Extract MergeRequestDiffCommit deletions

Projects will have at least one merge_request_diff_commit for every commit
contained in every MR, which deleting via `project.destroy!` and
cascading deletes may exceed statement timeouts, causing failures.
(see https://gitlab.com/gitlab-org/gitlab/-/issues/346166)

Changelog: fixed

app/services/projects/destroy_service.rb

@@ -140,6 +140,10 @@ module Projects
       destroy_project_bots!
       destroy_ci_records!
 
+      if ::Feature.enabled?(:extract_mr_diff_commit_deletions, default_enabled: :yaml)
+        destroy_mr_diff_commits!
+      end
+
       # Rails attempts to load all related records into memory before
       # destroying: https://github.com/rails/rails/issues/22510
       # This ensures we delete records in batches.
@@ -154,6 +158,33 @@ module Projects
       log_info("Attempting to destroy #{project.full_path} (#{project.id})")
     end
 
+    # Projects will have at least one merge_request_diff_commit for every commit
+    #   contained in every MR, which deleting via `project.destroy!` and
+    #   cascading deletes may exceed statement timeouts, causing failures.
+    #   (see https://gitlab.com/gitlab-org/gitlab/-/issues/346166)
+    #
+    # rubocop: disable CodeReuse/ActiveRecord
+    def destroy_mr_diff_commits!
+      mr_batch_size = 100
+      delete_batch_size = 1000
+
+      project.merge_requests.each_batch(column: :iid, of: mr_batch_size) do |relation_ids|
+        loop do
+          inner_query = MergeRequestDiffCommit
+            .select(:merge_request_diff_id, :relative_order)
+            .where(merge_request_diff_id: MergeRequestDiff.where(merge_request_id: relation_ids).select(:id))
+            .limit(delete_batch_size)
+
+          deleted_rows = MergeRequestDiffCommit
+            .where('(merge_request_diff_commits.merge_request_diff_id, merge_request_diff_commits.relative_order) IN (?)', inner_query)
+            .delete_all
+
+          break if deleted_rows == 0
+        end
+      end
+    end
+    # rubocop: enable CodeReuse/ActiveRecord
+
     def destroy_ci_records!
       project.all_pipelines.find_each(batch_size: BATCH_SIZE) do |pipeline| # rubocop: disable CodeReuse/ActiveRecord
         # Destroy artifacts, then builds, then pipelines

config/feature_flags/development/extract_mr_diff_commit_deletions.yml

+---
+name: extract_mr_diff_commit_deletions
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/75963
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/347073
+milestone: '14.6'
+type: development
+group: group::code review
+default_enabled: false

spec/services/projects/destroy_service_spec.rb

@@ -97,6 +97,20 @@ RSpec.describe Projects::DestroyService, :aggregate_failures do
     end
   end
 
+  shared_examples_for "deleting a project with merge requests" do
+    let!(:merge_request) { create(:merge_request, source_project: project) }
+
+    it "deletes merge request and related records" do
+      merge_request_diffs = merge_request.merge_request_diffs
+      expect(merge_request_diffs.size).to eq(1)
+
+      mrdc_count = MergeRequestDiffCommit.where(merge_request_diff_id: merge_request_diffs.first.id).count
+
+      expect { destroy_project(project, user, {}) }
+      .to change { MergeRequestDiffCommit.count }.by(-mrdc_count)
+    end
+  end
+
   it_behaves_like 'deleting the project'
 
   it 'invalidates personal_project_count cache' do
@@ -105,6 +119,25 @@ RSpec.describe Projects::DestroyService, :aggregate_failures do
     destroy_project(project, user, {})
   end
 
+  context "extract_mr_diff_commit_deletions feature flag" do
+    context "with flag enabled" do
+      before do
+        stub_feature_flags(extract_mr_diff_commit_deletions: true)
+        allow(project).to receive(:destroy!).and_return(true)
+      end
+
+      it_behaves_like "deleting a project with merge requests"
+    end
+
+    context "with flag disabled" do
+      before do
+        stub_feature_flags(extract_mr_diff_commit_deletions: false)
+      end
+
+      it_behaves_like "deleting a project with merge requests"
+    end
+  end
+
   context 'with running pipelines' do
     let!(:pipelines)               { create_list(:ci_pipeline, 3, :running, project: project) }
     let(:destroy_pipeline_service) { double('DestroyPipelineService', execute: nil) }