Memoize the source_branch_exists method for a MR

Each time this will make a redis call, which is expensive if called
enough times. This was an issue when serializing large diffs for a merge
request.

Clear the memoized source_branch_exists on MR update
Clear memoized variables before proceeding with checks

Issue: gitlab.com/gitlab-org/gitlab/-/issues/209786
Merge Request: gitlab.com/gitlab-org/gitlab/-/merge_requests/34516

app/models/merge_request.rb

@@ -107,6 +107,7 @@ class MergeRequest < ApplicationRecord
 
   after_create :ensure_merge_request_diff
   after_update :clear_memoized_shas
+  after_update :clear_memoized_source_branch_exists
   after_update :reload_diff_if_branch_changed
   after_commit :ensure_metrics, on: [:create, :update], unless: :importing?
   after_commit :expire_etag_cache, unless: :importing?
@@ -862,6 +863,10 @@ class MergeRequest < ApplicationRecord
     clear_memoization(:target_branch_head)
   end
 
+  def clear_memoized_source_branch_exists
+    clear_memoization(:source_branch_exists)
+  end
+
   def reload_diff_if_branch_changed
     if (saved_change_to_source_branch? || saved_change_to_target_branch?) &&
         (source_branch_head && target_branch_head)
@@ -1109,9 +1114,17 @@ class MergeRequest < ApplicationRecord
   end
 
   def source_branch_exists?
-    return false unless self.source_project
+    if Feature.enabled?(:memoize_source_branch_merge_request, project)
+      strong_memoize(:source_branch_exists) do
+        next false unless self.source_project
+
+        self.source_project.repository.branch_exists?(self.source_branch)
+      end
+    else
+      return false unless self.source_project
 
-    self.source_project.repository.branch_exists?(self.source_branch)
+      self.source_project.repository.branch_exists?(self.source_branch)
+    end
   end
 
   def target_branch_exists?

changelogs/unreleased/209786-improve-performance-of-diffs_batch-diffs-metadata-source-branch-ex.yml

+---
+title: Further improve the performance for loading large diffs on a Merge request
+merge_request: 34516
+author:
+type: performance

spec/models/merge_request_spec.rb

@@ -1217,6 +1217,59 @@ RSpec.describe MergeRequest do
     end
   end
 
+  describe "#source_branch_exists?" do
+    let(:merge_request) { subject }
+    let(:repository) { merge_request.source_project.repository }
+
+    context 'when memoize_source_branch_merge_request feature is enabled' do
+      before do
+        stub_feature_flags(memoize_source_branch_merge_request: true)
+      end
+
+      context 'when the source project is set' do
+        it 'memoizes the value and returns the result' do
+          expect(repository).to receive(:branch_exists?).once.with(merge_request.source_branch).and_return(true)
+
+          2.times { expect(merge_request.source_branch_exists?).to eq(true) }
+        end
+      end
+
+      context 'when the source project is not set' do
+        before do
+          merge_request.source_project = nil
+        end
+
+        it 'returns false' do
+          expect(merge_request.source_branch_exists?).to eq(false)
+        end
+      end
+    end
+
+    context 'when memoize_source_branch_merge_request feature is disabled' do
+      before do
+        stub_feature_flags(memoize_source_branch_merge_request: false)
+      end
+
+      context 'when the source project is set' do
+        it 'does not memoize the value and returns the result' do
+          expect(repository).to receive(:branch_exists?).twice.with(merge_request.source_branch).and_return(true)
+
+          2.times { expect(merge_request.source_branch_exists?).to eq(true) }
+        end
+      end
+
+      context 'when the source project is not set' do
+        before do
+          merge_request.source_project = nil
+        end
+
+        it 'returns false' do
+          expect(merge_request.source_branch_exists?).to eq(false)
+        end
+      end
+    end
+  end
+
   describe '#default_merge_commit_message' do
     it 'includes merge information as the title' do
       request = build(:merge_request, source_branch: 'source', target_branch: 'target')

spec/services/merge_requests/conflicts/list_service_spec.rb

@@ -30,6 +30,7 @@ RSpec.describe MergeRequests::Conflicts::ListService do
     it 'returns a falsey value when one of the MR branches is missing' do
       merge_request = create_merge_request('conflict-resolvable')
       merge_request.project.repository.rm_branch(merge_request.author, 'conflict-resolvable')
+      merge_request.clear_memoized_source_branch_exists
 
       expect(conflicts_service(merge_request).can_be_resolved_in_ui?).to be_falsey
     end