Possible memory leak with Promise.each

[tdzienniak]

I'm using bluebird v3.3.4 running on node v4.4.0. Leaky code:

const Promise = require('bluebird');

Promise.each(new Array(3000), () => {
    const longString = (new Array(1000000)).join('1');

    return Promise.resolve(longString);
});

Not leaky code:

const Promise = require('bluebird');

Promise.each(new Array(3000), () => {
    const longString = (new Array(1000000)).join('1');

    return Promise.resolve(longString).then(() => void 0);
});

It seems that the results of the promises are stored somewhere, but they shouldn't be, at least this is what I understand reading the docs of Promise.each:

Resolves to the original array unmodified, this method is meant to be used for side effects.

Issue is happening on earlier versions of bluebird too.

[tdzienniak]

I've noticed, that second snippet is also leaking memory when long stack traces are enabled. Is this desired behaviour?

[benjamingr]

@tdzienniak this is because each is currently implemented through mapSeries https://github.com/petkaantonov/bluebird/blob/master/src/each.js#L14-L17 - the behavior of each was modified in 3.0 and then reverted when people complained.

Note that this is not a memory leak - it is merely wasteful behavior.

[benjamingr]

We can implement directly with _iterate and overriding shouldCopyValues

bluebird/src/promise_array.js

Lines 164 to 166 in 283412e

	PromiseArray.prototype.shouldCopyValues = function () {
	return true;
	};

- petka, I can take a shot at it but it's probably faster for you to fix it than to me to fix it and you to verify.

[tdzienniak]

@benjamingr yes, I've noticed that it is implemented through mapSeries, which is why with conjunction with long stack traces is wasting a lot of memory. I spent a lot of time trying to figure out why my node process is running out of memory. Maybe a note in the docs describing this behaviour will be helpful for others?

And well, this in not memory leak per se, but in context of desired Promise.each behaviour we can call it that.

[benjamingr]

Ping @petkaantonov

[petkaantonov]

@benjamingr feel free to give a shot at fixing it

[benjamingr]

@tdzienniak if you feel like it - I would appreciate it if you take a look at the code with the fix and see if it fixes the issue for you.

[tdzienniak]

@benjamingr ok, I can test the examples from my initial post.