Promise.coroutine() is difficult to debug

[petehunt]

First of all thanks for your great work on bluebird.

The following code sample (at scale) is very difficult to debug:

var Promise = require('bluebird');

Promise.longStackTraces();

var coro = Promise.coroutine(function*() {
  yield 7;
});

coro();

[petehunt@petehunt-mbp tmp]$ node --harmony break.js
Possibly unhandled TypeError: A value was yielded that could not be treated as a promise

    See http://goo.gl/4Y4pDk

From previous event:
From previous event:
    at Object.<anonymous> (/Users/petehunt/tmp/break.js:9:1)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:349:32)
    at Function.Module._load (module.js:305:12)
    at Function.Module.runMain (module.js:490:10)
    at startup (node.js:124:16)

I understand that it's likely impossible to capture a perfect stack trace, but I spent hours tracking down this bug and was hoping to get a better error message. I took a rough pass at an improvement here: petehunt@d15a93c

Is this something worth cleaning up and trying to get in? Or is this something you'd want to do? Or is this not worth doing?

[benjamingr]

Very awesome of you to open this issue we were just debating yesterday why people are getting TypeError: A value was yielded that could not be treated as a promise.

We take the ability to debug code very seriously. What do you think a meaningful stack trace would be here? None of the userland functions here are named.

[petehunt]

w/ my patch above on a modified test file (https://github.com/petehunt/bluebird/blob/d15a93ca57c24c8a935be35b8c62a6b2123e03a7/demo.js) I was able to get it to this (note the "from coroutine" bit):

Possibly unhandled TypeError: A value 7 was yielded that could not be treated as a promise

    See http://goo.gl/4Y4pDk

From coroutine:
    at Object.<anonymous> (/Users/petehunt/Projects/bluebird/demo.js:5:21)
From previous event:
    at /Users/petehunt/Projects/bluebird/demo.js:10:9
    at GeneratorFunctionPrototype.next (native)
From previous event:
    at /Users/petehunt/Projects/bluebird/demo.js:10:9
    at GeneratorFunctionPrototype.next (native)
From previous event:
From previous event:
    at Object.<anonymous> (/Users/petehunt/Projects/bluebird/demo.js:13:1)
    at Module._compile (module.js:449:26)
    at Object.Module._extensions..js (module.js:467:10)
    at Module.load (module.js:349:32)
    at Function.Module._load (module.js:305:12)
    at Function.Module.runMain (module.js:490:10)
    at startup (node.js:124:16)

This is I think about as good as we can get without using a syntax transform to capture the stack trace at the point of yielding (unless you guys have some magic for introspecting where a generator currently is at).

[petkaantonov]

Thanks for this, as @benjamingr says debuggability is very important and we are constantly trying to find improvement in that area.

I don't see any harm in it, the coroutine function should not be called at runtime anyway so it can do expensive setup stuff.

Before pull request remove the js/browser/bluebird.js from the commit (this is only commited in release commits for a download link, I should get a real CDN)

Also call util.toString on value, before passing it to .replace, it will handle the case where value could be Object.create(null) which throws when converted to string

[benjamingr]

It's also worth to check for performance degradation since stacks are passed here - and possibly only pass the stack along under .longStackTraces().

[spion]

@benjamingr should be fine though, the stack is pre-computed only once when Promise.coroutine is called

[petehunt]

I'll try to wrap this up end of this week. Pretty crazy time right now.