Avoid re-fetching merge-base SHA from Gitaly unnecessarily

app/models/compare.rb

 class Compare
+  include Gitlab::Utils::StrongMemoize
+
   delegate :same, :head, :base, to: :@compare
   attr_reader :project
@@ -11,9 +13,10 @@ class Compare
     end
   end
-  def initialize(compare, project, straight: false)
+  def initialize(compare, project, base_sha: nil, straight: false)
     @compare = compare
     @project = project
+    @base_sha = base_sha
     @straight = straight
   end
@@ -22,40 +25,41 @@ class Compare
   end
   def start_commit
-    return @start_commit if defined?(@start_commit)
-    commit = @compare.base
-    @start_commit = commit ? ::Commit.new(commit, project) : nil
+    strong_memoize(:start_commit) do
+      commit = @compare.base
+      ::Commit.new(commit, project) if commit
+    end
   end
   def head_commit
-    return @head_commit if defined?(@head_commit)
-    commit = @compare.head
-    @head_commit = commit ? ::Commit.new(commit, project) : nil
+    strong_memoize(:head_commit) do
+      commit = @compare.head
+      ::Commit.new(commit, project) if commit
+    end
   end
   alias_method :commit, :head_commit
   def base_commit
-    return @base_commit if defined?(@base_commit)
-    @base_commit = if start_commit && head_commit
-                     project.merge_base_commit(start_commit.id, head_commit.id)
-                   else
-                     nil
-                   end
+    strong_memoize(:base_commit) do
+      return unless start_commit && head_commit
+      return OpenStruct.new(sha: @base_sha) if @base_sha
+      project.merge_base_commit(start_commit.id, head_commit.id)
+    end
   end
   def start_commit_sha
-    start_commit.try(:sha)
+    start_commit&.sha
   end
   def base_commit_sha
-    base_commit.try(:sha)
+    base_commit&.sha
   end
   def head_commit_sha
-    commit.try(:sha)
+    commit&.sha
   end
   def raw_diffs(*args)

app/services/compare_service.rb

@@ -10,9 +10,14 @@ class CompareService
     @start_ref_name = new_start_ref_name
   end
-  def execute(target_project, target_ref, straight: false)
+  def execute(target_project, target_ref, base_sha: nil, straight: false)
     raw_compare = target_project.repository.compare_source_branch(target_ref, start_project.repository, start_ref_name, straight: straight)
-    Compare.new(raw_compare, target_project, straight: straight) if raw_compare
+    return unless raw_compare
+
+    Compare.new(raw_compare,
+                target_project,
+                base_sha: base_sha,
+                straight: straight)
   end
 end

changelogs/unreleased/osw-stop-recalculating-merge-base-on-mr-loading.yml

+---
+title: Avoid re-fetching merge-base SHA from Gitaly unnecessarily
+merge_request:
+author:
+type: performance

lib/gitlab/diff/diff_refs.rb

@@ -44,7 +44,11 @@ module Gitlab
           project.commit(head_sha)
         else
           straight = start_sha == base_sha
-          CompareService.new(project, head_sha).execute(project, start_sha, straight: straight)
+
+          CompareService.new(project, head_sha).execute(project,
+                                                        start_sha,
+                                                        base_sha: base_sha,
+                                                        straight: straight)
         end
       end
     end

spec/models/compare_spec.rb

@@ -59,6 +59,36 @@ describe Compare do
     end
   end
+  describe '#base_commit_sha' do
+    it 'returns @base_sha if it is present' do
+      expect(project).not_to receive(:merge_base_commit)
+
+      sha = double
+      service = described_class.new(raw_compare, project, base_sha: sha)
+
+      expect(service.base_commit_sha).to eq(sha)
+    end
+
+    it 'fetches merge base SHA from repo when @base_sha is nil' do
+      expect(project).to receive(:merge_base_commit)
+        .with(start_commit.id, head_commit.id)
+        .once
+        .and_call_original
+
+      expect(subject.base_commit_sha)
+        .to eq(project.repository.merge_base(start_commit.id, head_commit.id))
+    end
+
+    it 'is memoized on first call' do
+      expect(project).to receive(:merge_base_commit)
+        .with(start_commit.id, head_commit.id)
+        .once
+        .and_call_original
+
+      3.times { subject.base_commit_sha }
+    end
+  end
+
   describe '#diff_refs' do
     it 'uses base_commit sha as base_sha' do
       expect(subject).to receive(:base_commit).at_least(:once).and_call_original