presto-main/src/main/java/io/prestosql/operator/DriverContext.java/DriverContext/recordBlocked
class DriverContext:    public void recordBlocked(ListenableFuture<?> blocked)
    {
        requireNonNull(blocked, "blocked is null");

        BlockedMonitor monitor = new BlockedMonitor();

        BlockedMonitor oldMonitor = blockedMonitor.getAndSet(monitor);
        if (oldMonitor != null) {
            oldMonitor.run();
        }

        blocked.addListener(monitor, notificationExecutor);
    }