Using async to properly handle exceptions when multiple threads are run

[qrilka]

No description provided.

[qrilka]

@tmcgilchrist @markhibberd is something required for this to be merged?
We were bitten by this bug in production

[tmcgilchrist]

Would you mind putting some reasonable bounds on this dependency?

[qrilka]

amended the commit with >= 2.0.0.0

[tmcgilchrist]

Do you have some investigation notes from the bug you encountered?

If this change fixed it I will merge but I'm still not convinced the code is doing the right thing under exceptions. We've been bitten badly with async exceptions lately and I'm suspicious of everything :-)

[qrilka]

@tmcgilchrist that was 3 months ago and unfortunately I haven't done detailed notes regarding this bug.
But as far as I remember the main problem was that our data processing code reading Riak sometimes was just stuck without giving any logs or something. This patch fixed the situation.
The problem in our code was cause by getMany function with was calling pipe under the hood.
And in unpatched pipe any exception in replicateM_ numReqs $ writeChan ch =<< receive conn will just kill sending thread leaving the main one waiting forever for results in channel ch.
The patch adds more safety as asyncBoth re-throws child thread exception if it happens.

[tmcgilchrist]

Thanks for the details, I'll push out a new version once the builds are all good.

[tmcgilchrist]

Merged here #77