Fix no avatar not showing in user selection box

After upgrading to Ruby 2.5.3, we switched `URI.join` in favor of
`Gitlab::Utils.append_path`. However,
ActionController::Base.helpers.image_path can return a full URL if a CDN
host is present.

Rails provides a way to generate the full URL using the asset path, but
that doesn't appear to work because `request` is nil`.

Revert to the previous behavior to handle CDNs and relative URLs.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56268

changelogs/unreleased/sh-fix-gon-helper-avatar.yml

+---
+title: Fix no avatar not showing in user selection box
+merge_request: 24346
+author:
+type: fixed

lib/gitlab/gon_helper.rb

@@ -8,10 +8,7 @@ module Gitlab
 
     def add_gon_variables
       gon.api_version            = 'v4'
-      gon.default_avatar_url     =
-        Gitlab::Utils.append_path(
-          Gitlab.config.gitlab.url,
-          ActionController::Base.helpers.image_path('no_avatar.png'))
+      gon.default_avatar_url     = default_avatar_url
       gon.max_file_size          = Gitlab::CurrentSettings.max_attachment_size
       gon.asset_host             = ActionController::Base.asset_host
       gon.webpack_public_path    = webpack_public_path
@@ -50,5 +47,15 @@ module Gitlab
       # use this method to push multiple feature flags.
       gon.push({ features: { var_name => enabled } }, true)
     end
+
+    def default_avatar_url
+      # We can't use ActionController::Base.helpers.image_url because it
+      # doesn't return an actual URL because request is nil for some reason.
+      #
+      # We also can't use Gitlab::Utils.append_path because the image path
+      # may be an absolute URL.
+      URI.join(Gitlab.config.gitlab.url,
+               ActionController::Base.helpers.image_path('no_avatar.png')).to_s
+    end
   end
 end

spec/lib/gitlab/gon_helper_spec.rb

@@ -29,4 +29,13 @@ describe Gitlab::GonHelper do
       helper.push_frontend_feature_flag(:my_feature_flag, 10)
     end
   end
+
+  describe '#default_avatar_url' do
+    it 'returns an absolute URL' do
+      url = helper.default_avatar_url
+
+      expect(url).to match(/^http/)
+      expect(url).to match(/no_avatar.*png$/)
+    end
+  end
 end