Don't delete a branch involved in an open merge request in "Delete all merged branches" service

Customers were surprised by the previous behavior, which destroyed branches
even though an open merge request existed for it.

Closes #29427

app/services/delete_merged_branches_service.rb

@@ -8,9 +8,20 @@ class DeleteMergedBranchesService < BaseService
 
     branches = project.repository.branch_names
     branches = branches.select { |branch| project.repository.merged_to_root_ref?(branch) }
+    # Prevent deletion of branches relevant to open merge requests
+    branches -= merge_request_branch_names
 
     branches.each do |branch|
       DeleteBranchService.new(project, current_user).execute(branch)
     end
   end
+
+  private
+
+  def merge_request_branch_names
+    # reorder(nil) is necessary for SELECT DISTINCT because default scope adds an ORDER BY
+    source_names = project.origin_merge_requests.opened.reorder(nil).uniq.pluck(:source_branch)
+    target_names = project.merge_requests.opened.reorder(nil).uniq.pluck(:target_branch)
+    (source_names + target_names).uniq
+  end
 end

changelogs/unreleased/sh-issue-29247-fix.yml

+---
+title: Don't delete a branch involved in an open merge request in "Delete all merged
+  branches" service
+merge_request:
+author:

spec/factories/merge_requests.rb

@@ -40,6 +40,10 @@ FactoryGirl.define do
       state :closed
     end
 
+    trait :opened do
+      state :opened
+    end
+
     trait :reopened do
       state :reopened
     end

spec/services/delete_merged_branches_service_spec.rb

@@ -42,6 +42,19 @@ describe DeleteMergedBranchesService, services: true do
         expect { described_class.new(project, user).execute }.to raise_error(Gitlab::Access::AccessDeniedError)
       end
     end
+
+    context 'open merge requests' do
+      it 'does not delete branches from open merge requests' do
+        fork_link = create(:forked_project_link, forked_from_project: project)
+        create(:merge_request, :reopened, source_project: project, target_project: project, source_branch: 'branch-merged', target_branch: 'master')
+        create(:merge_request, :opened, source_project: fork_link.forked_to_project, target_project: project, target_branch: 'improve/awesome', source_branch: 'master')
+
+        service.execute
+
+        expect(project.repository.branch_names).to include('branch-merged')
+        expect(project.repository.branch_names).to include('improve/awesome')
+      end
+    end
   end
 
   context '#async_execute' do