Fix DNSCache race-condition

[socketpair]

Are there changes in behavior for the user?

Related issue number

#2620

Checklist

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[codecov-io]

Codecov Report

Merging #2621 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #2621      +/-   ##
==========================================
+ Coverage   97.92%   97.92%   +<.01%     
==========================================
  Files          38       38              
  Lines        7265     7268       +3     
  Branches     1259     1259              
==========================================
+ Hits         7114     7117       +3     
  Misses         47       47              
  Partials      104      104

Impacted Files	Coverage Δ
aiohttp/connector.py	96.99% <100%> (+0.01%)	⬆️

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[pfreixes]

Why not keep the original round robin but making sure that we get rid of the corner case ? such as

>>> addrs = cycle([1, 2, 3, 4])
>>> list(islice(addrs, 4))
[1, 2, 3, 4]
>>> next(addrs)
1
>>> list(islice(addrs, 4))
[2, 3, 4, 1]

Following your idea of using an ad-hoc list per call to next_addrs it's just a matter of make a next after built the list to make sure that we always use a shifted list per call.

[socketpair]

Because, anyway, cycle will start from actually random element. Why not to randomize explicitly ? Why we should stick on round-robin ?

In [7]: timeit list(islice(xxx, 4))
The slowest run took 15.44 times longer than the fastest. This could mean that an intermediate result is being cached.
1000000 loops, best of 3: 447 ns per loop

In [8]: timeit shuffle(qwe)
The slowest run took 5.42 times longer than the fastest. This could mean that an intermediate result is being cached.
100000 loops, best of 3: 2.82 µs per loop

Well, it seems you are right....

[socketpair]

So, I fixed.

[pfreixes]

So, ready to be merged? flake8 is failing for some reason. Up to yo to merge it once it is fixed.

[asvetlov]

Let me review soon.
P.S. Travis is red still :(

[asvetlov]

Looks good

[lock]

This thread has been automatically locked since there has not been any recent activity after it was closed. Please open a [new issue] for related bugs.
If you feel like there's important points made in this discussion, please include those exceprts into that [new issue].
[new issue]: https://github.com/aio-libs/aiohttp/issues/new