presto-main/src/main/java/io/prestosql/operator/ExchangeClient.java/ExchangeClient/notifyBlockedCallers
class ExchangeClient:    private synchronized void notifyBlockedCallers()
    {
        List<SettableFuture<?>> callers = ImmutableList.copyOf(blockedCallers);
        blockedCallers.clear();
        for (SettableFuture<?> blockedCaller : callers) {
            // Notify callers in a separate thread to avoid callbacks while holding a lock
            scheduler.execute(() -> blockedCaller.set(null));
        }
    }