Extract `Repository.memoize_method` method

And reuse `Gitlab::Utils::StrongMemoize`.

There is a subtle behavior change required to reuse StrongMemoize in
this case. The early fallback check now occurs *before* reading the
memoized value instead of after.

I think this is fine since a memoized value should only exist if
`exists?` is also already memoized as `true`.

1 files changed, 58 insertions, 0 deletions

diff --git a/spec/lib/gitlab/repository_cache_adapter_spec.rb b/spec/lib/gitlab/repository_cache_adapter_spec.rb
index 5bd4d6c6a48..703e73f1a9b 100644
--- a/spec/lib/gitlab/repository_cache_adapter_spec.rb
+++ b/spec/lib/gitlab/repository_cache_adapter_spec.rb
@@ -65,6 +65,64 @@ describe Gitlab::RepositoryCacheAdapter do
     end
   end
 
+  describe '#memoize_method_output' do
+    let(:fallback) { 10 }
+
+    context 'with a non-existing repository' do
+      let(:project) { create(:project) } # No repository
+
+      subject do
+        repository.memoize_method_output(:cats, fallback: fallback) do
+          repository.cats_call_stub
+        end
+      end
+
+      it 'returns the fallback value' do
+        expect(subject).to eq(fallback)
+      end
+
+      it 'avoids calling the original method' do
+        expect(repository).not_to receive(:cats_call_stub)
+
+        subject
+      end
+
+      it 'does not set the instance variable' do
+        subject
+
+        expect(repository.instance_variable_defined?(:@cats)).to eq(false)
+      end
+    end
+
+    context 'with a method throwing a non-existing-repository error' do
+      subject do
+        repository.memoize_method_output(:cats, fallback: fallback) do
+          raise Gitlab::Git::Repository::NoRepository
+        end
+      end
+
+      it 'returns the fallback value' do
+        expect(subject).to eq(fallback)
+      end
+
+      it 'does not set the instance variable' do
+        subject
+
+        expect(repository.instance_variable_defined?(:@cats)).to eq(false)
+      end
+    end
+
+    context 'with an existing repository' do
+      it 'sets the instance variable' do
+        repository.memoize_method_output(:cats, fallback: fallback) do
+          'block output'
+        end
+
+        expect(repository.instance_variable_get(:@cats)).to eq('block output')
+      end
+    end
+  end
+
   describe '#expire_method_caches' do
     it 'expires the caches of the given methods' do
       expect(cache).to receive(:expire).with(:rendered_readme)