S3 Hackage mirror for Travis builds

Yesterday, I noticed a bunch of Travis build failures with the cabal error message "does not exist." As I covered last month, this is typically due to a download failure when trying to install packages. This leaves some ambiguity as to where the download failure originated from: was the client (in this case, Travis) having network issues, or was there a problem on Hackage's end? (Or, of course, any one of dozens of other possible explanations.)

I have no hard data to prove that the failure is from the Hackage side, but anecdotal evidence suggests that it's Hackage. In particular, Gregory Collins reports that the Snap build bot is also having issues. When we had these issues in FP Haskell Center, we resolved it by switching to an S3 mirror, so I've decided to start migrating my Travis jobs over to this mirror as well. This is fairly straightforward. First, add a new script to your repo containing:

#!/bin/sh

set -eux

mkdir -p $HOME/.cabal
cat > $HOME/.cabal/config <<EOF
remote-repo: hackage.haskell.org:http://hackage.fpcomplete.com/
remote-repo-cache: $HOME/.cabal/packages
jobs: \$ncpus
EOF

I called this file .travis-setup.sh, but you can give it whatever name you want. Just make sure to set it as executable. Then, just make sure to run this script from your .travis.yml script, likely in the before_install section, e.g.:

before_install:
 - export PATH=$HOME/.cabal/bin:/opt/ghc/$GHCVER/bin:/opt/cabal/$CABALVER/bin:$PATH
 - ./.travis-setup.sh

My first rollout for this is the stack repository. My plan is to keep this blog post up-to-date with the best instructions for doing this, so if you have recommendations on how to improve this setup, please report back to me (or just send a pull request.

Well Typed's TUF implementation is planned to allow cabal to download from mirrors automatically, which will hopefully fix this problem in the future. But this change is a useful stop-gap measure. The only downside I'm aware of is that the mirror can take up to 30 minutes to sync with Hackage, so you may end up with a slightly older package index than if you used Hackage itself.