Database: Send a 500 status and no-cache headers when using db-error.php or maintenance.php drop-ins

[dineshinau]

Currently, when a site uses a db-error.php or maintenance.php drop-in to show a custom outage page, WordPress skips the status_header() and nocache_headers() calls it normally sends via wp_die(). This means a 500/503 page can get cached as a 200 by a CDN or reverse proxy (e.g. Varnish), leaving a stale error page served long after the outage is resolved.

This adds status_header( 500 ) + nocache_headers() before loading db-error.php, and status_header( 503 ) + nocache_headers() + the existing Retry-After header before loading maintenance.php.

Note there are two separate places that load db-error.php: the dead_db() function in wp-includes/functions.php (used when a query fails mid-request), and a duplicated inline check in WPDB::db_connect() (used when the initial connection attempt fails). Both needed the same fix, since they don't share code.

Tested by forcing a connection failure via invalid wp-config.php credentials (exercises class-wpdb.php) and by creating a .maintenance file (exercises wp_maintenance()), confirming via curl -I that the correct status code and Cache-Control: no-cache headers are now sent in both cases.

Trac ticket: https://core.trac.wordpress.org/ticket/66080

Opened from the WordPress Contributor Toolkit.

[github-actions]

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[github-actions]

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[westonruter]

Note suggestions.

[dineshinau]

@westonruter Addressed your requested changes — fixed the PHPCS issues and corrected the status header from 500 to 503 in the comment above wp_maintenance(). Ready for another look whenever you have time.