lib/workflow-in-memory-backend.js
function isRunnerIdle(runner_id, callback) {
            if (!runners[runner_id] || (runners[runner_id].idle === true)) {
                return callback(true);
            } else {
                return callback(false);
            }
        }
function isRunnerIdle(runner_id, callback) {
            if (!runners[runner_id] || (runners[runner_id].idle === true)) {
                return callback(true);
            } else {
                return callback(false);
            }
        }
Check if the given runner is idle - runner_id - String, unique identifier for runner - callback - f(boolean)