I upgraded to celery v4.0.2 from an old v3.x version and now I'm facing a weird issue with groups.
So here is my tasks file to reproduce the issue:

from celery import group, chord
from worker import app


@app.task(name='task')
def task(i):
    return i


@app.task(name='dummy')
def dummy(result):
    print result


@app.task(name='mainTask')
def mainTask():
    groupTask = group([task.s(i) for i in range(0,10)])
    job = chord(groupTask, dummy.s())()
    return job

The expected output of the variable result is:

[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]

and this is indeed what I'm getting when using amqp as result backend. But when I use redis, the output gets messed up. Basically, the redis backend is serving the results in the order the group tasks get finished, and not in the order its assigned. A sample would be:

[0, 1, 2, 3, 4, 8, 6, 9, 7, 5]

Is there something I'm missing? I've posted the same thing over in stackoverflow as well.