z.js

var Writable = require('stream').Writable;
var util = require('util');

// a lowlevel stderr writer
var TTY = process.binding('tty_wrap').TTY;
var handle = new TTY(2, false);


util.inherits(W, Writable);

function W(opts) {
  Writable.call(this, opts);
}

W.prototype._write = function(chunk, cb) {
  var str = util.format('chunk %j', chunk.toString()) + '\n';
  var req = handle.writeUtf8String(str);
  // here's the problem.
  // it needs to tell the Writable machinery that it's ok to write
  // more, but that the current buffer length is handle.writeQueueSize
  req.oncomplete = afterWrite
  req.cb = cb;
  console.error('before write', handle.writeQueueSize);
}
function afterWrite(status, handle, req) {
  console.error('afterWrite', status, handle.writeQueueSize);
  req.cb();
}

var w = new W
w.write('foo');
w.write('bar');
w.write('baz');
process.exit();