Rejection of a Promise that depends on another promise is extremely slow in V8/Chrome.

[dtinth]

The Story

First, I created a Promise, called x.

var x = new Promise(function(resolve) {
  setTimeout(resolve, 1000)
})

Then I created a function that processes a task, but it depends on x, but they all fail:

function y(task) {
  return x.then(function() { throw new Error('it failed') })
}

I have 1000 tasks, and I process them all.

Promise.all(tasks.map(y)).then(...).catch(...)

The Problem

At the moment x is resolved, on Google Chrome with a minified build, the browser freezes for ~10 seconds.

The Test Case

https://rawgit.com/dtinth/39520b129f143c66bd40/raw/index.html

The Weirdness

• From my testing, this only happens in Google Chrome, including Chrome Canary.
  • It runs smoothly on Firefox and Safari.
• When using native promises, this problem doesn't happen.
• It only happens in minified builds.
  • When I use bluebird.js instead of bluebird.min.js, the problem disappears.
  • Nevertheless, when I use bluebird in webpack environment, the problem still appears.

The Profile

On my MacBook, it takes 9 seconds to settle the Promise.

Upon zooming, most time is spent on _rejectCallback.

The Variations

If y returns a promise that's eventually fulfilled (instead of rejected), this problem doesn't happen.

If each y depends on different x, this problem doesn't happen. What I mean is that x and y looks like this instead:

function x() {
  return new Promise(function(resolve) {
    setTimeout(resolve, 1000)
  })
}

function y(task) {
  return x().then(function() { throw new Error('it failed') })
}

[benjamingr]

Can you reproduce this in node/io
?

[dtinth]

Yes, this is reproducable in Node.js (v0.10.32) using the following test code:

var Promise = require('bluebird')

var x = new Promise(function(resolve) {
  setTimeout(function() {
    console.log('will now resolve!')
    resolve()
  }, 1000)
})

function y(task) {
  return x.then(function() { throw new Error('it failed') })
}

var tasks = []
for (var i = 0; i < 1000; i ++) tasks.push('Task ' + i)

Promise.all(tasks.map(y))
.then(function() { console.log('done') })
.catch(function() { console.log('caught') })

After will now resolve! is printed, it takes ~8 seconds until the word caught is displayed.

If the function y is modified to return a fulfilling promise, the 1000 tasks are resolved instantly.

[phpnode]

@dtinth that particular test case did not reproduce the bug for me on iojs / node 0.11.x / node 0.10.x

[dtinth]

@phpnode

It also happens on iojs 1.0.3. I reproduced this on Mac OS X 10.10.1 with Bluebird 2.9.9.

This is also reproducible on Ubuntu 13.10 running Node v0.10.28 and Bluebird 2.9.9.

[phpnode]

@dtinth ah, sorry, you're right, I was testing on 2.4.x, was able to reproduce with 2.9.9

[dtinth]

By trying to install various versions, this problem started appearing since 2.8.2.

▹ npm install bluebird@2.8.1
bluebird@2.8.1 node_modules/bluebird

▹ time node test.js
will now resolve!
caught
node test.js  0.12s user 0.03s system 12% cpu 1.143 total

▹ npm install bluebird@2.8.2
bluebird@2.8.2 node_modules/bluebird

▹ time node test.js
will now resolve!
caught
node test.js  8.98s user 0.15s system 89% cpu 10.224 total

[petkaantonov]

@dtinth I will push a 2.9.10 release as soon as the browser tests come back out, no need to revert to 2.8.1 :)

[petkaantonov]

@dtinth Fixed in 2.9.10

[dtinth]

Thank you for fixing this very quickly. :)