Optimize merge request refresh by using the database to check commit SHAs

Previously for a given merge request, we would:

1. Create the array of commit SHAs involved in the push (A)
2. Request all merge request commits and map the SHA (B)
3. Reload the diff if there were any common commits between A and B

We can avoid additional database querying and overhead by
checking with the database whether the merge request contains any
of the commit SHAs.

Relates to https://gitlab.com/gitlab-org/gitlab-ce/issues/53213

app/models/merge_request.rb

@@ -353,6 +353,15 @@ class MergeRequest < ActiveRecord::Base
     end
   end
 
+  # Returns true if there are commits that match at least one commit SHA.
+  def includes_any_commits?(shas)
+    if persisted?
+      merge_request_diff.commits_by_shas(shas).exists?
+    else
+      (commit_shas & shas).present?
+    end
+  end
+
   # Calls `MergeWorker` to proceed with the merge process and
   # updates `merge_jid` with the MergeWorker#jid.
   # This helps tracking enqueued and ongoing merge jobs.

app/models/merge_request_diff.rb

@@ -140,6 +140,12 @@ class MergeRequestDiff < ActiveRecord::Base
     merge_request_diff_commits.map(&:sha)
   end
 
+  def commits_by_shas(shas)
+    return [] unless shas.present?
+
+    merge_request_diff_commits.where(sha: shas)
+  end
+
   def diff_refs=(new_diff_refs)
     self.base_commit_sha = new_diff_refs&.base_sha
     self.start_commit_sha = new_diff_refs&.start_sha

app/services/merge_requests/refresh_service.rb

@@ -87,11 +87,8 @@ module MergeRequests
       filter_merge_requests(merge_requests).each do |merge_request|
         if branch_and_project_match?(merge_request) || @push.force_push?
           merge_request.reload_diff(current_user)
-        else
-          mr_commit_ids = merge_request.commit_shas
-          push_commit_ids = @commits.map(&:id)
-          matches = mr_commit_ids & push_commit_ids
-          merge_request.reload_diff(current_user) if matches.any?
+        elsif merge_request.includes_any_commits?(push_commit_ids)
+          merge_request.reload_diff(current_user)
         end
 
         merge_request.mark_as_unchecked
@@ -104,6 +101,10 @@ module MergeRequests
     end
     # rubocop: enable CodeReuse/ActiveRecord
 
+    def push_commit_ids
+      @push_commit_ids ||= @commits.map(&:id)
+    end
+
     def branch_and_project_match?(merge_request)
       merge_request.source_project == @project &&
         merge_request.source_branch == @push.branch_name

changelogs/unreleased/sh-optimize-mr-commit-sha-lookup.yml

@@ -0,0 +1,5 @@
+---
+title: Optimize merge request refresh by using the database to check commit SHAs
+merge_request: 22731
+author:
+type: performance

spec/models/merge_request_diff_spec.rb

@@ -211,4 +211,25 @@ describe MergeRequestDiff do
       expect(diff_with_commits.commits_count).to eq(29)
     end
   end
+
+  describe '#commits_by_shas' do
+    let(:commit_shas) { diff_with_commits.commit_shas }
+
+    it 'returns empty if no SHAs were provided' do
+      expect(diff_with_commits.commits_by_shas([])).to be_empty
+    end
+
+    it 'returns one SHA' do
+      commits = diff_with_commits.commits_by_shas([commit_shas.first, Gitlab::Git::BLANK_SHA])
+
+      expect(commits.count).to eq(1)
+    end
+
+    it 'returns all matching SHAs' do
+      commits = diff_with_commits.commits_by_shas(commit_shas)
+
+      expect(commits.count).to eq(commit_shas.count)
+      expect(commits.map(&:sha)).to match_array(commit_shas)
+    end
+  end
 end

spec/models/merge_request_spec.rb

@@ -2611,6 +2611,32 @@ describe MergeRequest do
     end
   end
 
+  describe '#includes_any_commits?' do
+    it 'returns false' do
+      expect(subject.includes_any_commits?([Gitlab::Git::BLANK_SHA])).to be_falsey
+    end
+
+    it 'returns true' do
+      expect(subject.includes_any_commits?([subject.merge_request_diff.head_commit_sha])).to be_truthy
+    end
+
+    it 'returns true even when there is a non-existent comit' do
+      expect(subject.includes_any_commits?([Gitlab::Git::BLANK_SHA, subject.merge_request_diff.head_commit_sha])).to be_truthy
+    end
+
+    context 'unpersisted merge request' do
+      let(:new_mr) { build(:merge_request) }
+
+      it 'returns false' do
+        expect(new_mr.includes_any_commits?([Gitlab::Git::BLANK_SHA])).to be_falsey
+      end
+
+      it 'returns true' do
+        expect(new_mr.includes_any_commits?([subject.merge_request_diff.head_commit_sha])).to be_truthy
+      end
+    end
+  end
+
   describe '#can_allow_collaboration?' do
     let(:target_project) { create(:project, :public) }
     let(:source_project) { fork_project(target_project) }