Fix performance regression in Rails helper

Previously reported performance regression #234 was a red herring. The
`Engine` class and addition of adding `app/` asset path appears to be a
big problem, but the reason it's slow is because then it would run
through `precompiled_assets` where without that code it skips
`precompiled_assets` thus skipping the code that is actually slow.

Although `precompiled_assets` was memoized the cache wasn't actually
working and the method cache was being thrown away every single time.
This became very obvious when our test suite (Basecamp) went from 54
seconds to 320 seconds to run on sprockets-rails master.

The problem is that when the env is reset the `precompiled_assets` gets
reset instead of keeping the cache. I moved this method onto the config
to behave like the other methods, for example `assets_manifest`.

Benchmarks:
On a brand new application with a single show test that simply renders a
view with a few assets.

Before this change:
```
Calculating -------------------------------------
     Running test...    27.000  i/100ms
-------------------------------------------------
     Running test...    305.554  (± 8.2%) i/s -      1.539k
```

After this change:
```
Calculating -------------------------------------
     Running test...    55.000  i/100ms
-------------------------------------------------
     Running test...    558.135  (± 5.4%) i/s -      2.805k
```

Author: eileencodes

lib/sprockets/rails/helper.rb

@@ -21,7 +21,7 @@ def initialize(source)
       include Sprockets::Rails::Utils
 
       VIEW_ACCESSORS = [:assets_environment, :assets_manifest,
-                        :assets_precompile,
+                        :assets_precompile, :precompiled_assets,
                         :assets_prefix, :digest_assets, :debug_assets]
 
       def self.included(klass)
@@ -227,11 +227,6 @@ def lookup_debug_asset(path, options = {})
 
           asset
         end
-
-        # Internal: Generate a Set of all precompiled assets logical paths.
-        def precompiled_assets
-          @precompiled_assets ||= assets_manifest.find(assets_precompile || []).map(&:logical_path)
-        end
     end
   end
 end

lib/sprockets/rails/utils.rb

@@ -6,6 +6,11 @@ module Utils
       def using_sprockets4?
         Gem::Version.new(Sprockets::VERSION) >= Gem::Version.new('4.0.0')
       end
+
+      # Internal: Generate a Set of all precompiled assets logical paths.
+      def build_precompiled_list(manifest, assets)
+        manifest.find(assets || []).map(&:logical_path)
+      end
     end
   end
 end

lib/sprockets/railtie.rb

@@ -26,6 +26,9 @@ class Configuration
 
     # Returns Sprockets::Manifest for app config.
     attr_accessor :assets_manifest
+
+    # Returns array of already precompiled assets
+    attr_accessor :precompiled_assets
   end
 
   class Engine < Railtie
@@ -150,8 +153,11 @@ def self.build_manifest(app)
         app.routes.prepend do
           mount app.assets => config.assets.prefix
         end
+        app.assets_manifest = build_manifest(app)
+        app.precompiled_assets = build_precompiled_list(app.assets_manifest, config.assets.precompile)
+      else
+        app.assets_manifest = build_manifest(app)
       end
-      app.assets_manifest = build_manifest(app)
 
       ActionDispatch::Routing::RouteWrapper.class_eval do
         class_attribute :assets_prefix
@@ -176,6 +182,7 @@ def self.build_manifest(app)
 
         self.assets_environment = app.assets
         self.assets_manifest = app.assets_manifest
+        self.precompiled_assets = app.precompiled_assets
       end
     end
   end

test/test_helper.rb

@@ -25,6 +25,7 @@ def setup
     @view.assets_manifest    = @manifest
     @view.assets_prefix      = "/assets"
     @view.assets_precompile  = %w( manifest.js )
+    @view.precompiled_assets = @view.build_precompiled_list(@manifest, @view.assets_precompile)
     @view.request = ActionDispatch::Request.new({
       "rack.url_scheme" => "https"
     })
@@ -729,6 +730,7 @@ def test_stylesheet_link_tag_integrity
 class AssetUrlHelperLinksTarget < HelperTest
   def test_precompile_allows_links
     @view.assets_precompile = ["url.css"]
+    @view.precompiled_assets = @view.build_precompiled_list(@manifest, @view.assets_precompile)
     assert @view.asset_path("url.css")
     assert @view.asset_path("logo.png")