Use Heroku's cache for downloaded jemalloc binaries

GitHub's releases service recently had an issue where downloads were
failing. This attempts to fix two main issues: resilience for deploys
and improved error messages.

Using Heroku's cache we should only need to download each version once.
This will allow deploys should GitHub have issues in the future.

Before this change, if there was an error downloading the bundle then
the buildpack would fail with an error about bzip file formats. This now
checks the http status code and prints a more useful error should it
fail.

Fixes #13

bin/compile

@@ -15,16 +15,45 @@ if [ -f $ENV_DIR/JEMALLOC_VERSION ]; then
   version=$(cat $ENV_DIR/JEMALLOC_VERSION)
 fi
 
-url="https://github.com/gaffneyc/heroku-buildpack-jemalloc/releases/download/$STACK/jemalloc-$version.tar.bz2"
+# dest is the path in the application that jemalloc will be extracted to.
 dest="$BUILD_DIR/vendor/jemalloc"
 
-echo "-----> Vendoring jemalloc $version"
-echo "       Fetching $url"
+# bundle is the full path to the cached jemalloc binaries for this version.
+bundle=$CACHE_DIR/jemalloc/$version
+
+function download_jemalloc() {
+  url="https://github.com/gaffneyc/heroku-buildpack-jemalloc/releases/download/$STACK/jemalloc-$version.tar.bz2"
+
+  # Disable exit on command failure so we can provide better error messages
+  set +e
+
+  echo "       Fetching $url"
+  status=$(curl -sL -f  -w "%{http_code}" -o /tmp/jemalloc.tar.bz2 $url)
+
+  if [[ $status -ge 300 ]]; then
+    echo " !     jemalloc: Server returned HTTP $status"
+    exit 1
+  fi
+
+  # Reenable exit on failure
+  set -e
+
+  mkdir -p $bundle
+  tar -xj -f /tmp/jemalloc.tar.bz2 -C $bundle
+}
+
+echo "-----> jemalloc: Vendoring $version"
+
+# Check if this version of jemalloc is in the cache and download it if it
+# doesn't exist.
+if [[ ! -f $bundle ]]; then
+  download_jemalloc
+fi
 
 mkdir -p $dest
-curl -sL $url | tar -C $dest -xj
+cp -r $bundle -T $dest/
 
-echo "-----> Building runtime environment"
+echo "-----> jemalloc: Building runtime environment"
 mkdir -p $BUILD_DIR/.profile.d
 
 cat <<'EOF' > $BUILD_DIR/.profile.d/jemalloc.sh