Add Stats to XML-RPC Errors

class.jetpack-heartbeat.php

@@ -120,6 +120,12 @@ public static function generate_stats_array( $prefix = '' ) {
 		$return["{$prefix}plugins"]        = implode( ',', Jetpack::get_active_plugins() );
 		$return["{$prefix}manage-enabled"] = true;
 
+		$xmlrpc_errors = Jetpack_Options::get_option( 'xmlrpc_errors', array() );
+		if ( $xmlrpc_errors ) {
+			$return["{$prefix}xmlrpc-errors"] = implode( ',', array_keys( $xmlrpc_errors ) );
+			Jetpack_Options::delete_option( 'xmlrpc_errors' );
+		}
+
 		// Missing the connection owner?
 		$connection_manager = new Manager();
 		$return["{$prefix}missing-owner"] = $connection_manager->is_missing_connection_owner();

class.jetpack.php

@@ -574,6 +574,8 @@ private function __construct() {
 			};
 		} );
 
+		add_action( 'jetpack_verify_signature_error', array( $this, 'track_xmlrpc_error' ) );
+
 		$this->connection_manager = new Connection_Manager();
 		$this->connection_manager->init();
 
@@ -4306,6 +4308,35 @@ function admin_notices() {
 <?php endif;
 	}
 
+	/**
+	 * We can't always respond to a signed XML-RPC request with a
+	 * helpful error message. In some circumstances, doing so could
+	 * leak information.
+	 *
+	 * Instead, track that the error occurred via a Jetpack_Option,
+	 * and send that data back in the heartbeat.
+	 * All this does is increment a number, but it's enough to find
+	 * trends.
+	 *
+	 * @param WP_Error $xmlrpc_error The error produced during
+	 *                               signature validation.
+	 */
+	function track_xmlrpc_error( $xmlrpc_error ) {
+		$code = is_wp_error( $xmlrpc_error )
+			? $xmlrpc_error->get_error_code()
+			: 'should-not-happen';
+
+		$xmlrpc_errors = Jetpack_Options::get_option( 'xmlrpc_errors', array() );
+		if ( isset( $xmlrpc_errors[ $code ] ) && $xmlrpc_errors[ $code ] ) {
+			// No need to update the option if we already have
+			// this code stored.
+			return;
+		}
+		$xmlrpc_errors[ $code ] = true;
+
+		Jetpack_Options::update_option( 'xmlrpc_errors', $xmlrpc_errors, false );
+	}
+
 	/**
 	 * Record a stat for later output.  This will only currently output in the admin_footer.
 	 */

packages/options/legacy/class.jetpack-options.php

@@ -61,6 +61,7 @@ public static function get_option_names( $type = 'compact' ) {
 					'static_asset_cdn_files',      // (array) An nested array of files that we can swap out for cdn versions.
 					'mapbox_api_key',              // (string) Mapbox API Key, for use with Map block.
 					'mailchimp',                   // (string) Mailchimp keyring data, for mailchimp block.
+					'xmlrpc_errors',               // (array) Keys are XML-RPC signature error codes. Values are truthy.
 				);
 
 			case 'private':