Cache Repository#root_ref within a request

When an empty project is loaded in the UI, there are 15 separate Gitaly
FindDefaultBranch calls to determine the root_ref. Previously, it was
not cached even within the request. This change caches it within the
request so only a single FindDefaultBranch RPC is needed.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58684

app/models/repository.rb

@@ -534,10 +534,9 @@ class Repository
   end
 
   def root_ref
-    # When the repo does not exist, or there is no root ref, we raise this error so no data is cached.
-    raw_repository&.root_ref or raise Gitlab::Git::Repository::NoRepository # rubocop:disable Style/AndOr
+    raw_repository&.root_ref
   end
-  cache_method :root_ref
+  cache_method_asymmetrically :root_ref
 
   # Gitaly migration: https://gitlab.com/gitlab-org/gitaly/issues/314
   def exists?

changelogs/unreleased/sh-cache-root-ref-asymetrically.yml

+---
+title: Cache Repository#root_ref within a request
+merge_request: 25903
+author:
+type: performance

spec/models/repository_spec.rb

@@ -1095,65 +1095,69 @@ describe Repository do
     end
   end
 
-  describe '#exists?' do
-    it 'returns true when a repository exists' do
-      expect(repository.exists?).to be(true)
-    end
-
-    it 'returns false if no full path can be constructed' do
-      allow(repository).to receive(:full_path).and_return(nil)
-
-      expect(repository.exists?).to be(false)
-    end
-
-    context 'with broken storage', :broken_storage do
-      it 'should raise a storage error' do
-        expect_to_raise_storage_error { broken_repository.exists? }
-      end
-    end
-
+  shared_examples 'asymmetric cached method' do |method|
     context 'asymmetric caching', :use_clean_rails_memory_store_caching, :request_store do
       let(:cache) { repository.send(:cache) }
       let(:request_store_cache) { repository.send(:request_store_cache) }
 
       context 'when it returns true' do
         before do
-          expect(repository.raw_repository).to receive(:exists?).once.and_return(true)
+          expect(repository.raw_repository).to receive(method).once.and_return(true)
         end
 
         it 'caches the output in RequestStore' do
           expect do
-            repository.exists?
-          end.to change { request_store_cache.read(:exists?) }.from(nil).to(true)
+            repository.send(method)
+          end.to change { request_store_cache.read(method) }.from(nil).to(true)
         end
 
         it 'caches the output in RepositoryCache' do
           expect do
-            repository.exists?
-          end.to change { cache.read(:exists?) }.from(nil).to(true)
+            repository.send(method)
+          end.to change { cache.read(method) }.from(nil).to(true)
         end
       end
 
       context 'when it returns false' do
         before do
-          expect(repository.raw_repository).to receive(:exists?).once.and_return(false)
+          expect(repository.raw_repository).to receive(method).once.and_return(false)
         end
 
         it 'caches the output in RequestStore' do
           expect do
-            repository.exists?
-          end.to change { request_store_cache.read(:exists?) }.from(nil).to(false)
+            repository.send(method)
+          end.to change { request_store_cache.read(method) }.from(nil).to(false)
         end
 
         it 'does NOT cache the output in RepositoryCache' do
           expect do
-            repository.exists?
-          end.not_to change { cache.read(:exists?) }.from(nil)
+            repository.send(method)
+          end.not_to change { cache.read(method) }.from(nil)
         end
       end
     end
   end
 
+  describe '#exists?' do
+    it 'returns true when a repository exists' do
+      expect(repository.exists?).to be(true)
+    end
+
+    it 'returns false if no full path can be constructed' do
+      allow(repository).to receive(:full_path).and_return(nil)
+
+      expect(repository.exists?).to be(false)
+    end
+
+    context 'with broken storage', :broken_storage do
+      it 'should raise a storage error' do
+        expect_to_raise_storage_error { broken_repository.exists? }
+      end
+    end
+
+    it_behaves_like 'asymmetric cached method', :exists?
+  end
+
   describe '#has_visible_content?' do
     before do
       # If raw_repository.has_visible_content? gets called more than once then
@@ -1271,6 +1275,8 @@ describe Repository do
       repository.root_ref
       repository.root_ref
     end
+
+    it_behaves_like 'asymmetric cached method', :root_ref
   end
 
   describe '#expire_root_ref_cache' do