Deprecate asset fallback

README.md

@@ -48,9 +48,12 @@ Each asset task will invoke `assets:environment` first. By default this loads th
 
 Also see [Sprockets::Rails::Task](https://github.com/rails/sprockets-rails/blob/master/lib/sprockets/rails/task.rb) and [Rake::SprocketsTask](https://github.com/rails/sprockets/blob/master/lib/rake/sprocketstask.rb).
 
-
 ### Initializer options
 
+**`config.assets.unknown_asset_fallback`**
+
+When set to a truthy value, a result will be returned even if the requested asset is not found in the asset pipeline. When set to a falsey value it will raise an error when no asset is found in the pipeline. Defaults to `true`.
+
 **`config.assets.precompile`**
 
 Add additional assets to compile on deploy. Defaults to `application.js`, `application.css` and any other non-js/css file under `app/assets`.

lib/sprockets/rails/helper.rb

@@ -6,6 +6,8 @@
 module Sprockets
   module Rails
     module Helper
+      class AssetNotFound < StandardError; end
+
       class AssetNotPrecompiled < StandardError
         include Sprockets::Rails::Utils
         def initialize(source)
@@ -33,7 +35,8 @@ def initialize(source)
         :assets_environment, :assets_manifest,
         :assets_precompile, :precompiled_asset_checker,
         :assets_prefix, :digest_assets, :debug_assets,
-        :resolve_assets_with, :check_precompiled_asset
+        :resolve_assets_with, :check_precompiled_asset,
+        :unknown_asset_fallback
       ]
 
       def self.included(klass)
@@ -68,12 +71,26 @@ def assets_environment
         end
       end
 
+      # Writes over the built in ActionView::Helpers::AssetUrlHelper#compute_asset_path
+      # to use the asset pipeline.
       def compute_asset_path(path, options = {})
         debug = options[:debug]
 
         if asset_path = resolve_asset_path(path, debug)
           File.join(assets_prefix || "/", legacy_debug_path(asset_path, debug))
         else
+          message =  "The asset #{ path.inspect } is not present in the asset pipeline."
+          raise AssetNotFound, message unless unknown_asset_fallback
+
+          if respond_to?(:public_compute_asset_path)
+            message << "Falling back to an asset that may be in the public folder.\n"
+            message << "This behavior is deprecated and will be removed.\n"
+            message << "To bypass the asset pipeline and preserve this behavior,\n"
+            message << "use the `skip_pipeline: true` option.\n"
+
+            call_stack = respond_to?(:caller_locations) ? caller_locations : caller
+            ActiveSupport::Deprecation.warn(message, call_stack)
+          end
           super
         end
       end

lib/sprockets/railtie.rb

@@ -122,6 +122,7 @@ def configure(&block)
     config.assets.cache_limit = 50.megabytes
     config.assets.gzip        = true
     config.assets.check_precompiled_asset = true
+    config.assets.unknown_asset_fallback  = true
 
     config.assets.configure do |env|
       config.assets.paths.each { |path| env.append_path(path) }
@@ -245,7 +246,7 @@ def self.build_manifest(app)
         self.resolve_assets_with = config.assets.resolve_with
 
         self.check_precompiled_asset = config.assets.check_precompiled_asset
-
+        self.unknown_asset_fallback  = config.assets.unknown_asset_fallback
         # Expose the app precompiled asset check to the view
         self.precompiled_asset_checker = -> logical_path { app.asset_precompiled? logical_path }
       end

test/test_helper.rb

@@ -28,6 +28,7 @@ def setup
     @view.assets_precompile   = %w( manifest.js )
     precompiled_assets = @manifest.find(@view.assets_precompile).map(&:logical_path)
     @view.check_precompiled_asset = true
+    @view.unknown_asset_fallback  = true
     @view.precompiled_asset_checker = -> logical_path { precompiled_assets.include? logical_path }
     @view.request = ActionDispatch::Request.new({
       "rack.url_scheme" => "https"
@@ -827,6 +828,37 @@ def test_index_files
   end
 end
 
+class DeprecationTest < HelperTest
+  def test_deprecations_for_asset_path
+    @view.send(:define_singleton_method, :public_compute_asset_path, -> {})
+    assert_deprecated do
+      @view.asset_path("does_not_exist.noextension")
+    end
+  ensure
+    @view.instance_eval('undef :public_compute_asset_path')
+  end
+
+  def test_deprecations_for_asset_url
+    @view.send(:define_singleton_method, :public_compute_asset_path, -> {})
+
+    assert_deprecated do
+      @view.asset_url("does_not_exist.noextension")
+    end
+  ensure
+    @view.instance_eval('undef :public_compute_asset_path')
+  end
+
+  def test_deprecations_for_image_tag
+    @view.send(:define_singleton_method, :public_compute_asset_path, -> {})
+
+    assert_deprecated do
+      @view.image_tag("does_not_exist.noextension")
+    end
+  ensure
+    @view.instance_eval('undef :public_compute_asset_path')
+  end
+end
+
 class RaiseUnlessPrecompiledAssetDisabledTest < HelperTest
   def test_check_precompiled_asset_enabled
     @view.check_precompiled_asset = true