Use persisted/memoized value for MRs shas instead of doing git lookups

app/controllers/projects/merge_requests_controller.rb

@@ -187,7 +187,7 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
       begin
         @merge_request.environments_for(current_user).map do |environment|
           project = environment.project
-          deployment = environment.first_deployment_for(@merge_request.diff_head_commit)
+          deployment = environment.first_deployment_for(@merge_request.diff_head_sha)
           stop_url =
             if environment.stop_action? && can?(current_user, :create_deployment, environment)

app/models/environment.rb

@@ -99,8 +99,8 @@ class Environment < ActiveRecord::Base
     folder_name == "production"
   end
-  def first_deployment_for(commit)
-    ref = project.repository.ref_name_for_sha(ref_path, commit.sha)
+  def first_deployment_for(commit_sha)
+    ref = project.repository.ref_name_for_sha(ref_path, commit_sha)
     return nil unless ref

app/models/merge_request.rb

@@ -375,15 +375,27 @@ class MergeRequest < ActiveRecord::Base
   end
   def diff_start_sha
-    diff_start_commit.try(:sha)
+    if persisted?
+      merge_request_diff.start_commit_sha
+    else
+      target_branch_head.try(:sha)
+    end
   end
   def diff_base_sha
-    diff_base_commit.try(:sha)
+    if persisted?
+      merge_request_diff.base_commit_sha
+    else
+      branch_merge_base_commit.try(:sha)
+    end
   end
   def diff_head_sha
-    diff_head_commit.try(:sha)
+    if persisted?
+      merge_request_diff.head_commit_sha
+    else
+      source_branch_head.try(:sha)
+    end
   end
   # When importing a pull request from GitHub, the old and new branches may no
@@ -646,7 +658,7 @@ class MergeRequest < ActiveRecord::Base
     !ProtectedBranch.protected?(source_project, source_branch) &&
       !source_project.root_ref?(source_branch) &&
       Ability.allowed?(current_user, :push_code, source_project) &&
-      diff_head_commit == source_branch_head
+      diff_head_sha == source_branch_head.try(:sha)
   end
   def should_remove_source_branch?

changelogs/unreleased/mr-commit-optimization.yml

+---
+title: Use persisted/memoized value for MRs shas instead of doing git lookups
+merge_request: 17555
+author:
+type: performance

spec/models/environment_spec.rb

@@ -142,15 +142,15 @@ describe Environment do
     let(:commit)        { project.commit.parent }
     it 'returns deployment id for the environment' do
-      expect(environment.first_deployment_for(commit)).to eq deployment1
+      expect(environment.first_deployment_for(commit.id)).to eq deployment1
     end
     it 'return nil when no deployment is found' do
-      expect(environment.first_deployment_for(head_commit)).to eq nil
+      expect(environment.first_deployment_for(head_commit.id)).to eq nil
     end
     it 'returns a UTF-8 ref' do
-      expect(environment.first_deployment_for(commit).ref).to be_utf8
+      expect(environment.first_deployment_for(commit.id).ref).to be_utf8
     end
   end