[close #797] Do not carry asset pipeline fragment cache into runtime

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 +66,13 @@ def default_assets_cache
     "tmp/cache/assets"
   end
 
+  def cleanup
+    super
+    return if assets_compile_enabled?
+    return unless Dir.exist?(default_assets_cache)
+    FileUtils.remove_dir(default_assets_cache)
+  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