[close heroku#797] Do not carry asset pipeline fragment cache into ru…

…ntime

When someone is using the asset pipeline, and they've disabled the config that allows them to build assets in production.

So when

```
config.assets.compile = false
```

Then the `tmp/cache/assets/sprockets/` folder is not needed at runtime and should be safe to delete, which will lower the overall slug size.

Do not merge until after Dreamforce.

CHANGELOG.md

@@ -1,5 +1,7 @@
 ## Master
 
+* Delete the sprockets temp directory for a smaller runtime slug if they are not building assets at runtime (https://github.com/heroku/heroku-buildpack-ruby/pull/812)
+
 ## v195 (10/18/2018)
 
 * Default Ruby version is now 2.4.5 (https://github.com/heroku/heroku-buildpack-ruby/pull/821)

hatchet.json

@@ -79,6 +79,9 @@
     "sharpstone/active_storage_local",
     "sharpstone/sprockets_asset_compile_true"
   ],
+  "heroku": [
+    "heroku/ruby-getting-started"
+  ],
   "multibuildpack": [
     "sharpstone/node_multi"
   ],

hatchet.lock

@@ -27,6 +27,8 @@
   - 540ec776eff3111ad9bb01af204c62725c94a11d
 - - "./repos/ci/ruby_no_rails_test"
   - 3916137106d59b008b67d738abe6a1438f8fbde6
+- - "./repos/heroku/ruby-getting-started"
+  - 7abd8af85c7ba6486bb03a4964da9cea98766b06
 - - "./repos/jruby/jruby_naether"
   - c5081c07de64dcd4b480923e4b7e2a98a9b4a431
 - - "./repos/multibuildpack/node_multi"

lib/language_pack/rails4.rb

@@ -66,6 +66,11 @@ def default_assets_cache
     "tmp/cache/assets"
   end
 
+  def cleanup
+    super
+    FileUtils.remove_dir(default_assets_cache) unless assets_compile_enabled?
+  end
+
   def run_assets_precompile_rake_task
     instrument "rails4.run_assets_precompile_rake_task" do
       log("assets_precompile") do

lib/language_pack/ruby.rb

@@ -110,10 +110,14 @@ def compile
       end
       config_detect
       best_practice_warnings
+      cleanup
       super
     end
   end
 
+  def cleanup
+  end
+
   def config_detect
   end
 

spec/hatchet/getting_started_spec.rb

@@ -0,0 +1,9 @@
+require_relative '../spec_helper'
+
+describe "Heroku ruby getting started" do
+  it "clears runtime cache" do
+    Hatchet::Runner.new("ruby-getting-started").deploy do |app|
+      expect(app.run("ls tmp/cache/assets")).to_not match("sprockets")
+    end
+  end
+end