presto-main/src/main/java/io/prestosql/operator/OperatorContext.java/OperatorContext/recordBlocked
class OperatorContext:    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, executor);
        // Do not register blocked with driver context.  The driver handles this directly.
    }