Avoid re-fetching merge-base SHA from Gitaly unnecessarily

2 files changed, 27 insertions, 18 deletions

diff --git a/app/models/compare.rb b/app/models/compare.rb
index 3a8bbcb1acd..19ea2bc3065 100644
--- a/app/models/compare.rb
+++ b/app/models/compare.rb
@@ -1,4 +1,6 @@
 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)
+    strong_memoize(:start_commit) do
+      commit = @compare.base
 
-    commit = @compare.base
-    @start_commit = commit ? ::Commit.new(commit, project) : nil
+      ::Commit.new(commit, project) if commit
+    end
   end
 
   def head_commit
-    return @head_commit if defined?(@head_commit)
+    strong_memoize(:head_commit) do
+      commit = @compare.head
 
-    commit = @compare.head
-    @head_commit = commit ? ::Commit.new(commit, project) : nil
+      ::Commit.new(commit, project) if commit
+    end
   end
   alias_method :commit, :head_commit
 
   def base_commit
-    return @base_commit if defined?(@base_commit)
+    strong_memoize(:base_commit) do
+      return unless start_commit && head_commit
+      return OpenStruct.new(sha: @base_sha) if @base_sha
 
-    @base_commit = if start_commit && head_commit
-                     project.merge_base_commit(start_commit.id, head_commit.id)
-                   else
-                     nil
-                   end
+      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)
diff --git a/app/services/compare_service.rb b/app/services/compare_service.rb
index 1db91c3c90c..2a69a205629 100644
--- a/app/services/compare_service.rb
+++ b/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