Nodejs - promises, unhandled termination and memory leak

Question

seeking help from nodejs gurus out there on usage of promises. I have the following test program, in which I invoke an async "q" function that simply throws an exception. This program leaks memory pretty consistently; but the leak goes away if uncomment the .done() call.

Why does the leak happen when the promise is unterminated (i.e. no done() call)? I tried to follow the documentation, but have trouble understanding the explanation of the done() method. Thanks in advance for your help!

Here's my code:

(function() {
  var MAX_ITER_COUNT, Q, iterCount, maxMem, noop, qDoit, test;

  Q = require("q");

  iterCount = 0;

  MAX_ITER_COUNT = 10 * 1000;

  maxMem = 0;

  noop = function() {};

  qDoit = function() {
    var currentMem;
    currentMem = Math.round(process.memoryUsage().heapUsed / 1024 / 1024);
    if (currentMem > maxMem) {
      maxMem = currentMem;
    }
    console.log("" + iterCount + " - memory is: " + currentMem + "/" + maxMem + " MB");
    return Q(10).then(function() {
      throw new Error("X");
    });
  };

  test = function() {
    if (iterCount++ > MAX_ITER_COUNT) {
      console.log("DONE");
      return;
    }

    // ----  If I uncomment the done() call below the leak goes away ----
    return qDoit()["finally"](function() {
      return setImmediate(test);
    })
    //.done(noop, noop, noop);


  };

  Q.onerror = function() {};

  test();

}).call(this);

Answer 1

Answering my own question, hopefully it will help someone.

On digging a bit into the q library code, it looks like all unhandled exceptions are put in an array called unhandledRejections, by default. Not sure why it was implemented like this, but presumably to help developers track down un-handled exceptions. This behavior can be changed by calling Q.stopUnhandledRejectionTracking(). When I did this, the memory leak went away, even without the .done() call.