Prototype 50x response interceptor

5xx-interceptor.php

@@ -0,0 +1,36 @@
+<?php 
+/**
+ * This plugin intercepts the 50x status code and changes it to 58x
+ * 
+ * The status code will be rewritten at the edge.
+ * 
+ * This is a WPVIP-specific plugin and will only work on WPVIP infrastructure.
+ */
+namespace Automattic\VIP;
+
+use Automattic\VIP\Utils\Context;
+if ( ! ( Context::is_web_request() || Context::is_ajax() ) ) {
+	return;
+}
+
+// We hook on parse_request to side-step the handle_404 logic.
+// Strictly speaking send_headers would be but at that point WP()->handle_404() has already been called, potentially overriding any user 503 to 200. 
+add_action( 'parse_request', '\Automattic\VIP\rewrite_50x_headers', PHP_INT_MIN );
+function rewrite_50x_headers() {
+	$current_response_code = http_response_code();
+	$override_codes        = [
+		503 => [
+			'code'        => 583,
+			'description' => 'Service Unavailable',
+		],
+	];
+
+	// Technically headers_sent() should never be true at this moment.
+	if ( ! headers_sent() && isset( $override_codes[ $current_response_code ] ) ) {
+		// 404 handler may override a 404 with a 200, so we need to ensure that the 404 handler is not triggered.
+		add_filter( 'pre_handle_404', '__return_true' );
+		// The description is important for the core logic, otherwise the non-standard codes get discarded and replaced by 200.
+		// @see wp-includes/functions.php:status_header()
+		status_header( $override_codes[ $current_response_code ]['code'], $override_codes[ $current_response_code ]['description'] );
+	}
+}