@Override protected void doEnqueue(Callback<Void> callback) {
        exec.submit(() -> {
          try {
            callsLatch.await();
          } catch (InterruptedException e) {
            callback.onError(e);
          }
          if (fail) {
            callback.onError(new IOException());
          } else {
            callback.onSuccess(null);
          }
        });
      }
This shows that regardless of success or error, we block on completion
zipkin/src/test/java/zipkin2/internal/AggregateCallTest.java