Experiment with Travis's build stages.

See #4556.

.travis.yml

@@ -114,13 +114,25 @@ install:
  # We intentionally do not install anything before trying to build Cabal because
  # it should build with each supported GHC version out-of-the-box.
 
-# Here starts the actual work to be performed for the package under test; any
-# command which exits with a non-zero exit code causes the build to fail. Using
-# ./dist/setup/setup here instead of cabal-install to avoid breakage when the
-# build config format changed.
+# All jobs except 'meta' have two stages; the first stage is used to
+# warm up the cache. This is mostly useful when a new compiler gets
+# added to the build matrix: work done installing dependencies is not
+# wasted when we exceed the time limit for jobs.
+jobs:
+  include:
+    - stage: prepare cache
+      script: if [ "x$SCRIPT" != "xmeta" ]; then ./travis-precache.sh; fi
+      after_success: skip
+    - stage: test
+
+# Here starts the actual work to be performed for the package under
+# test; any command which exits with a non-zero exit code causes the
+# build to fail. Using ./dist/setup/setup here instead of
+# cabal-install to avoid breakage when the build config format
+# changed.
 script:
- - rm -rf dist-newstyle
- - ./travis-${SCRIPT}.sh -j2
+  - rm -rf dist-newstyle
+  - ./travis-${SCRIPT}.sh -j2
 
 cache:
  directories:

travis-precache.sh

@@ -0,0 +1,31 @@
+#!/bin/sh
+
+. ./travis-common.sh
+
+# ---------------------------------------------------------------------
+# Update the Cabal index
+# ---------------------------------------------------------------------
+
+timed cabal update
+
+# ---------------------------------------------------------------------
+# Setup our local project
+# ---------------------------------------------------------------------
+
+cp cabal.project.travis cabal.project.local
+
+# ---------------------------------------------------------------------
+# Install executables if necessary
+# ---------------------------------------------------------------------
+
+if ! command -v happy; then
+    timed cabal install $jobs happy
+fi
+
+# ---------------------------------------------------------------------
+# Warm up the cache
+# ---------------------------------------------------------------------
+
+# TODO: Don't build Cabal and hackage-security here, they'll be
+# rebuilt anyway.
+timed cabal new-build --only-dependencies cabal-install cabal-tests