presto-main/src/main/java/io/prestosql/transaction/InMemoryTransactionManager.java/InMemoryTransactionManager/cleanUpExpiredTransactions
class InMemoryTransactionManager:    private synchronized void cleanUpExpiredTransactions()
    {
        Iterator<Entry<TransactionId, TransactionMetadata>> iterator = transactions.entrySet().iterator();
        while (iterator.hasNext()) {
            Entry<TransactionId, TransactionMetadata> entry = iterator.next();
            if (entry.getValue().isExpired(idleTimeout)) {
                iterator.remove();
                log.info("Removing expired transaction: %s", entry.getKey());
                entry.getValue().asyncAbort();
            }
        }
    }