Validate that reference exists for atom format

* Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/351520
* Sentry error: https://sentry.gitlab.net/gitlab/gitlabcom/issues/3152528

**Problem**

We don't verify if provided ref exists if it has `.atom` suffix

**Solution**

Return 404 page when ref is missing.

Changelog: fixed

lib/extracts_path.rb

@@ -16,6 +16,8 @@ module ExtractsPath
     id_without_atom = id.sub(/\.atom$/, '')
     valid_refs = ref_names.select { |v| "#{id_without_atom}/".start_with?("#{v}/") }
 
+    raise InvalidPathError if valid_refs.blank?
+
     valid_refs.max_by(&:length)
   end
 

spec/controllers/projects/commits_controller_spec.rb

@@ -151,6 +151,20 @@ RSpec.describe Projects::CommitsController do
             expect(response.media_type).to eq('text/html')
           end
         end
+
+        context 'when the ref does not exist' do
+          before do
+            get(:show, params: {
+              namespace_id: project.namespace,
+              project_id: project,
+              id: 'unknown.atom'
+            })
+          end
+
+          it 'returns 404 page' do
+            expect(response).to be_not_found
+          end
+        end
       end
     end
 

spec/lib/extracts_path_spec.rb

@@ -209,8 +209,8 @@ RSpec.describe ExtractsPath do
       expect(extract_ref_without_atom('release/app/v1.0.0.atom')).to eq('release/app/v1.0.0')
     end
 
-    it 'returns nil if there are no matching refs' do
-      expect(extract_ref_without_atom('foo.atom')).to eq(nil)
+    it 'raises an error if there are no matching refs' do
+      expect { extract_ref_without_atom('foo.atom') }.to raise_error(ExtractsRef::InvalidPathError)
     end
   end
 end