also some remaining cases in core of stateful services that, in some configurations, result in incorrect error pages being generated. For that reason the feature defaults to disabled for now.

This feature is only relevant for site configurations loading error pages from a different Page ID.