Fix attachments using the wrong URLs in e-mails

Prior to https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/29889,
only the project context were set for the Markdown renderer. For a note
on an issuable, the group context was set to `nil` because
`note.noteable.try(:group)` attempted to get the issuable's group, which
doesn't exist.

To make group notifications work, now both the project and group context
are set. The context gets passed to `RelativeLinkFilter`, which
previously assumed that it wasn't possible to have both a group and a
project in the Markdown context. However, if a group were defined, it
would take precedence, and the URL rendered for uploads would be
`/group/-/uploads` instead of `/group/project/uploads/`. This led to
404s in e-mails.

However, now that we have both project and group in the context, we
render the Markdown giving priority to the project context if is set.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/63910

changelogs/unreleased/sh-fix-issue-63910.yml

@@ -0,0 +1,5 @@
+---
+title: Fix attachments using the wrong URLs in e-mails
+merge_request: 30197
+author:
+type: fixed

lib/banzai/filter/relative_link_filter.rb

@@ -56,10 +56,10 @@ module Banzai
       def process_link_to_upload_attr(html_attr)
         path_parts = [Addressable::URI.unescape(html_attr.value)]
 
-        if group
-          path_parts.unshift(relative_url_root, 'groups', group.full_path, '-')
-        elsif project
+        if project
           path_parts.unshift(relative_url_root, project.full_path)
+        elsif group
+          path_parts.unshift(relative_url_root, 'groups', group.full_path, '-')
         else
           path_parts.unshift(relative_url_root)
         end

spec/helpers/markup_helper_spec.rb

@@ -50,6 +50,43 @@ describe MarkupHelper do
         expect(markdown(actual, project: second_project)).to match(expected)
       end
     end
+
+    describe 'uploads' do
+      let(:text) { "![ImageTest](/uploads/test.png)" }
+      let(:group) { create(:group) }
+
+      subject { helper.markdown(text) }
+
+      describe 'inside a project' do
+        it 'renders uploads relative to project' do
+          expect(subject).to include("#{project.full_path}/uploads/test.png")
+        end
+      end
+
+      describe 'inside a group' do
+        before do
+          helper.instance_variable_set(:@group, group)
+          helper.instance_variable_set(:@project, nil)
+        end
+
+        it 'renders uploads relative to the group' do
+          expect(subject).to include("#{group.full_path}/-/uploads/test.png")
+        end
+      end
+
+      describe "with a group in the context" do
+        let(:project_in_group) { create(:project, group: group) }
+
+        before do
+          helper.instance_variable_set(:@group, group)
+          helper.instance_variable_set(:@project, project_in_group)
+        end
+
+        it 'renders uploads relative to project' do
+          expect(subject).to include("#{project_in_group.path_with_namespace}/uploads/test.png")
+        end
+      end
+    end
   end
 
   describe '#markdown_field' do