Fix 404s for snippet uploads when relative URL root used

Personal snippet uploads have neither a group nor a project. If a GitLab
instance were configured with a relative URL root (e.g. `/gitlab`), then
the Markdown filter would not include this root in the generated path.
We fix this by adding this root if there is no group or project.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/56280

changelogs/unreleased/sh-fix-upload-snippets-with-relative-url-root.yml

+---
+title: Fix 404s for snippet uploads when relative URL root used
+merge_request: 24588
+author:
+type: fixed

lib/banzai/filter/relative_link_filter.rb

@@ -58,6 +58,8 @@ module Banzai
           path_parts.unshift(relative_url_root, 'groups', group.full_path, '-')
         elsif project
           path_parts.unshift(relative_url_root, project.full_path)
+        else
+          path_parts.unshift(relative_url_root)
         end
 
         begin

spec/lib/banzai/filter/relative_link_filter_spec.rb

@@ -353,5 +353,59 @@ describe Banzai::Filter::RelativeLinkFilter do
         expect(doc.at_css('a')['href']).to eq 'http://example.com'
       end
     end
+
+    context 'to a personal snippet' do
+      let(:group) { nil }
+      let(:project) { nil }
+      let(:relative_path) { '/uploads/-/system/personal_snippet/6/674e4f07fbf0a7736c3439212896e51a/example.tar.gz' }
+
+      context 'with an absolute URL' do
+        let(:absolute_path) { Gitlab.config.gitlab.url + relative_path }
+        let(:only_path) { false }
+
+        it 'rewrites the link correctly' do
+          doc = filter(link(relative_path))
+
+          expect(doc.at_css('a')['href']).to eq(absolute_path)
+        end
+      end
+
+      context 'with a relative URL root' do
+        let(:gitlab_root) { '/gitlab' }
+        let(:absolute_path) { Gitlab.config.gitlab.url + gitlab_root + relative_path }
+
+        before do
+          stub_config_setting(relative_url_root: gitlab_root)
+        end
+
+        context 'with an absolute URL' do
+          let(:only_path) { false }
+
+          it 'rewrites the link correctly' do
+            doc = filter(link(relative_path))
+
+            expect(doc.at_css('a')['href']).to eq(absolute_path)
+          end
+        end
+
+        it 'rewrites the link correctly' do
+          doc = filter(link(relative_path))
+
+          expect(doc.at_css('a')['href']).to eq(gitlab_root + relative_path)
+        end
+      end
+
+      it 'rewrites the link correctly' do
+        doc = filter(link(relative_path))
+
+        expect(doc.at_css('a')['href']).to eq(relative_path)
+      end
+
+      it 'does not modify absolute URL' do
+        doc = filter(link('http://example.com'))
+
+        expect(doc.at_css('a')['href']).to eq 'http://example.com'
+      end
+    end
   end
 end