Adds different status code when user deletion fails

changelogs/unreleased/20890-soft-delete-user-api-status-code.yml

+---
+title: Adds 409 when user cannot be soft deleted through the API
+merge_request: 21037
+author:
+type: fixed

doc/api/users.md

@@ -430,7 +430,7 @@ e.g. when renaming the email address to some existing one.
 ## User deletion
 
 Deletes a user. Available only for administrators.
-This returns a `204 No Content` status code if the operation was successfully or `404` if the resource was not found.
+This returns a `204 No Content` status code if the operation was successfully, `404` if the resource was not found or `409` if the user cannot be soft deleted.
 
 ```
 DELETE /users/:id

lib/api/users.rb

@@ -452,6 +452,7 @@ module API
 
         user = User.find_by(id: params[:id])
         not_found!('User') unless user
+        conflict!('User cannot be removed while is the sole-owner of a group') unless user.can_be_removed? || params[:hard_delete]
 
         destroy_conditionally!(user) do
           user.delete_async(deleted_by: current_user, params: params)

spec/requests/api/users_spec.rb

@@ -1261,6 +1261,25 @@ describe API::Users do
       expect { Namespace.find(namespace.id) }.to raise_error ActiveRecord::RecordNotFound
     end
 
+    context "sole owner of a group" do
+      let!(:group) { create(:group).tap { |group| group.add_owner(user) } }
+
+      context "hard delete disabled" do
+        it "does not delete user" do
+          perform_enqueued_jobs { delete api("/users/#{user.id}", admin)}
+          expect(response).to have_gitlab_http_status(409)
+        end
+      end
+
+      context "hard delete enabled" do
+        it "delete user and group", :sidekiq_might_not_need_inline do
+          perform_enqueued_jobs { delete api("/users/#{user.id}?hard_delete=true", admin)}
+          expect(response).to have_gitlab_http_status(204)
+          expect(Group.exists?(group.id)).to be_falsy
+        end
+      end
+    end
+
     it_behaves_like '412 response' do
       let(:request) { api("/users/#{user.id}", admin) }
     end