Speed up avatar URLs with object storage

With object storage enabled, calling `#filename` on an upload does this:

1. Call the `#filename` method on the CarrierWave object.
2. Generate the URL for that object.
3. If the uploader isn't public, do so by generating an authenticated
   URL, including signing that request.

That's all correct behaviour, but for the case where we use `#filename`,
it's typically to generate a GitLab URL. That URL doesn't need to be
signed because we do our own auth.

Signing the URLs can be very expensive, especially in batch (say, we
need to get the avatar URLs for 150 users in one request). It's all
unnecessary work. If we used the `RecordsUploads` concern, we have
already recorded a `path` in the database. That `path` is actually
generated from CarrierWave's `#filename` at upload time, so we don't
need to recompute it - we can just use it and strip off the prefix if
it's available.

On a sample users autocomplete URL, at least 10% of the time before this
change went to signing URLs. After this change, we spend no time in URL
signing, and still get the correct results.

app/uploaders/records_uploads.rb

@@ -46,6 +46,10 @@ module RecordsUploads
       File.join(store_dir, filename.to_s)
     end
 
+    def filename
+      upload&.path ? File.basename(upload.path) : super
+    end
+
     private
 
     # rubocop: disable CodeReuse/ActiveRecord

changelogs/unreleased/stop-signing-avatar-paths.yml

+---
+title: Speed up generation of avatar URLs when using object storage
+merge_request:
+author:
+type: performance

spec/uploaders/records_uploads_spec.rb

@@ -94,4 +94,13 @@ describe RecordsUploads do
       expect { uploader.remove! }.to change { Upload.count }.from(1).to(0)
     end
   end
+
+  describe '#filename' do
+    it 'gets the filename from the path recorded in the database, not CarrierWave' do
+      uploader.store!(upload_fixture('rails_sample.jpg'))
+      expect_any_instance_of(GitlabUploader).not_to receive(:filename)
+
+      expect(uploader.filename).to eq('rails_sample.jpg')
+    end
+  end
 end