code: public final void __tryComplete(CountedCompleter caller) {
      CountedCompleter a = this, s = caller;
      for (int c;;) {
        if((c = a.pending) == 0) {
          a.onCompletion(s);
          if ((a = (s = a).completer) == null) {
            s.quietlyComplete();
            return;
          }
        }
        else if (U.compareAndSwapInt(a, PENDING, c, c - 1))
          return;
      }
    }
docs: H2O Hack to get distributed FJ behavior closer to the behavior on local node.
  It allows us to continue in "completion propagation" interrupted by remote task.
  Should *not* be called by anyone outside of RPC mechanism.
  
  In standard FJ, tryComplete is always called by the task itself and the task is thus it's own caller.
  Afterwards, the FJ framework will start propagating the completion up the task tree, walking up the list of completers(parents)
  each time calling onCompletion of the parent with the current node (the child which triggered the completion) being passed as  the "caller" argument.
  
  When there is a distributed task in the chain, the sequence is broken as the task is completed on a remote node.
  We want to be able to continue the completion chain, i.e. the remote task should now call onCompletion of its parent with itself passed as the caller argument.
  We can not call tryComplete on the task, since it has already been called on the remote.
  Instead, we explicitly set the caller argument in this overloaded tryComplete calls
  
  Example:
  
  new RPC(node,task).addCompletor(f(x) {...})
  
  When we receive the reponse, we want to pass task as x to f.
  We call f.__tryComplete(task)
  
  @param caller - The child task completing this