Upload assets to S3 with Cache-Control header set

We want to be able to redirect Asset Manager requests to the assets
stored in an S3 bucket. However, we want as much of the
externally-visible behaviour to remain the same and so we need to set
the 'Cache-Control' header to the same value as in
`MediaController#download`, i.e. the value generated by
`ActionController::ConditionalGet#expires_in` [1] using
`ApplicationController#set_expiry`.

Note that the header is set on the S3 object at the point where it is
uploaded. See the documentation for `Aws::S3::Object#upload_file` [2].

[1]: http://api.rubyonrails.org/v4.2.7.1/classes/ActionController/ConditionalGet.html#method-i-expires_in
[2]: http://docs.aws.amazon.com/sdkforruby/api/Aws/S3/Object.html#upload_file-instance_method

app/models/asset.rb

@@ -71,7 +71,7 @@ def accessible_by?(user)
   end
 
   def save_to_cloud_storage
-    Services.cloud_storage.save(self)
+    Services.cloud_storage.save(self, cache_control: AssetManager.cache_control.header)
   rescue => e
     Airbrake.notify_or_ignore(e, params: { id: self.id, filename: self.filename })
     raise

lib/cache_control_configuration.rb

@@ -10,4 +10,13 @@ def max_age
   def options
     @attributes.except(:max_age)
   end
+
+  def header
+    response = ActionDispatch::Response.new
+    @attributes.each do |key, value|
+      response.cache_control[key] = value
+    end
+    response.prepare!
+    response['Cache-Control']
+  end
 end

lib/s3_storage.rb

@@ -19,8 +19,8 @@ def initialize(bucket_name)
     @bucket_name = bucket_name
   end
 
-  def save(asset)
-    object_for(asset).upload_file(asset.file.path)
+  def save(asset, options = {})
+    object_for(asset).upload_file(asset.file.path, options)
   end
 
   def load(asset)

spec/lib/cache_control_configuration_spec.rb

@@ -18,4 +18,22 @@
       expect(subject.options).to eq(public: true)
     end
   end
+
+  describe '#header' do
+    context 'when attributes are supplied to constructor' do
+      let(:attributes) { { max_age: 1.hour, public: true, must_revalidate: true } }
+
+      it 'returns Cache-Control header value c.f. expires_in' do
+        expect(subject.header).to eq('max-age=3600, public, must-revalidate')
+      end
+    end
+
+    context 'when only max_age attribute is supplied to constructor' do
+      let(:attributes) { { max_age: 1.hour } }
+
+      it "returns Cache-Control header value with default visibility" do
+        expect(subject.header).to include('private')
+      end
+    end
+  end
 end

spec/lib/s3_storage_spec.rb

@@ -15,11 +15,17 @@
 
   describe '#save' do
     it 'uploads file to S3 bucket' do
-      expect(s3_object).to receive(:upload_file).with(asset.file.path)
+      expect(s3_object).to receive(:upload_file).with(asset.file.path, anything)
 
       subject.save(asset)
     end
 
+    it 'passes options to Aws::S3::Object#upload_file' do
+      expect(s3_object).to receive(:upload_file).with(anything, cache_control: 'cache-control-header')
+
+      subject.save(asset, cache_control: 'cache-control-header')
+    end
+
     context 'when bucket name is blank' do
       let(:bucket_name) { '' }
 

spec/models/asset_spec.rb

@@ -120,13 +120,22 @@
   describe "#save_to_cloud_storage" do
     let(:asset) { FactoryGirl.create(:clean_asset) }
     let(:cloud_storage) { double(:cloud_storage) }
+    let(:cache_control) { instance_double(CacheControlConfiguration) }
 
     before do
       allow(Services).to receive(:cloud_storage).and_return(cloud_storage)
+      allow(AssetManager).to receive(:cache_control).and_return(cache_control)
+      allow(cache_control).to receive(:header).and_return('cache-control-header')
     end
 
     it 'saves the asset to cloud storage' do
-      expect(cloud_storage).to receive(:save).with(asset)
+      expect(cloud_storage).to receive(:save).with(asset, anything)
+
+      asset.save_to_cloud_storage
+    end
+
+    it 'sets the Cache-Control header on the asset stored in the cloud' do
+      expect(cloud_storage).to receive(:save).with(anything, cache_control: 'cache-control-header')
 
       asset.save_to_cloud_storage
     end