Deprecate legacy /uploads route

The route `/uploads/:namespace/:project/:secret/:filename` has been
around for a long time, but it duplicates the
`/:namespace/:project/uploads/:secret/:filename` route. We'll redirect
the former to the second, canonical version in preparation for removing
this route entirely later.

app/controllers/projects/uploads_controller.rb

@@ -31,7 +31,8 @@ class Projects::UploadsController < Projects::ApplicationController
   end
 
   # Overrides ApplicationController#build_canonical_path since there are
-  # multiple routes that match project uploads
+  # multiple routes that match project uploads:
+  # https://gitlab.com/gitlab-org/gitlab/issues/196396
   def build_canonical_path(project)
     return super unless action_name == 'show'
     return super unless params[:secret] && params[:filename]

config/routes/uploads.rb

@@ -21,9 +21,10 @@ scope path: :uploads do
       as: 'appearance_upload'
 
   # Project markdown uploads
-  # Consider deprecating this in GitLab 13.x because this is redundant to show_namespace_project_uploads
+  # DEPRECATED: Remove this in GitLab 13.0 because this is redundant to show_namespace_project_uploads
+  # https://gitlab.com/gitlab-org/gitlab/issues/196396
   get ":namespace_id/:project_id/:secret/:filename",
-    to:           "projects/uploads#show",
+    to: redirect("%{namespace_id}/%{project_id}/uploads/%{secret}/%{filename}"),
     constraints:  { namespace_id: /[a-zA-Z.0-9_\-]+/, project_id: /[a-zA-Z.0-9_\-]+/, filename: %r{[^/]+} }, format: false, defaults: { format: nil }
 
   # create uploads for models, snippets (notes) available for now

spec/routing/uploads_routing_spec.rb

@@ -28,4 +28,12 @@ describe 'Uploads', 'routing' do
       expect(post("/uploads/#{model}?id=1")).not_to be_routable
     end
   end
+
+  describe 'legacy paths' do
+    include RSpec::Rails::RequestExampleGroup
+
+    it 'redirects project uploads to canonical path under project namespace' do
+      expect(get('/uploads/namespace/project/12345/test.png')).to redirect_to('/namespace/project/uploads/12345/test.png')
+    end
+  end
 end