Cache calls to Repository#merge_to_ref

It stands to reason that for a given target/source sha combination,
subsequent calls should result in the same result.. so why call it more
than once for a given combination (especially since this can be a slow,
weighty call to make to Gitaly?)

app/services/merge_requests/merge_to_ref_service.rb

@@ -66,6 +66,16 @@ module MergeRequests
     end
 
     def commit
+      if Feature.enabled?(:cache_merge_to_ref_calls, project, default_enabled: false)
+        Rails.cache.fetch(cache_key, expires_in: 1.day) do
+          extracted_merge_to_ref
+        end
+      else
+        extracted_merge_to_ref
+      end
+    end
+
+    def extracted_merge_to_ref
       repository.merge_to_ref(current_user,
         source_sha: source,
         branch: merge_request.target_branch,
@@ -76,5 +86,9 @@ module MergeRequests
     rescue Gitlab::Git::PreReceiveError, Gitlab::Git::CommandError => error
       raise MergeError, error.message
     end
+
+    def cache_key
+      [:merge_to_ref_service, project.full_path, merge_request.target_branch_sha, merge_request.source_branch_sha]
+    end
   end
 end

config/feature_flags/development/cache_merge_to_ref_calls.yml

+---
+name: cache_merge_to_ref_calls
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/67789
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/338251
+milestone: '14.2'
+type: development
+group: group::code review
+default_enabled: false

spec/services/merge_requests/merge_to_ref_service_spec.rb

@@ -36,6 +36,37 @@ RSpec.describe MergeRequests::MergeToRefService do
       expect(repository.ref_exists?(target_ref)).to be(true)
       expect(ref_head.id).to eq(result[:commit_id])
     end
+
+    context 'cache_merge_to_ref_calls flag enabled', :use_clean_rails_memory_store_caching do
+      before do
+        stub_feature_flags(cache_merge_to_ref_calls: true)
+
+        # warm the cache
+        #
+        service.execute(merge_request)
+      end
+
+      it 'caches the response', :request_store do
+        expect { 3.times { service.execute(merge_request) } }
+          .not_to change(Gitlab::GitalyClient, :get_request_count)
+      end
+    end
+
+    context 'cache_merge_to_ref_calls flag disabled', :use_clean_rails_memory_store_caching do
+      before do
+        stub_feature_flags(cache_merge_to_ref_calls: false)
+
+        # warm the cache
+        #
+        service.execute(merge_request)
+      end
+
+      it 'does not cache the response', :request_store do
+        expect(Gitlab::GitalyClient).to receive(:call).at_least(3).times.and_call_original
+
+        3.times { service.execute(merge_request) }
+      end
+    end
   end
 
   shared_examples_for 'successfully evaluates pre-condition checks' do