Detect asynchronous exceptions via their types #187

[snoyberg]

This commit uses the same async-exception detection mechanism as is used
by the safe-exceptions package, via checking if the given exception is
cast to a SomeAsyncException. (On older GHCs without SomeAsyncException,
it contains a hard-coded list of async exception types.) It then ensures
that:

• Throwing via throwChecked always generates a synchronous exception
• Catching via catchChecked (et al) never catches an asynchronous
  exception

Unfortunately, I don't currently have a reliable test case to ensure
that this fixes the problems described in #187. Hopefully with this
patch available we can begin testing cabal-install and Stack against the
change and see if it resolves the issues.

[snoyberg]

Note that I'm not particularly happy about the fallback here for older GHCs (base < 4.7), but there's not much choice in the matter. The alternative would be dropping support for building hackage-security with older versions of GHC. I'm not sure if that's an option, but it would be more reliable, especially with third-party libraries potentially defining their own async exception types (such as the async package).

[edsko]

LGTM. catchChecked was never intended to catch asynchronous exceptions; indeed, it was intended purely to catch checked exceptions, and asynchronous exceptions never are checked. Thanks for the PR!

[dcoutts]

LGTM. Thanks.

(I'd suggest we squash the second 2 patches into the first)

[snoyberg]

Agreed on squashing, let me know if you want me to do that, or you want to do it at merge time.

[snoyberg]

Thanks for merging this and #203 @hvr. We're deciding whether to move ahead with merging commercialhaskell/stack#3865 or wait until these changes are released on Hackage. Do you have an ETA on a Hackage release for these changes?