Fatal error handler references recovery mode email, which may not have actually been sent

[stevegrunwell]

In troubleshooting a client issue, I've stumbled upon a potential cause for some of the trac tickets regarding the recovery email not being sent (e.g. #52201, #51634, #52207, and others):

The recovery mode email is ​sent from within the WP_Recovery_Mode::handle_error() method, but that method is ​only called by the fatal error handler if WP_Recovery_Mode has been initialized.

This initialization ​occurs in wp-settings.php after the MU plugins have been loaded; as a result, WP_Recovery_Mode::handle_error() will not be called for any errors that occur during the loading of MU plugins (or anywhere else prior to initialization).

Meanwhile, ​the messaging for the default error template uses is_protected_endpoint() as its sole criteria for whether it should include the "Please check your site admin email inbox for instructions." sentence.

All together, this means that a fatal error that occurs on a protected endpoint before wp_recovery_mode()->initialize() is called — be it from core or an MU plugin — will trigger an error screen telling the user to look for an email that will never be sent.

It seems like there are two possible solutions to this problem:

1. Initialize recovery mode earlier in the process
2. Update the conditional within WP_Fatal_Error_Handler::display_default_error_template() to only mention the email if one has actually been sent.

[TimothyBlynJacobs]

Thanks for the ticket @stevegrunwell!

Yeah, it is intentional that recovery mode isn't initialized until after MU Plugins have been loaded since we can't "deactivate" MU Plugins.

I think your second suggestion is the right one. We could probably check wp_recovery_mode->is_initialized().

Trac ticket: https://core.trac.wordpress.org/ticket/52560

[jorbin]

In 51076:

Booststrap/Load: Only reference recovery mode email when it can be sent.

The recovery mode email is sent from within the WP_Recovery_Mode::handle_error() method, but that method is only called by the fatal error handler if WP_Recovery_Mode has been initialized. This adjusts the message to only say the email has been sent if it can be sent.

Props reynhartono, stevegrunwell for initial plan.
Fixes #52560.

Merged in https://core.trac.wordpress.org/changeset/51076