Fix extra merge request versions created from forked merge requests

When a forked merge request was created with the same branch name as the
target name, MergeRequests::RefreshService would always create a new
diff even though nothing had changed. For example, on GitLab.com:

1. There were a number of merge requests in the gitlab-ce and www-gitlab-com
projects that had old merge requests from the community.
2. These merge requests originated from forked projects and used the
source branch master.
3. When someone pushed to master in the main repository, MergeRequests::RefreshService
would see that master matched the merge requests in question and generated a new
diff.
4. This led to an explosion of merge request diffs and slowed down the "Changes"
tab considerably.

This change alters MergeRequests::RefreshService so that it will only
refresh the diff if the merge request's source project and branch
match. Otherwise, the refresh will only happen if a pushed commit
contains a commit relevant to the existing merge request.

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/53153

app/services/merge_requests/refresh_service.rb

@@ -85,7 +85,7 @@ module MergeRequests
         .where.not(target_project: @project).to_a
 
       filter_merge_requests(merge_requests).each do |merge_request|
-        if merge_request.source_branch == @push.branch_name || @push.force_push?
+        if branch_and_project_match?(merge_request) || @push.force_push?
           merge_request.reload_diff(current_user)
         else
           mr_commit_ids = merge_request.commit_shas
@@ -104,6 +104,11 @@ module MergeRequests
     end
     # rubocop: enable CodeReuse/ActiveRecord
 
+    def branch_and_project_match?(merge_request)
+      merge_request.source_project == @project &&
+        merge_request.source_branch == @push.branch_name
+    end
+
     def reset_merge_when_pipeline_succeeds
       merge_requests_for_source_branch.each(&:reset_merge_when_pipeline_succeeds)
     end

changelogs/unreleased/sh-fix-issue-53153.yml

+---
+title: Fix extra merge request versions created from forked merge requests
+merge_request: 22611
+author:
+type: fixed

spec/services/merge_requests/refresh_service_spec.rb

@@ -306,6 +306,66 @@ describe MergeRequests::RefreshService do
       end
     end
 
+    context 'forked projects with the same source branch name as target branch' do
+      let!(:first_commit) do
+        @fork_project.repository.create_file(@user, 'test1.txt', 'Test data',
+                                             message: 'Test commit',
+                                             branch_name: 'master')
+      end
+      let!(:second_commit) do
+        @fork_project.repository.create_file(@user, 'test2.txt', 'More test data',
+                                             message: 'Second test commit',
+                                             branch_name: 'master')
+      end
+      let!(:forked_master_mr) do
+        create(:merge_request,
+               source_project: @fork_project,
+               source_branch: 'master',
+               target_branch: 'master',
+               target_project: @project)
+      end
+      let(:force_push_commit) { @project.commit('feature').id }
+
+      it 'should reload a new diff for a push to the forked project' do
+        expect do
+          service.new(@fork_project, @user).execute(@oldrev, first_commit, 'refs/heads/master')
+          reload_mrs
+        end.to change { forked_master_mr.merge_request_diffs.count }.by(1)
+      end
+
+      it 'should reload a new diff for a force push to the source branch' do
+        expect do
+          service.new(@fork_project, @user).execute(@oldrev, force_push_commit, 'refs/heads/master')
+          reload_mrs
+        end.to change { forked_master_mr.merge_request_diffs.count }.by(1)
+      end
+
+      it 'should reload a new diff for a force push to the target branch' do
+        expect do
+          service.new(@project, @user).execute(@oldrev, force_push_commit, 'refs/heads/master')
+          reload_mrs
+        end.to change { forked_master_mr.merge_request_diffs.count }.by(1)
+      end
+
+      it 'should reload a new diff for a push to the target project that contains a commit in the MR' do
+        expect do
+          service.new(@project, @user).execute(@oldrev, first_commit, 'refs/heads/master')
+          reload_mrs
+        end.to change { forked_master_mr.merge_request_diffs.count }.by(1)
+      end
+
+      it 'should not increase the diff count for a new push to target branch' do
+        new_commit = @project.repository.create_file(@user, 'new-file.txt', 'A new file',
+                                                     message: 'This is a test',
+                                                     branch_name: 'master')
+
+        expect do
+          service.new(@project, @user).execute(@newrev, new_commit, 'refs/heads/master')
+          reload_mrs
+        end.not_to change { forked_master_mr.merge_request_diffs.count }
+      end
+    end
+
     context 'push to origin repo target branch after fork project was removed' do
       before do
         @fork_project.destroy