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

1 files changed, 11 insertions, 0 deletions

diff --git a/app/services/delete_merged_branches_service.rb b/app/services/delete_merged_branches_service.rb
index 1b5623baebe..3b611588466 100644
--- a/app/services/delete_merged_branches_service.rb
+++ b/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