Add a upper limit for merge request diffs versions

Contributes to https://gitlab.com/gitlab-org/gitlab/-/issues/517497

**Problem**

The upper limit for merge request diffs version is missing.

**Solution**

* Stop creating new diffs versions after reaching the limit.
* Display an error message to the user

app/controllers/projects/merge_requests_controller.rb

@@ -411,6 +411,8 @@ def show_merge_request
       )
     end
 
+    display_max_limit_warning
+
     respond_to do |format|
       format.html do
         # use next to appease Rubocop
@@ -685,6 +687,14 @@ def diffs_stream_resource_url(merge_request, offset, diff_view)
       view: diff_view
     )
   end
+
+  def display_max_limit_warning
+    return unless @merge_request.reached_versions_limit?
+
+    flash[:alert] = format(
+      _("This merge request has reached the maximum limit of %{limit} versions and cannot be updated further. " \
+        "Close this merge request and create a new one instead."), limit: MergeRequest::DIFF_VERSION_LIMIT)
+  end
 end
 
 Projects::MergeRequestsController.prepend_mod_with('Projects::MergeRequestsController')

app/models/merge_request.rb

@@ -39,6 +39,7 @@ class MergeRequest < ApplicationRecord
   SORTING_PREFERENCE_FIELD = :merge_requests_sort
   CI_MERGE_REQUEST_DESCRIPTION_MAX_LENGTH = 2700
   MERGE_LEASE_TIMEOUT = 15.minutes.to_i
+  DIFF_VERSION_LIMIT = 1_000
 
   belongs_to :target_project, class_name: "Project"
   belongs_to :source_project, class_name: "Project"
@@ -2454,6 +2455,12 @@ def squash_option
 
   delegate :squash_always?, :squash_never?, :squash_enabled_by_default?, :squash_readonly?, to: :squash_option
 
+  def reached_versions_limit?
+    return false if Feature.disabled?(:merge_requests_diffs_limit, target_project)
+
+    merge_request_diffs.count >= DIFF_VERSION_LIMIT
+  end
+
   private
 
   def merge_base_pipelines

app/services/merge_requests/reload_diffs_service.rb

@@ -9,6 +9,9 @@ def initialize(merge_request, current_user)
 
     def execute
       old_diff_refs = merge_request.diff_refs
+
+      return if merge_request.reached_versions_limit?
+
       new_diff = merge_request.create_merge_request_diff
 
       clear_cache(new_diff)

config/feature_flags/gitlab_com_derisk/merge_requests_diffs_limit.yml

+---
+name: merge_requests_diffs_limit
+feature_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/517497
+introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/183063
+rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/521970
+milestone: '17.10'
+group: group::source code
+type: gitlab_com_derisk
+default_enabled: false

locale/gitlab.pot

@@ -59421,6 +59421,9 @@ msgstr ""
 msgid "This merge request does not have codequality reports"
 msgstr ""
+msgid "This merge request has reached the maximum limit of %{limit} versions and cannot be updated further. Close this merge request and create a new one instead."
+msgstr ""
+
 msgid "This merge request is closed. To apply this suggestion, edit this file directly."
 msgstr ""

spec/requests/projects/merge_requests_controller_spec.rb

@@ -25,6 +25,30 @@
 
       it { is_expected.to have_gitlab_http_status(:not_found) }
     end
+
+    context 'when diff version limit is reached' do
+      before do
+        stub_const('MergeRequest::DIFF_VERSION_LIMIT', 1)
+      end
+
+      it 'displays a warning' do
+        get project_merge_request_path(project, merge_request)
+
+        expect(flash[:alert]).to include('This merge request has reached the maximum limit')
+      end
+
+      context 'when "merge_requests_diffs_limit" feature flag is disabled' do
+        before do
+          stub_feature_flags(merge_requests_diffs_limit: false)
+        end
+
+        it 'does not display a warning' do
+          get project_merge_request_path(project, merge_request)
+
+          expect(flash[:alert]).to be_blank
+        end
+      end
+    end
   end
 
   describe 'GET #index' do

spec/services/merge_requests/reload_diffs_service_spec.rb

@@ -32,6 +32,26 @@
       subject.execute
     end
 
+    context 'when the number of diff versions reaches the limit' do
+      before do
+        stub_const('MergeRequest::DIFF_VERSION_LIMIT', 1)
+      end
+
+      it 'does not create a new diff' do
+        expect { subject.execute }.not_to change { merge_request.merge_request_diffs.count }
+      end
+
+      context 'when "merge_requests_diffs_limit" feature flag is disabled' do
+        before do
+          stub_feature_flags(merge_requests_diffs_limit: false)
+        end
+
+        it 'creates new merge request diff' do
+          expect { subject.execute }.to change { merge_request.merge_request_diffs.count }.by(1)
+        end
+      end
+    end
+
     context 'cache clearing' do
       it 'clears the cache for older diffs on the merge request' do
         expect_next_instance_of(Gitlab::Diff::FileCollection::MergeRequestDiff) do |instance|