Improve performance of the /projects/:id/repository/branches API endpoint

Mitigate a N+1 requests to Gitaly problem. Still one left.

Signed-off-by: Rémy Coutable <remy@rymai.me>

4 files changed, 14 insertions, 8 deletions

diff --git a/changelogs/unreleased/37442-api-branches-id-repository-branches-is-calling-gitaly-n-1-times-per-request.yml b/changelogs/unreleased/37442-api-branches-id-repository-branches-is-calling-gitaly-n-1-times-per-request.yml
new file mode 100644
index 00000000000..11a11a289bf
--- /dev/null
+++ b/changelogs/unreleased/37442-api-branches-id-repository-branches-is-calling-gitaly-n-1-times-per-request.yml
@@ -0,0 +1,5 @@
+---
+title: Improve performance of the /projects/:id/repository/branches API endpoint
+merge_request: 15215
+author:
+type: performance
diff --git a/lib/api/branches.rb b/lib/api/branches.rb
index 19152c9f395..cdef1b546a9 100644
--- a/lib/api/branches.rb
+++ b/lib/api/branches.rb
@@ -29,12 +29,11 @@ module API
         use :pagination
       end
       get ':id/repository/branches' do
-        branches = ::Kaminari.paginate_array(user_project.repository.branches.sort_by(&:name))
+        repository = user_project.repository
+        branches = ::Kaminari.paginate_array(repository.branches.sort_by(&:name))
+        merged_branch_names = repository.merged_branch_names(branches.map(&:name))
 
-        # n+1: https://gitlab.com/gitlab-org/gitlab-ce/issues/37442
-        Gitlab::GitalyClient.allow_n_plus_1_calls do
-          present paginate(branches), with: Entities::Branch, project: user_project
-        end
+        present paginate(branches), with: Entities::Branch, project: user_project, merged_branch_names: merged_branch_names
       end
 
       resource ':id/repository/branches/:branch', requirements: BRANCH_ENDPOINT_REQUIREMENTS do
diff --git a/lib/api/entities.rb b/lib/api/entities.rb
index 398a7906dcb..df78e49b8cf 100644
--- a/lib/api/entities.rb
+++ b/lib/api/entities.rb
@@ -244,7 +244,7 @@ module API
       expose :merged do |repo_branch, options|
         # n+1: https://gitlab.com/gitlab-org/gitlab-ce/issues/37442
         Gitlab::GitalyClient.allow_n_plus_1_calls do
-          options[:project].repository.merged_to_root_ref?(repo_branch.name)
+          options[:project].repository.merged_to_root_ref?(repo_branch.name, options[:merged_branch_names])
         end
       end
 
diff --git a/lib/api/v3/branches.rb b/lib/api/v3/branches.rb
index 69cd12de72c..b201bf77667 100644
--- a/lib/api/v3/branches.rb
+++ b/lib/api/v3/branches.rb
@@ -14,9 +14,11 @@ module API
           success ::API::Entities::Branch
         end
         get ":id/repository/branches" do
-          branches = user_project.repository.branches.sort_by(&:name)
+          repository = user_project.repository
+          branches = repository.branches.sort_by(&:name)
+          merged_branch_names = repository.merged_branch_names(branches.map(&:name))
 
-          present branches, with: ::API::Entities::Branch, project: user_project
+          present branches, with: ::API::Entities::Branch, project: user_project, merged_branch_names: merged_branch_names
         end
 
         desc 'Delete a branch'