test-genny-co-stacks

test.js

var co = require('co');
var genny = require('genny');

genny.longStackSupport = true;

function errorish() { 
  return function throwy(cb) {
    setTimeout(function() { cb(new Error("Oops")); });
  }
}

function delay(t) { return function(cb) { setTimeout(cb, t); } }

co(function *(){
  try {
    var a = yield delay(500);
    var b = yield errorish(); 
    var c = yield delay(500);
    console.log("OK");
  } catch (e) {
      console.error("--- co:");
      console.error(e.stack);
  }
})();

genny.run(function* (resume) {
  try {
    var a = yield setTimeout(resume(), 500);
    var b = yield errorish()(resume()); 
    var c = yield setTimeout(resume(), 500);
    console.log("OK");
  } catch (e) {
      console.error("--- genny:");
      console.error(e.stack);
  }
});

testOutput.txt

➜  co  node --harmony index.js
--- co:
Error: Oops
    at null._onTimeout (/home/spion/Documents/tests/co/index.js:8:12)
    at Timer.listOnTimeout (timers.js:124:15)

--- genny:
Error: Oops
    at null._onTimeout (/home/spion/Documents/tests/co/index.js:8:12)
    at Timer.listOnTimeout (timers.js:124:15)
From generator:
    at /home/spion/Documents/tests/co/index.js:29:30

How is it possible to get the correct stack trace here? Is it possible with current harmony support in node?

You may want to emulate it with a decorator or something. V8 stack-tracing you out of a setTimeout sounds unlikely.

@Marak yes.

Its a techique pioneered (afaik) by Q, and addopted by genny as well as [Bluebird]((//github.com/petkaantonov/bluebird) . Bluebird takes this the furthest (has some clever optimizations, supports an entire chain of previous events)

It uses Error.captureStackTrace (or simply generating a new Error()) when resume() is called or when executing a promise. The stack traces are kept in memory. If a real error happens, they're appended to that real error's stack trace.

There is a performance and memory penalty involved - its most likely not usable in production. But its still super-useful in development.